CHAP. I.

THe first Explication of mine, that the Learned Doctor ani­madverts upon in his 12th Chapter, is, that which I give in the 33th of my Physico-Mechanical Experiments, touching the Spring and Weight of the Air; where I relate, that the Sucker in the Air-Pump of our En­gine, having been forcibly depress'd to the lower part of the Brass Cylin­der, which yet was carefully closed at the top, so that the cavity of the Cylinder was empty of Air; this Sucker, I say, would in this case ap­pear spontaneously to remount to­wards the top of the Cylinder, though [Page 7]it were clogg'd with a hundred pound weight to hinder its ascent. Which Phaenomenon I ascrib'd to this, that the Sucker being, by the withdraw­ing of the Air in the Cylinder, freed from the wonted force of the springy Air that endeavour'd to depress the internal part of it, was not inabled by the appendant weight to resist the pressure of an Atmospherical Cylin­der equal in diameter to it, which, pressing against its lower or exter­nal surface, endeavour'd to impel it up.

Now the Doctor having in the two first Paragraphs made a Description of my Engine, (which I shall now pass over) does in the third teach us, that the Corporeal cause, if there be any, of the ascent of the Sucker, must be, either in the Sucker it self, or in the almost exhausted cavity of the Cylinder, or lastly in the exter­nal [Page 8]Air: Which premised, he does in the same third Section, and in the fourth, endeavour to prove at large, that the cause is to be derived neither from the one, nor from the other of the two first. And therefore I, that maintain neither of the Opinions he disputes against, shall leave those Pa­ragraphs of his untouch'd. Nor shall I meddle with the fifth, sixth, and se­venth, where he argues against the explications of some, that would solve the Phaenomenon upon some Cartesian grounds, and as well amply as par­ticularly against the solution that he supposes would be given of it con­gruously to his own Sentiments by the Learned Regius. These Discourses, I say, of the Doctors I leave un­touch'd; because 'tis at length in the eighth Paragraph, that he im­pugnes that solution of the Phaeno­menon, which he ascribes to me, whose [Page 9]Opinion he first delivers, though not just in the terms I would express it my self; yet I dare say very sincere­ly, and so near my sence, that I shall forthwith pass from the eighth Secti­on to the beginning of the ninth, where he begins to propose his Ob­jections, which he is pleas'd to usher in with a complement to me, that I should be very vain if I looked upon as any thing more than a Comple­ment.

To his first Objection, propos'd in these words, 139. Primò enim, si haec solutio verè mechanica sit, quae tandem Causa verè mechanica assi­gnari potest gravitationis singularum particularum, totiúsque atmosphaerae in suis locis? Nam quod materiam subtilem attinet, &c. I answer, that I did not in that Book intend to write a whole Systeme, or so much as the Elements of Natural Philosophy; but [Page 10]having sufficiently proved, that the Air, we live in, is not devoid of weight, and is endowed with an Ela­stical Power or springiness, I endea­vour'd by those two Principles to ex­plain the Phaenomena exhibited in our Engine, and particularly that now under debate, without recourse to a Fuga Vacui, or the Anima Mun­di, or any such unphysical Principle. And since such kind of Explications have been of late generally called Mechanical, in respect of their be­ing grounded upon the Laws of the Mechanicks; I, that do not use to contend about Names, suffer them quietly to be so: And to entitle my now examined Explication to be Me­chanical, as far as I pretend, and in the usual sence of that expression, I am not obliged to treat of the cause of Gravity in general; since many Propositions of Archimedes, Stevinus, [Page 11]and those others that have written of Staticks, are confessed to be Mathe­matically or Mechanically demon­strated, though those Authors do not take upon them to assign the true cause of Gravity, but take it for granted, as a thing universally ac­knowledged, that there is such a quality in the Bodies they treat of. And if in each of the Scales of an or­dinary and just Ballance, a pound weight, for instance, be put; he that shall say, that the Scales hang still in Aequilibrium, because the equal weights counterpoise one another: and in case an ounce be put into one of the Scales, and not into the oppo­site; he that shall say that the loaded Scale is depress'd, because 'tis urged by a greater weight than the other, will be thought to have given a Me­chanical Explication of the Aequili­brium of the Scales, and their losing [Page 12]it; though he cannot give a true cause, why either of those Scales tends towards the Center of the Earth. Since then the assigning of the true cause of Gravity is not required in the Staticks themselves, though one of the principal and most known of the Mechanical Disci­plines; Why may not other Pro­positions and Accounts, that suppose Gravity in the Air, (nay prove it, though not à priori) be look'd on as Mechanical?

CHAP. II.

THe next thing the Doctor op­poses to my Explication, is a resolute Denial, that there is any such Gravitation, as I pretend, of Bodies, or their Particles, in their [Page 13]proper places. But because, for the proof of his negation, he refers us to the next Chapter, we shall hereafter have a fitter place than this to con­sider it in.

Thirdly, he tells us, we may justly doubt of the equal diffusion of the Springy power, or the Pressure of the Air every way. In what sence, in some cases, I admit of a small inaequa­lity between the pressure of Fluids against differing parts of a surrounded body, I have See the Hy­drostatical Paradoxes, esp. cially Parad. 7. elsewhere declared, and need not here discourse of; since in the case before us, and in the like, that Pressure is inconsiderable enough to be safely neglected. And whereas our Author thus ar­gues, p. 139. Semotâ vi Elasticâ, par­ticulae tamen Atmosphaerae deorfum tenderent. Est igitur depressio quae­dam deorsum praeter vim Elasticam [Page 14]ipsi superaddita; sursum non item sed elastica sola, éstque suppar ratio in pressionious transversis & obliquis. I presume, he did not sufficiently con­sider our Hypothesis and the nature of the pressure of Fluid Bodies that have weight: For Water, to which no Springiness is ascribed, as there is to Air, but which acts by its weight and fluidity, is able upon the score of those Qualities to buoy up great Ships, that the ebbing Tide often leaves upon the strand.

And whereas the Learned Exami­ner proposes a fourth Objection in these terms, p. 139. Quibus omnibus addas, difficile esse intellectu, si unius Cylindri Atmosphaerae pondus ae­qualis diametri cum Embolo reflectione in fundum Emboli derivetur, cur non quinque alii Cylindri Aeris qui cir­cumstant Embolum in ejus fundum eo­dem modo simul agere possunt, ita ut [Page 15]vis sursum impellens Embolum sextuplo major sit quàm hactenus ab bujus opi­nionis fautoribus existim ita est. Quod si sit, tunc certe, siquo artificio fieri possit ut unius solius Cylindri actio in Embolum admitteretur, re iquorum quinque exclusa, & pari tamen faci­litate Embolus ascenderet, manifestum indicium esset, ne unum quidem Cylin­drum Atsmosphaerae agere in fundum Emboli, sed totam Hypothesin, inge­niosam tantummodo esse fictionem. I presume, Hydrostaticians will think, this might have been spared. For they will tell him, that there can no more of a fluid press directly upward against the Cylindrical Orifice of a Body immers'd in that fluid, than a Cylinder of that fluid of the same dia­meter with the Orifice (the lateral pressures bearing against the lateral parts of the Cylinder.) And there­fore if you invert, for instance, a [Page 16]Pipe open at both ends, and filled to a certain height with Oyl, into com­mon water; the Oyl that is kept up by the pressure of the water upwards, will keep at the same height as to sense, whether the Vessel that con­tains the Water be broad or narrow, provided it be somewhat larger than the Orifice of the Pipe.

And now, to invalidate yet further the precedent Objections, made by the Doctor, I shall add, that it need not be thought incredible, that the Atmosphere by its weight, or the Spring of the Air compress'd by that weight, should be able to raise up fourscore or a hundred pound, hang­ing at the Sucker: Since I have See Continuat. of New Exper. Physico-Mechan. Exp. 48. p. 165. manifested two or three years ago by a clear and cogent Expe­riment, that a little air included in a Bladder will by its meer Spring be [Page 17]able to heave up a weight of a hun­dred Pound, and this without the help of any rarefaction by heat. By which Experiment may be also con­firmed, what I deliver'd a while since about the endeavour of the Air, that is wont to be included in our brass Cylinder, by expanding it self to thrust away the Sucker (which, in regard of the structure of the Pump, it can do no other­wise than downwards,) with a de­pressing force, aequivalent to the pressure upwards of the Atmosphere against the external part of the same Sucker.

CHAP. III.

BUt I shall not insist upon the foregoing Objections, because the Learned Doctor himself tells us, [Page 18]that their attempts may seem to be but light skirmishes in comparison of that which follows. Whereunto I shall therefore apply my atten­tion.

This grand Objection our Learned Adversary takes from the already often-mention'd ascent of the Sucker clogg'd with a hundred pound weight, and recommends by this introdu­ction. p. 140. Etenim ex ipsis Phae­nomeni visceribus robustissimum jam contra omnem mechanicam illius solutionem Argumentum eruo, & quod non solum contra vim aeris elasticam suprà dicto modo explicatam militat, sed etiam contra Cartesianum illum aeris conatum nixúmque, &c. Which premis'd, the Argument it self is thus propos'd: p. 140. Est enim (says he) juxta hujus experimenti Phaeno­menon, vis illa aeris elastica (nixus­que expansorius) major multo quàm [Page 19]quae fieri potest à rerum natura, quám­que quotidianis illis Phaenomenis con­gruit. Nam si nixus hic elasticus tan­tam vim elasticam haberet ut plus cen­tum pondo plumbum sursum possit pro­pellere, omnes profecto rerum terre­strium compages tantâ violentiâ com­primerentur, ut nullae, nisi quae ad­modum firmiter compactae sint, tantae compressioni resistere possent, quin con­fringerentur, vel partium collisione ita contererentur ut brevi tempore peri­rent, &c.

Though this Objection be specious enough, yet it presents me with no difficulty, that I was not well aware of; as I presume you will easily per­ceive by what you will meet with in the following Papers, especially that, which consists of Experiments and Considerations about the differing Pressures of Solids, Weights, and Am­bient Fluids. The nature of which [Page 20]Pressure and its aequality (as far as in our controversie 'tis needful to be suppos'd) will, I hope, satisfie you of the invalidity of the propos'd ob­jections; especially since the Do­ctrine it impugns, namely the Weight and Pressure of the Atmosphere, is not a bare Hypothesis, but a truth made out by divers Experiments, by which even profess'd Opposers of it have publickly acknowledg'd themselves to be convinced.

CHAP. IV.

IN the next Paragraph (which is the 11th.) the Learned Doctor adds a further Objection, wherein he supposes, that there is laid upon a wooden Scale, of the same diameter with the above-mention'd Sucker, a lump of Butter of the same largeness [Page 21]with the Scale. Whence he argues, that if our Hypothesis take place, the Butter must be press'd against by two Cylinders of Air, the one pressing it upwards, the other downwards, and the pressure of them both amounting to two hundred pounds. But, sayes he, the Butter is not press'd at all, as appears by this, that no serous hu­mour is squeez'd out of it towards the edges, not so much as in those parts that lie parallel to the Hori­zon, whence the Conclusion seems easie to be deduc'd.

But in the 12th Paragraph, the Doctor himself proposes a Solution, which he might easily foresee I would employ to invalidate his Argument; Namely, that the Air pressing, as well against the sides of the Butter, as against the top and bottom, hin­ders the Mass from horizontally ex­tending it self. And whereas, by [Page 22]way of reply to this subterfuge, as 'tis call'd in the margent, he sub­joynes, p. 142. Cui respondeo, quòd tamen hoc nihil prohibet quo mi­nùs in omnes partes horizontales ex­primatur humor serosus & lacteus, si revera esset ulla hujusmodi pressura elastica qualis fingitur. The Reply is easie, that the pressure of the am­bient Air, which is a fluid more sub­til than Butter-milk, will as well hin­der the starting out of that liquor as of the parts of the Butter it self: As he will easily grant, that attentively considers the nature of the thing, and remembers how Air keeps Water from running out at the little holes of a Gardeners Watering-pot clos'd at the top. What the Objector adds about the extrusion of what he calls a sub­tiler Element (supposed to be har­bour'd in the Butter) by the pres­sure of the Atmosphere, in case it had [Page 23]any, I think it would not be difficult to answer, if we consider'd, that a great and undeniable pressure ap­plied to water does not sensibly con­dense it, or deprive it of its fluidity, because of the grossness and strength of its parts. But the Argument be­ing but transiently mention'd by the Author, and grounded upon a Car­tesian supposition that I never em­ployed, I leave it to those that may think themselves concern'd (which I am not) to make a solemn answer to.

And whereas our Learned Exami­ner superadds, p. 142. Quod tametsi butyri massa in disci lignei spe­ciem reducta, cujus margo centum vi­cibus areâ sit minor, interque duas laminas ligneas ejusdem formae ac lati­tudinis posita, filis sus [...] enderetur in aere tanquam in lance, ita ut pressura aeris elastica quà ab infra, quà de­super [Page 24]ducentis fere vicibus excessura sit pressionem in marginem butyri, buty­rum tamen nihilo arctiùs comprimetur per vim aeris elasticam, nec aliter hîc afficietur quàm antea: He seems not to have sufficiently consider'd the Laws of the Hydrostaticks, according to which, supposing the pressure of the Atmosphere that he rejects, the Butter ought not to be deprived of its shape. For the pressure of the am­bient Air, being equal on all sides, if we suppose the superficies of the But­ter to be distinguish'd into a multi­tude of little equal portions, each of these, whether they be scituated Ho­rizontally, or on the edges, can be press'd against but by an Atmosphe­rical Pillar equal to its Basis; and the Horizontal portions, if I may so call them, cannot be thrust out of place, without there be at the same time squeez'd out some of the Lateral por­tions, [Page 25]which yet cannot be so dis­placed, because they also are with equal force press'd (inwards) by little aerial Pillars, whose Bases are contiguous to them, and bear against them. Which Answer, though of it self sufficient, may be much confirm'd by the Instance, you will hereafter meet with, of a lump of Butter that kept its irregular shape, in spight of a great and manifest pressure of the wa­ter that surrounded it.

And this Answer may suffice to dis­prove, what the Doctor annexes in the beginning of the 13th Paragraph, about the vast excess of Pressure, which the Air exercises upon the flat and Horizontal surfaces of the above-mention'd lump of Butter, in com­parison of the pressure the Marginal parts of its surface can be exposed to. What he adds, and illustrates with a Scheme, about the hands being [Page 26]assisted with the pressure of the Air, it concerns not me to answer. But whereas among the places where the Elastical power of the Air is under­stood not to reach, he reckons a Pail full of water, with a lump of butter put in it; he supposes that, which our Hydrostaticks will by no means allow, and which is disproved by se­veral both of our former Experiments, and by those you will meet with in the following Papers. By which it appears, that the pressure of the At­mosphere is exercised, as indeed I do not see what should hinder it from being, even upon Bodies that are quite immers'd under water; and by which, added to what has been hi­therto discours'd in answer to the Learned Doctors Objections, you will easily judge, how deservedly he shuts up the Arguments, we have been examining, with this Conclu­sion. [Page 27] p. 143. Adeo ut extra omnem controversiam positum videa­tur, quòd nulla est ejusmodi vis ela­stica in aere, qualem è doctis non­nulli supponunt, multoque minus tam fortis ut centum librarum pondus su­peret. Quod erat Demonstrandum.

CHAP. V.

BUt this is not all the Doctor ur­ges against me in this Chapter; for in the 14 ^th Paragraph he seconds his former argument by another, drawn from this Experiment of mine, That having taken two round Mar­bles, whose surfaces, that were to be contiguous, were as well ground ve­ry flat as carefully polish'd; and ha­ving placed them one directly upon the other, they did in a horizontal [Page 28]posture so firmly cohere, without the help of any Glue, See the Hist. of Flu­idity and Firmness, p. 222. of the second E [...]ition. or viscous Body, that the upper Marble being pull'd up, would take up the lower, though clogg'd with a weight of fourscore and odd pounds.

This Experiment, when I many years ago first publish'd it, I referr'd to the action of the Atmosphere, which pressing equally and strongly against the surfaces of both the Mar­bles, except where they were conti­guous, the higher could not be drawn directly upwards from the lower (and consequently must be follow'd by it) by a less force than that which was equivalent to the weight of as great a Cylinder of the Atmosphere as lean'd upon the upper Marble.

This Experiment thus explain'd, though it hath been judged a very fa­vourable [Page 29]one to the Hypothesis on whose behalf I alledged it, does yet to the justly famous Doctor seem a very considerable Argument against it, though for this judgement of his he urges only this reason, That if the force, with which the Air presses the lower Marble against the upper, be able to sustain that Marble, though clogg'd with the great weight above mention'd, the same pressure of Air would much more easily support a Plate of wood brought to a true plain, and not loaded with any weight, if the wooden Plate were substituted to the lower Marble, and instead of it applied to the upper.

But since the Experiment, as I pro­posed it, did upon tryal succeed ve­ry well, it had not been amiss if the Learned Examiner had consider'd it as it was really and successfully made, and shewed why the pressure of the [Page 30]ambient Air was not able to hinder the separation of the Marbles: And his needless substitution of a Wooden plate instead of the lower Marble ea­sily suggests a suspition, that there may lie some fallacy, though not in­tended by him, in the variation he proposes of the Experiment. And he seems to have himself had thoughts of this kind, by taking notice, that it may be answer'd on our behalf, that a Wooden Plate cannot be so exactly applied to the upper Marble, but that there will be a little Air intercepted between it and the bottom of that stone. And though having granted that it may be so, he employes two pages to shew, that this intermediate Air could not keep the pressure of the Atmosphere from supporting the un­clogg'd plate of wood, if it had been That pressure, which, when there was no such intermediate Air, had sustain'd [Page 31]the lower Marble with all the appen­dant weight; yet I confess his Proofs seem not to me to be answerable to the Assurance he uses in speaking of them. His Examples taken from Gunpowder and Wind, you will easily judge not to be very proper, where we are not considering a force that acts by a sudden and vanishing Im­petus, but a constant and equal pres­sure. And as to his other Instance, which is taken from five men that thrust against the sixth (standing with his back to a Wall) who is but as strong as any one of them: I answer, that neither is this example near e­nough of kin to our case. For each of these five men is supposed to have an equal power of thrusting, proper to himself, and independent from all or any of the other four. And the sixth man is likewise supposed to re­sist but by his own single force, with­out [Page 32]having his power of reacting in­creas'd by the force wherewith the o­thers thrust against him. But in our case the thing is quite otherwise; for supposing that some aerial particles be so placed that a solid Body hinders them to recoil or expand themselves, we are to consider, that, as the conti­guous corpuscles of air press against them not by their own single weight or pressure, but as they transmit the action of all the other particles of the air which by their weight or pressure thrust them on; so the aerial parti­cles, contiguous to the solid Body, resist not barely by that force which they would have if they were not com­press'd, but by vertue of the Sprin­giness they acquire upon the score of the forcible inflection they sustain from the action of the corpuscles, that either mediately or immediately thrust against them; and consequently, in [Page 33]proportion to that external force, the Elasticity of these compress'd Parti­cles will be increased, as we see that a Bow or other Springy body, the more it is bent by an external force, the greater power it has to resist further compression. Upon which grounds it need to be no wonder, that a small portion of Air, being almost included in a solid Body, and having for some (though but very little) time been exposed to the outward air, should be capable of resisting the pressure of as much of the whole Atmosphere, as can come to press against it. For, this pressure of the Atmosphere being continual, if the Springiness of the aerial particles were not now great enough to resist that pressure, they must necessarily have been before­hand inflected or compress'd by it, till the endeavours of the one and the other were reduced to an Aequipol­lency. [Page 34]Of which I shall give you an instance in so obvious a body as a Bubble at the top of water: For though there be but a little Air in­cluded in a very thin and transparent film of water; yet this little air is so well able to resist the weight of all the Atmosphere that can come to bear a­gainst it, that all the pressure of it is not able to make the film shrink, or become wrinkled; which it would do, if the corpuscles of the Internal air were not reduced to a Springiness, which makes its power of resisting equal to the endeavour of the Exter­nal Atmosphere to compress it. And to let you see, that we may well con­ceive such a Springiness of the air included in the Bubbles, I have else­where related, how by barely with­drawing the pressure of the ambient air from Glass-Bubbles hermetically sealed with air in them not compress'd [Page 35]beyond its usual state, the Spring of the Internal air would make the Bubbles fly in pieces: And this will happen to stronger Glasses than Bub­bles, as you will find in one of the former Experiments See the Tract about the Pres­sure of the Airs Spring on bo­dies under wa­ter.. And if we would illustrate what we are debating of by an Example; it should not be by considering, as the Doctor does, the endeavour of five men against the sixth that hath his back to the Wall; but that of five Bladders full of air, pil'd up, and resting upon a sixth. For in this case, whatever force or power of pres­sing we suppose in the incumbent Bladders, they all bear jointly upon the lower, which continuing at a stand, must thereby be so compress'd as to be able to resist their joint endeavours, as 'tis manifest; because otherwise it would not continue in that state, but [Page 36]be farther compress'd; which is against the supposition.

This Notion about Pressure and Resistance I have the more particu­larly deduced, because I found many modern Naturalists, and even Hydro­staticians themselves to be great stran­gers to it. For which reason I shall add, that I have evinc'd it by pur­posely devised Experiments in the Continuation of the Physico-Mecha­nical Experiments Exper. 25. and elsewhere. a­bout the Air. Were it not for this, I should per­haps have spared my self the labour of setting down these thoughts as not necessary to the solution of the Do­ctors Objections. For he admits a Layer, or (as he aptly speaks) an Area of Aerial Particles to be inter­posed between the upper Marble and the Wooden Plate; and therefore the flatness and stiffness of those two Bo­dies [Page 37]must keep them from an imme­diate contact as well at the edges as by the help of the same Area they do elsewhere; and consequently that in­terposed Air may communicate with the ambient Air. From whence the Laws of the Hydrostaticks (which I have elsewhere shewn) will allow me to conclude, that the weight of the Atmosphere endeavours to depress the upper surface of the wooden Plate; and so what the Examiner urges of the inconsiderable resistance, that the few Aerial Particles interposed be­tween the flat Bodies can make to the great pressure of the Column of Air that thrusts the Woodden against the Marble Plate, would not conclude, though our former answer could not have been made; since the resistance, made by the interposed Aerial Par­ticles to the pressure upwards of the Atmosphere, is not in our present sup­position [Page 38]made by those Particles a­lone, but by the weight of the lateral and superior part of the Atmosphere exercised by the intervention of these Particles. Which being so, what the Learned Doctor adds, that the weight of the wooden Plate it self is here of no consideration, must needs be a mi­stake. For the two equal Atmosphe­rical Pressures, the one against the upper surface of the woodden Plate, and the other against the lower, coun­tervailing and consequently frustra­ting the endeavour of each other, the gravity of the wood it self will suffice to make it fall, as well as if it were press'd against by neither of them. And from this Discourse you will ea­sily judge, whether the Doctor had reason to say as he does, p. 146. Quam ab omni ratione (igitur) ab­sonum est, ut superficies illa sive area aerearum particularum, quae insinuant [Page 39]se laminam ligneam inter & marmor, solidam columnam hujusmodi particu­larum, vi elastica sursum enitentium, contra laminam ligneam obnitendo vincat, ipsamque laminam in terram de­turbet.

CHAP. VI.

WHat he adds in the sixteenth number against those that fancy the Aerial Particles to be en­dow'd with Perception, and to act with Design pro re nata, does not at all concern me; and what he adds in the next Paragraph, wherewith he concludes his twelfth Chapter, I shall altogether pass by, as far as it con­cerns the extravagant conceit he op­poses. But because at the close of the Paragraph, he makes an Inference, [Page 40]which comprises our Opinion also; since he concludes, that the Experi­ment by him alledged, p. 150. Cer­tissimum est indicium, particulas Aerias nec cum consilio nec sine con­silio inferius marmor sustinere nec suf­fulcire: It will not be amiss to shew, that our Opinion is undeservedly in­cluded in the Inference; which I shall do by briefly solving the Phae­nomenon the Doctor layes so much weight on. For if we conceive with him, that the two flat Marbles for­merly mention'd be suspended, and that to the lower of them a flat wood­den plate of the same shape and extent be applied; I see no cause to wonder, why the two Marbles should stick to­gether, and not the lower of them to the woodden plate. For, as I lately noted, there being an Area or Bed of Aerial Particles interpos'd be­twixt the Marble and the Wood, the [Page 41]weight of the Atmosphere, exercised by the intervention of those Aerial Corpuscles, ought to be aequipollent to the pressure of the Atmospherical Cylinder, that bears against the lower surface of the Plate; which conse­quently by its own weight must drop down: whereas there being no such Layer of Aerial Particles interposed betwixt the two Marbles, the pres­sure of the ambient Atmosphere, which touches them every where, save where their polish'd surfaces are contigu­ous, must keep them strongly cohe­rent.

I presume I need not mind you, that hitherto I have discours'd upon supposition, that the Doctor experi­mentally knows, what he delivers concerning the Non-adhesion of an exactly smooth woodden Plate to a Marble one: And upon his conces­sion, that, because of the want of [Page 42]sufficient congruity between the sur­faces of two Bodies, there is a bed of Aerial Corpuscles interposed between them. But now I think, it will not be unfit to take notice to you, that though, to illustrate on this occa­sion a subject that is generally so little understood, as the exercise of Pression among fluid Bodies, I have answer'd my Learned Adversaries Objections, as if I had nothing more to say for my Explication of the Suspension of coherent Marbles, than what I many years since deliver'd in the little Tract by him cited; yet I have since abundantly confirmed that Explication by the 50th of the Expe­riments publish'd in my Continua­tion; which if the Doctor had been pleased to read, perhaps he would have received the same satisfaction that other Learned men have done: since there I experimentally shew, [Page 43]that the undermost Marble without the accustomed Clog, would, upon the bare withdrawing of the sustain­ing air, drop off from the upper. And whereas the two Marbles in our Va­cuum would not cohere; as soon as the formerly excluded Air was let in upon them, it did by its supervening pressure make them stick together ve­ry strongly.

CHAP. I.

I Proceed now to the second of those two Chapters, that I am interess'd to consider, in which the Learned Examiner is pleased to attaque three or four of my Hydrostatical Opinions and Explications; in the defence whereof, I hope, I shall be the less put to exercise your Patience, because the Learned Doctor himself is pleased to grant me almost as much as I need desire concerning the Truth of the Hypothesis, whereon my Paradoxes and Explications are founded. For whereas the main thing I suppos'd in my Hydrostatical Papers, is, that in water, though stagnant, the superior parts do actually, though not always [Page 45] prevalently, gravitate upon the infe­rior, or (if you will) press upon them even when they do not sensibly depress them; the Doctor in divers places allows this Hypothesis to be consonant to the Principles of the Mechanical Philosophy; and accord­ingly having shew'd, that in a suspend­ed tub of water the whole liquor gra­vitates upon the bottom of the tub; He subjoyns, P. 161. Jam verò cum tota haec aqua constet ex parti­culis aqueis non compactis vel concretis, sed solutis à se invicem, impossibile est ut omnes fundum situlae premant, nisi in­fima quaeque ab omnibus superioribus prematur, quemadmodum clarè demon­stravimus in secunda sectione hujus ca­pitis; nempe, si nullae causae nisi purè Mechanicae (quales sunt Motus localis, Magnitudo, Figura, &c.) in edendo hoc Phaenomeno se intermiscent.

And elsewhere in the same Chap­ter [Page 46]he speaks thus of the gravitation of liquors (towards the close of the second Paragraph.) p. 152. Necesse utique est, ut partes singulae gravitent, cum totius sit gravitatio, si non sit aliquid immateriale Princi­pium in rerum natura, &c. And adds, at the beginning of the next Number; Atque sanè huic externi motûs Hypo­thesi, & gravitationis Elementorum in propriis locis inde necessariò emergen­tis, apprimè consonum est primum il­lud Experimentum, quod Scriptor pro­fert in Paradoxis suis Hydrostaticis.

And now, Sir, I presume you do not much wonder, if I think these con­cessions reach the main thing I pre­tend to. For though I do as freely and heartily, as the Doctor himself, (who, I dare say, does it very sin­cerely,) admit or rather assert an In­corporeal Being that made and go­verns the world; yet all that I have [Page 47]endeavour'd to do in the Explication of what happens among Inanimate Bodies, is to shew, that, supposing the World to have been at first made and to be continually preserv'd by Gods divine Power and Wisdome; and supposing his General concourse to the maintenance of the Laws he has established in it, the Phaenomena, I strive to explicate, may be solv'd Me­chanically, that is, by the Mechani­cal affections of Matter, without re­course to Natures abhorrence of a Va­cuum, to Substantial Forms, or to o­ther Incorporeal Creatures. And therefore, if I have shewn, that the Phaenomena, I have endeavour'd to account for, are explicable by the motion, bigness, gravity, shape, and other Mechanical affections of the small parts of liquors, I have done what I pretended; which was not to prove, that no Angel, or other imma­terial [Page 48] Creature could interpose in these cases; For concerning such A­gents, all that I need say, is, that in the cases propos'd we have no need to recurr to them. And this, being agreeable to the generally own'd rule about Hypotheses, that Entia non sunt multiplicanda absque Necessitate, has been by almost all the modern Philo­sophers of different Sects thought a sufficient reason to reject the agency of Intelligences, after Aristotle and so many Learned men, both Mathema­ticians and others, had for many a­ges believ'd them the Movers of the Celestial Orbs.

CHAP. II.

But you will tell me, that the Do­ctors Concessions will not avail me, since he urges against the Gra­vitation of the Elements in their pro­per places, which (gravitation) he would have to be suspended by his Incorporeal Principle, an Experi­ment, which he says is most manifestly repugnant to our Hypothesis. He conceives then, that in a tub or pail full of water with a perfectly Cylin­drical cavity, whose Diameter is of sixty two parts, there is violently kept at the bottom, by the help of a stick, a round Plate of wood, whose Dia­meter amounts but to sixty one of those parts; and that, as soon as ever the stick is removed, the woodden plate will emerge to the top and [Page 50]float. Quod (says he) prorsus im­possibile esset, si omnes partes aquae ab (FG) ad (HJ) non solùm junctim fundum vasis, sed singulae singulas in eadem serie subjectas actu premetent. To which assertion he immediately subjoyns this Argument to prove it by; p. 155. Cum Diameter laminae ligneae (H M) partes 61. habeat aequales, Diameter vasis (HI) habeat 62, manifestum est, quod super­ficies fundi vasis ad superficiem lami­nae se habet ut 3844. ad 3721; quo­rum differentia est 123. Itaque rotun­dum intervallum inter latera vasis & marginem laminae ligneae habet se ad aream laminae ut 123. ad 3721, hoc est, area laminae ligneae excedit aream dicti intervalli plusquam triginta vi­cibus. Ac proinde aqua incumbens lig­neae laminae excedit magnitudine a­quam incumbentem dicto intervallo inter marginem laminae & later a vasis [Page 51]plus quàm triginta vicibus, pondúsque sive pressio hujus, alterius pondus pres­sionémque vincit plusquam triginta vi­cibus. Adeò ut impossibile sit, ut aqua incumbens praedicto intervallo ita pre­mat aquam ipsi subjectam, ut hujus vi sublevetur lamina, quam vis tricies major deprimit. Quod (says he, by way of inference) aeque absonum at­que absurdum Phaenomenon esset, &c.

How little this Ratiocination a­grees with the Experiments I have formerly told you of, about the cases wherein Light bodies will be detain'd under water, or emerge to the top of it, you will easily perceive, if you compare the one with the other, which you may quickly do, if you please to compare the Doctors discourse with the following Narra­tives of those Tryals See the Tract of the Positive or Relative Levity of Bodies un­der water. Exp. 1. &c., to which alone I might therefore refer you. [Page 52]But yet in the mean time, you may, if you think fit, consider a little, whe­ther the Argument, whereon the Do­ctor lays so much stress, be any more than a Paralogism.

First then, since according to his computation the Area of the interval between the sides of the Vessel and the edges of the round boards, is 123 of such parts, whereof the Area of the board amounts to 3721; 'tis evident, that there must be room enough for the water to pass between the sides of the vessel and the edges of the board, which is suppos'd on all hands to be of some wood lighter in specie than water, since else it would not emerge upon the withdrawing of the stick.

Next, this Board or woodden Plate is not here intimated or supposed to be (and indeed in practice can scarce be) made exactly congruous to the bottom of the Vessel; and consequently [Page 53]the water may get in between them; for which cause 'tis necessary to keep the woodden Plate forcibly down with a stick, which else were needless. And consequently this interposed water will communicate with the laterally superior water in the Vessel, which superior water may, according to the Laws Hydrostatical, by the interven­tion of the interposed, exercise its pressure upwards against the lower surface of the woodden Plate.

Thirdly, the Doctors Scheme al­lows and assists us to conceive, (which we may do however,) an imaginary Plane of water to be parallel to the bottom of the Vessel, and to pass a­long the bottom of the Board; so that, of the water that lies between this Plane and the bottom of the Vessel, one part is cover'd by the woodden Plate; and the other, between the edges of that and the sides of the [Page 54]tub, is cover'd with the incumbent water only.

CHAP. III.

THese things being premis'd, I thus argue: 'Tis manifested by Hydrostaticians after Archimedes, that in water, those parts that are most press'd, will thrust out of place those that are less press'd: which both a­grees with the common apprehensi­ons of men, and might, if it were needful, be confirm'd by Experi­ments. 'Tis also evident, that that part of the above-mention'd imagi­nary Plane, that is cover'd by the woodden Plate, must be pressed by a less weight than the other part of the same Plane; because the wood being bulk for bulk lighter than water, the [Page 55]aggregate of the wood and water in­cumbent on the cover'd part of the same Plane must be lighter in specie, than the water alone that is incum­bent on the uncover'd part of the same Plane; and consequently this uncover'd part being more press'd than the other part of the Plane, the heavier must displace the lighter, which it cannot do but by thrusting up the board, as it does, when the ex­ternal force that kept it down is re­moved. And, to add this upon the by, this greater pressure against the bottom than against the top of bo­dies immers'd in water specifically heavier than they, is a true reason of their emersion, as I have elsewhere shewn. So that there happens no more in this case than what usually happens in the ascension of bodies in liquors specifically heavier than themselves, on the account of the [Page 56]newly mention'd difference of Pres­sure. And 'tis with an (express or suppos'd) exception of such a diffe­rence, which in many other cases may be safely neglected, that (which I desire you to take notice of,) in most places of this discourse I speak of the Pressure of ambient Fluids on im­mersed Solids as uniform or every way equal.

'Tis true, that according to the Doctors supputation, if the solid Cylinder, consisting of the woodden Plate, and all the water directly in­cumbent on it, were put into an or­dinary ballance, it would there many times out-weigh the hollow Cylinder of water alone that leans upon the un­cover'd part of the imaginary Plane. And that is it that seems to have de­ceiv'd the Learned Doctor. But there are divers Hydrostatical Cases, where­in the Phaenomenon depends not so [Page 57]much upon the absolute weight of the compared Bodies, as upon their re­spective and their specifick Gravity; on whose account it is, that a small Pible, for instance, that weighs not a quarter of an Ounce, will readily sink to the bottom of the river, on whose surface a log of wood of a hun­dred pound in weight will float. 'Tis a Rule in Hydrostaticks, that when two portions of water or any other Homogeneous liquor press against each other, the prevalency will goe, not according to the absolute weight, but the perpendicular height of those Portions. And accordingly we find, that if a slender pipe of glass, being fill'd with water, have its lower ori­fice unstop'd at the bottom of a ves­sel of water, which contains much more of that liquor than the pipe; yet if this last named water were, for instance, two foot high, and that in [Page 58]the Vessel but one, the water in the pipe will readily subside, till it come almost to a level with the external water, though it cannot do so with­out raising the whole mass of water that stagnated in the vessel.

And now I shall subjoin an Experi­ment, which, though at first it may seem slight, and was made in lesser glasses & quantitys than I would have imploy'd if I could have procur'd better Accommodations, has the ad­vantage of requiring no curious in­struments, and yet I hope will serve for an ocular proof of the fallacious­ness of that reasoning the Doctor is so strangely confident of.

We took an open mouth'd glass, such as some call Jarrs, and Ladys often use to keep sweet meats in, which was three inches and a half or better in Diameter, and somewhat less in depth; and had the figure of its cavity [Page 59]Cylindrical enough. Into this hav­ing put some water to cover the pro­tuberance, wont to be at the bottom of such glasses, we took a convenient quantity of Bees-wax, and having just melted it, we poured it cautiously into the glass, warm'd before-hand to prevent its cracking, till it reach'd to a convenient height. This vessel and the contained liquors we set aside to cool, in expectation, that when the heat, that had dilated the wax, was gone, it would shrink from the glass, and consequently leave a little inter­val every where between the concave superficies of the vessel, and convex of the harden'd wax; which accor­dingly came to pass, and sav'd me the labour of getting the wax shap'd for my purpose with tooles; which might have been done but not with­out trouble and less exactness. And now 'twas easie for me to try the ex­periment [Page 60]I design'd; for, pouring in warily some water between the glass and the wax, so that it fill'd all the in­terval, left between those two bodys both at the bottom and the sides, the wax was made presently to float, be­ing visibly lifted up from the bottom, and its upper part appearing a little above the level of the water, which was no more than I did, and had rea­son to expect, according to the true Principles of Hydrostaticks. For water being somewhat, though but lit­tle, heavier, in specie, than wax, and that which was poured into the bottom and stagnated there, being press'd by the collateral water, every way interpos'd between the concave part of the Glass and the convex of the Wax (so that this col­lateral liquor answer'd what I lately called a hollow Cylinder of water in the Doctors Experiment) that part [Page 61]of the stagnant water, that was lean'd upon by the wax, being less press'd than the other part of the same stag­nant water was by the water incum­bent on it; this latter must displace the former, which it could not doe but by raising up the wax that lean'd upon it. And yet this collateral wa­ter was so far from being heavier than the wax its pressure impell'd up, that both the collateral, and the stagnant water all together, being weigh'd in good scales, amounted to little above a quarter of the weight of the wax, which happen'd by rea­son of the narrowness of the Vessel, which, if it had been wide enough, I doubt not but the experiment would have succeeded, though the wax had outweigh'd the collateral water ten times more than in our experiment it did. But that the solid body excee­ded almost four times the weight, [Page 62]not onely of the collateral but the stagnant liquor too, does sufficient­ly overthrow the Doctors ratiocina­tion. Whose fallaciousness will yet further appear by two other improve­ments, among others, which I made of one Experiment.

For I. though we pour'd in more and more water, as long as the Ves­sel would contain any, the Cylinder of wax was but lifted higher and higher from the bottom of the glass, but did not appeare rais'd more than at the first, above the upper surface of the water; which argues, that 'twas not at all the Quantity of the inferior water, which was continually increas'd, but the pressure of the col­lateral water, which continued still at the same height in reference to that wax, that caus'd the elevation of the body.

And II. to manifest yet more [Page 63]clearly the Doctors mistake, I devi­sed the following tryal. We took a round plate of Lead about the thickness of a shilling, and having made it stick fast to the bottom of the Cylinder of wax, to make this body sink the more direct­ly, we placed one after another upon the upper part of the wax divers grain weights (first wetted to keep them from floating) till we had put on enough to make the wax subside to the bottom: For the facilitating whereof we had par'd off its edges; by this means, the glass having been at first almost fill'd with water, there swam about an inch or better of that liquor above the upper surface of the wax. And lastly, we took off by de­grees the grain weights that we had put on, till we saw the wax, not­withstanding the adhering Lead, rise, by degrees, to the top of the water, [Page 64]above which some part of it was visi­bly extant.

From this experiment I thus ar­gue: 'Tis manifest, that, according to the Doctors supposition, here was in­cumbent upon the wax a Cylinder of an inch in height and of the same Di­ameter or breadth with the round sur­face of the wax, whereas upon the removing part of the water, that lay at the bottom when the wax began to rise, there was incumbent no grea­ter weight than that of the collateral water, and as much of the superior and stagnant, as was directly imcum­bent upon that collateral water (and would have deserv'd the same name, if we had suppos'd the convex sur­face of the wax to have been con­tinued upwards as high as the glass reach'd.) But now, whereas, according to the Doctors ratiocination, this Cy­linder of water incumbent on the [Page 65]wax, being an inch deep, and a good deal above three inches broad, must press the wax with a greater weight by several times, than that which the lateral and hollow Cylinder of this stagnant water could have upon the rest of the collateral water; yet the height of this aggregate of collateral waters being the same with that of the wax and the water swimming up­on it, the difference of the pressure was so small, that barely taking off a weight of four or five grains, the wax would, notwithstanding the pres­sure of the water incumbent on it, be impell'd up and made to float: And by the like weight, put again upon it, it would be made to sink, and by ano­ther removal of such a weight, (for I purposely reiterated the tryal more than once,) it would, though slowly, reascend. And these Phaenomena do so much depend upon a Mechanical [Page 66]aequipollence of pressure, that even four grains would not have been ne­cessary to make the wax rise or sink, if it had not been for some little ac­cidental impediments, that are easily met with in such narrow glasses; for otherwise in a larger Vessel we have made the same Lump of Wax readi­ly enough sink or float, by the put­ting in or taking off a single grain or perhaps less.

By this you may see, that for the Regulation of Hydrostatical things, Nature has her ballance too as well as Art, and that in the ballance of Na­ture the Statical Laws are nicely e­nough observ'd.

You may also take notice, upon the by, how little the weight of the Cylinder of water upon a body im­mers'd in stagnant water is conside­rable, whilst there is a pressure of collateral water to counterballance [Page 67]it; since in this last tryal, though the Cylinder of incumbent water did con­tinually increase or decrease in length, whilst the lump of Wax was sinking or emerging; yet the same despicable weight of a grain or less, that was just able to depress it beneath the upper surface of the water, did by its pressure or removal procure its sinking to the very bottom, or rising again to the top, and on both occasi­ons with an equal slowness, bating that little acceleration of motion, that ought to happen upon another ac­count, and which therefore is to be observ'd in the wax, during its ri­sing as well as during its sinking.

CHAP. IV.

SOme other Phaenomena I produc'd, by varying the hitherto menti­on'd experiment, which are very fa­vorable to our Notions about Hydro­staticks. But, since they do not di­rectly concern the present Controver­sie, I shall in this place only annex a couple, the former whereof affords an easie confirmation of that Paradox, which we lay as the ground of di­vers others, and the contrary where­of is maintan'd not only by Doctor More, but by many other famous and Learned men, namely, that in stagnant water the upper parts do actually press the lower.

Wee took then a very slender pipe of glass, whose Cavity was nar­rower than that of an ordinary Goos­quill, that heterogeneous Liquors [Page 69]may not be able to get by one ano­ther in it. This Pipe near one end was bent upwards like a Syphon, that it might have a short leg as Paral­lel as the Artificer could make it to the longer. Into this crooked Pipe we put a little oyl, and then held it perpendicularly in a somewhat deep and wide-mouth'd Glass fill'd partly with Water and partly with a Lump of Wax, of the bigness and shape of that already mention'd; that so the pressure of the incumbent Water upon the open orifice of the shorter Leg, might impel the oyl in­to the longer Leg, somewhat above the surface of the water in the Ves­sel; which 'twas convenient should be done, that we might the better see the motions of the Oyl, and which we knew must be done by the course we took; both because Oyl is lighter in specie than Water, and consequent­ly [Page 70]required not an equal height of Water to counterballance it; and be­cause in very slender Pipes, Water is wont to ascend a little above the Level of the External Water, where­into they are immers'd. The Pipe being, as was said, held upright, 'twas easie to take notice by a mark, fix'd on the outside, to what height the Oyl reach'd in it.

Now if we conceive a Horizontal Plane, Parallel to the bottom of the Vessel, to pass by the Basis of the floating Wax, 'tis evident by what has been formerly shewn, that, of this Imaginary Plane, that part on which the Wax is incumbent is as strongly press'd by the weight of the Wax, as the Lateral part of the same Plane is by the weight of the Water incumbent on it; (otherwise these Pressures would not be aequipollent, but the Wax would be raised:) And consequent­ly [Page 71]that part of this Plane, that is placed directly over the Orince of the shorter Leg of the Pipe, is no more pressed, than any equal portion of that part of the same Plane that is co­ver'd by the Wax. This Body being taken out of the Water, the Liquor subsided a great way in the Ves­sel, and so did proportionably the Oyl in the longer Leg of the Pipe. And lastly, having weigh'd out in a good pair of Scales as much Water, as we found the Wax to amount to, this Liquor was, instead of the Wax, poured into that which remained in the Glass; whereupon the Oyl, in the longer leg of the Pipe, was again impell'd up (very near) to the for­mer Marke to which it had been rai­sed by the Wax. Whence we may gather, that the Water newly put in, though in the Air it weigh'd no more than the Wax; yet it did as much [Page 72]press the Water, that lay beneath the foremention'd Imaginary Plane, and consequently that, which was direct­ly over the shorter Leg of the Pipe, as the Wax, that had been taken out, had done. And since we have already proved, that the Wax did confi­derably press that Plane, it ought not to be denyed, that the Water also (which instead of it was able to im­pell up the Oyl in the Pipe) did in like manner press that Plane; and consequently that Water may gravi­tate in Water, as well as a solid Body, such as Wax is, can. And this is the first additional use I told you I would make of our Experiment.

But, (to come now to the second) there is another Phaenomenon of it, viz. the abovemention'd tenderness of Nature's Ballance, whose use seems to be of no less general concernment to the true Doctrine of the Hydrosta­ticks. [Page 73]For, by duely considering that Phaenomenon, and reasoning a while upon it, we may be help'd to rectifie that plausible Mistake, which has long deluded both Philosophers and Mathematicians, and does yet im­pose on most of them; namely, that a Body does not actually gravitate when it does not descend. For we have seen already, and shall further shew by and by, that the sunken Wax and the Brass grains that lie on it, do actually press or gravitate upon the subjacent Water and Bottom of the Vessel on which 'tis incumbent; and conse­quently its pressure being not sur­mounted by that of the Collateral Water, which is unable to raise it, must be as great, as that of this colla­teral Water. Therefore, when upon the removal of a single Grain, the Wax with its incumbent weight is made to ascend, and that but very [Page 74]slowly, 'tis evident, that 'twas so far from not gravitating before, because it did not actually descend, that it retain'd its Gravity even whilst it a­scends: As may appear not only by the slowness of its motion upwards, proceeding from its being in Nature's Ballance very little less heavy than it need be to countervail the pressure of the Collateral Water; but by this al­so, that if but a single grain be laid on it when it begins to rise, its ascension will be check'd and hindred, which could not be done by the addition of so inconsiderable a weight, if the Wax and the adhering Metall did not, even during their ascent, retain their for­mer gravity, though that were fru­strated as to the act of descending, or so much as keeping their station by the prevailing pressure of the colla­teral Water: So that, since, (as we found) the Wax and adhering Me­tall [Page 75]amounted to a good deal above 4000 Grains, it did in the Ballance of Nature weigh, whilst it was ascen­ding, not so much as a 4000th part less than it did, whilst it was actually descending.

CHAP. V.

I Should beg your pardon, Sir, for having detain'd you so long with my Reply to a single Objection of the Doctors, how pompously soever propos'd; but that I thought it not amiss to do some service to the true Theory of Hydrostaticks, by taking this occasion to present you some things that I thought not unlikely to illustrate some parts of that Theory; though above what was necessary to answer the Doctors Argument; to which I confess I was troubled to see [Page 76]so Learned a man subjoin the fol­lowing conclusion: Haec tam lucu­lenta Demonstratio contra Gravita­tionem particularum aquae inter se quamvis junctae situlae fundum ur­geant, si non sit vera atque solida, e­quidem nec mei ipsius nec ullius un­quam mortalis in posterum ratiocini­is credam. But I hope he will not be as bad as his word, but will be pleas'd to consider as well as I do for him, that a man may be very happy in other parts of Learning and of greater moment, that has had the mis­fortune to mistake in Hydrostaticks, a discipline which very few Scholars have been at all vers'd in, and about which divers of those few have had the misfortune to err, not only in the conclusions they have drawn, but in the very Principles they have em­braced.

To the foregoing Argument the [Page 77]Doctor, though he declares he thinks it needless, adds in the 5th Para­graph another, taken from the Last experiment of my Hydrostatical Pa­radoxes, by which he ingenuously ac­knowledges, that I seem at first sight to have demonstrated what I pretend to, about the gravitation of the up­per parts of stagnant water upon the lower. And I am sorry that I can­not in return acknowledge, that his objection at first sight seem'd to me a cogent one: For, neither at the se­cond nor third perusal can I clearly discern where his Ratiocination lyes, supposing it to be meant for an an­swer to my experiment. And though I consulted with some Learned Mem­bers of the Royal Society, whereof two are Mathematicians, and one his particular friend; yet they all con­fess'd he had not sufficiently ex­plain'd himself on this occasion, nor [Page 78]could they shew me to what argumen­tation I might properly direct my reply. Only one of the Doctors Correspondents, having seriously per­us'd his discourse and the annex'd scheme, told me that what seem'd the most probable to him, was, that though the Doctor was too Civil to give me, in ter ninis, the Lye; yet he did indeed deny the matter of fact to be true. Which I cannot easily think, the Experiment having been tryed both before our whole Society, and very Critically, by its Royal Founder his Majesty himself. But, since you have your self seen and made it more than once, I need not spend words to convince you that the matter of fact is true.

But after I had in vain sought the Doctors meaning where I expected it, chancing lately to cast my Eyes on another place, where I saw my Scheme [Page 79]repeated, I find this passage in the Explication he endeavours to give of the Phaenomenon by his Hylarchical Principle; Cùm verò tam profundè immergitur tubus, ut obturaculum tan­gat Superficiem V. W, vis retractionis Aeris ita augetur ut etiam ponderis ap­pensi superadditam depressione [...] su­peret. Videtur igitur quasi quaed [...]m sursum-suctio Aeris in tubo contenti, & conformis ac contemporanea aquae compulsio in obturaculum, quo tam fir­miter in os valvulae comprimitur, ibi­que cum appenso pondere sustentatur. What considerable interest the sup­posed, but unprov'd, Retraction of the Valve or the Air it self can have in this Phaenomenon, I confess I do not discern, not being able to see, but that the experiment would suc­ceed when tri'd in vacuo, although all the Atmosphetical Air were anni­hilated. But if I mistake the Doctors [Page 80]meaning I am to be excused, since I do it not willingly, and his own ob­scurity has been accessary to it. Nor am I very apprehensive of being una­ble to defend my account of an expe­riment, which (as you know) has had the good fortune to recommend the Doctrine, for the Proof whereof I devis'd it, to many Learned and curi­ous Persons, several of which were sufficiently indispos'd to admitt it.

And to avoid all mistakes and dis­putes that may arise (which I think they must do needlessly) upon the score of the Valve imploy'd in our Ex­periment, I shall remind you of an­other, that I remember I have some times shew'n you and divers other Virtuosi, though I remember not whe­ther I have mention'd it in any of my publish'd writings. The Summ of this tryal is, that an arbitrary Quan­tity of Quicksilver, being by Suction [Page 81]rais'd into a very slender glass-pipe, whose upper Orifice is stop'd with the Experimenters finger, to keep the Mercury from falling before its time, the open end of the pipe with the Mercury in it is thrust into a compe­tently deep glass of water till the little Cylinder of Mercury have, be­neath the surface of the water, at­tain'd to a depth, that is at least 14 times as great as the Mercurial Cylin­der has of height. For then, the fin­ger being remov'd from the upper orifice, the glass-pipe will be open at both ends, and there will be nothing to hinder the Quicksilver's falling down to the bottom, but the resi­stance of the Cylinder of water, that is under it, which Cylinder can re­sist but by vertue of the weight or pressure of the stagnant water that is superior to it, though but collateral­ly plac'd above it: And yet this [Page 82]water being by the pipe, whose up­per part is higher than its surface, and accessible only to the air, kept from pressing against the Mercury any where but at the bottom of the Pipe, and being about a 14th part of the weight of an equal bulk of Mercury, it is able at that depth to make the subjacent water press up­ward against the Mercury, which is but a 14th part as high as the water is deep, with a force equivalent to that of the gravity wherewith the Mercury tends downwards. And to manifest, that this Phaenomenon de­pends meerly upon the Aequilibrium of the two liquors; if you gently raise the lower end of the pipe to­wards the surface of the water, this liquor, being not then able to exer­cise such a pressure as it could at a further and greater depth, the Mer­cury preponderating will, in part, [Page 83](more or less, as the pipe is more or less rais'd) fall out to the bottom of the glass. But if, when the Quicksilver is at the first depth, instead of raising the pipe you thrust it down farther under the water, the pressure of that liquor against the Mercury increasing with its depth, will not only sustain the Mercury, but impell it up in the pipe to a considerable distance from the lower orifice of it, and keep it near about the same distance from the surface of the laterally superior water. And this experiment may not only serve for the purpose, for which I here alledge it; but also, if duely consi­der'd and applyed, may very much both illustrate & confirm the Expli­cation formerly given of the seeming­ly spontaneous ascent of the clogg'd sucker in our exhausted Air-pump.

The last Argument, the Doctor urges against the Gravitation of water [Page 84]in what they call its proper place, is deduc'd from what happens to the Divers, who in the mid'st of the Sea, though the salt water of that be much heavier than that of freshwater Ri­vers, do not find themselves oppress'd, or so much as feel themselves harm'd or compress'd by the vast load of the incumbent water.

But that the Equality of the pres­sures of an ambient fluid will goe a great way towards the solving of this Difficulty, you will find, by the Ex­periments and considerations you will meet with in the following The Author means the New experiments of the differing pressure of hea­vy solids, & fluids, Papers, to which, for that reason, I referr you. And though the Doctor in this same Paragraph objects, Tametsi haec pressio aequalis sit, ni­hil tamen impedit quò minùs subtiliores partes corporis magisque fluidas expri­mat & elidat. I remember I answer'd [Page 85]that exception before, by saying, that those liquors that he supposes should be squeez'd out, cannot be so, because there is as great a pressure against those parts at which they should is­sue, as against any of the rest, if the parts that should be squeez'd out be not too spirituous and subtile, which if they be, I should gladly learn how the Doctor knows that no such mi­nute and spirituous particles are really expell'd: especially if that be observ'd, which we shall soon have occasion to relate, that a small animal, being vehemently compress'd in wa­ter, seem'd a little, though but a little, to shrink.

But that we may the more distinct­ly consider this grand argument, taken from the experience of the Divers, that is wont to be employ'd by the Schooles and others for the vulgar Opinion, and is now urg'd by the [Page 86]Learned Doctor to prove His; 'twill be convenient to observe, that it does, at once, both propose a Question, and contain an Objection, grounded upon the surmis'd insolubleness of that Question.

And to begin with the Probleme, Whence it is, that Divers are so far from being kill'd or oppress'd by the weight of the incumbent water, that they are not so much as hurt by it, nay, that they scarce feel it at all? We may take notice, that there is in it somewhat suppos'd, as well as some­what demanded. For, in the Que­stion 'tis taken for granted, that Di­vers, though at never so great a depth, feel no pressure exercised against them by the water; which is an affirmation in point of fact, of whose truth I make some question, for the reasons I shall ere long have occasion to mention.

But it will clear the way for what [Page 87]is to follow, if I here divide the noble and difficult Problem, we are to consider, into two Questions; the first, why a Diver should not be op­pressed and crush'd to death by the pressure of the Incumbent and Am­bient water. And the second, why at least he should not be made sensibly to feel it by suffering some conside­rable inconvenience from it.

In answer to the first of these Que­stions, you will easily perceive, that divers things may be pertinently ap­plyed, that you will meet with in the following Paper, to shew the differ­ence betwixt the pressure of Fluid and that of Solid bodies. And that de facto the pressure of water may be exceeding great without destroying an Animal quite surrounded with that liquor; I have long since shewn in another The Author points at the Appendix to the Hydrostatical Paradoxes. Trea­tise, by the experiment [Page 88]of a little Tadpole, which being, toge­ther with the water it swam in, inclu­ded in a bent Glass seal'd at one end, the animal was not kill'd or sensibly hurt, but only (according to what was lately noted by anticipation) seem'd to shrink into somewhat (and but little) lesser dimensions.

If it be here alledged, that this Experiment makes rather against me than for me, the Learned Doctor ha­ving made use of it with a Scheme to explain it in his 16th. Paragraph; it will be fit for me to consider his Ob­jection. Having then recited the matter of fact newly deliver'd, he adds, Quod certè fieri non posset nisi juxte legem quartam contrusio particularum aquae contra se invicem Principio Hy­larchico inhiberetur & eluderetur. Atque hinc fit, ut quamvis Aqua is tubo (A B C) vi trudis (G F) aliquantò facta sit condensatior, partes [Page 89]tamen sic compressae ut propiùs ad se invicem accedant, nihilo inde inter se fiunt comprimentiores. And then sub­joining the following passage; Ne­que emim sequitur ex earum contactu quod premant se invicem, quandoqui­dem particulae, uti fit in duris Corpo­ribus, in unum coalescere possunt & ta­men non mutuò se premere; (Where­in are some things that might be question'd if it were necessary;) He thus pursues his Discourse: Cùm verò hîc particulae aquae si omninò pre­merent se invicem, pressura in Gyri­num, columnae aqueae, ducentos vel tre­centos pedes, aeneae verò, plus viginti vel triginta pedes altae, pressionem ad­aequaret, luculentum est indicium quod revera particulae se invicem non premant. Nam planè est incredibile, columnum aeneam pro corpore quidem gyrini latam, sed altam viginti vel triginta pedes & amplius, Gyrinóque [Page 90]ad perpendiculum incumbente [...]n omnia viscera tam tenellae Gelatinae no [...] esse elisuram. Notwithstanding which al­legation I am apt to think, you will judge the Argument from this expe­riment to be more probable on my side than on the Doctors. For there being in our case an animal, exceed­ingly much more tender than a man, expos'd to a pressure which he affirms is so great, that if it were exercis'd on the Tadpole, it ought to squeeze out all his guts, I think I may pre­tend to have given a pertinent in­stance, that a Diver may be at a con­siderable depth under water preserv'd from being crush'd to death by the weight of it. And whereas the Doctor tells us, that the cause of the Inco­lumity of the Tadpole is, that the pres­sure or contrusion of the particles of the water against one another is hin­der'd or frustrated by the Principium [Page 91]Hylarchicum, I reply; That what I affirm is matter of Fact, and evident, (namely, that there was a great ex­ternal force duly and yet ineffectu­ally applyed to press to Death by means of the water the animal swim­ming in it;) but that this Mechani­cal force was suspended or made ineffectual by some invisible and im­material Agent, is but the Doctors Hypothesis, and a thing, which, whe­ther it be true or no, is at least not manifest.

Having said thus much about the first Question; I now proceed to the second, Why Divers though at never so great a depth complain not of the pressure of the water, nor suffer any harm nor inconvenience by it?

And here, Sir, the Question highly meriting a particular Curiosity, I shall not scruple in the more full en­quiry, I am now entring upon, as well [Page 92]sometimes to employ and inlarge par­ticulars already mentioned in the last of the following Papers, as often­times to strengthen them with new ones. And I shall also for a while suspend my difference with the Do­ctor, and addressing my self to you, who, I am sure, will allow me that water weighs in water, propose, ac­cording to my custom, not as a Dog­matist, but as an Inquirer, some par­ticulars, that may tend to the Solution of a Problem, which I take to be as difficult as noble. Not that I doubt but it must and will be expli­cated upon the Mechanical Principles; but partly, because the application of them to the Solution will not offer it self to every seeker; and partly, because we are not yet well furnished, either with experiments made on bo­dies under water, or so much as with so competent an account of the matter [Page 93]of fact, as I think may keep wary men from hesitations about it. For, what is commonly reported concern­ing the Divers, is (as has above been intimated) grounded but upon their own Relations and answers, perhaps amplified or procur'd by leading Questions from persons, who are ge­nerally either slaves or ignorant men, taken from the less sober part of the illiterate vulgar, and prepossest with the common opinion of the non-gravitation of water in its own place; and consequently are not like to make over-accurate observations, but prone to refer the inconvenient alterations, they feel, to any other cause than the pressure of the water, which they are taught to be none at all. If obser­vations about Diving were made by Philosophers and Mathematicians, or, at least, intelligent men, who would mind more the bringing up out of [Page 94]the Sea instructive observations than shipwrack'd goods, we should per­haps have an account of what hap­pens to men under water differing enough from the common reports.

You will in one of the following Pa­pers find mention of a Learned Phy­sician of my acquaintance, that, upon his diving leisurely, perceived a constriction to be made of his Thorax by the action of the surrounding Sea­water.

A Spanish Prelate, that liv'd long in America, speaking of the deplo­rable condition of those wretched In­dians that were employed by their inhumane Masters about the fishing for Pearls, gives us this account of them: See Purch. Tom. IV. Lib. 8. p. 1587. It is impossible that men should be able to live any long season under the water without taking breath, the continual cold piercing them; and so they dye [Page 95]commonly parbreaking of blood at the mouth, and of the bloody flux caused by the stomach. Their hair, which are by nature cole-black, alter and become afterwards a branded russet, like to the hairs of Sea-wolves, &c.

And a General of the English in the East-Indies, being by them employed on an Embassy to the Emperour of Japan, has this passage concerning some female Divers that he met with in his voyage: Purch. Tom. I. Lib. 4. C. 1. All along this coast & so up to Ozaca, we found women Divers, that liv'd with their houshold and family in boats upon the water, as in Holland they do the like. These women would catch fish by Diving, which by net & line they miss'd, and that in eight fathom depth. Their eyes by continually diving grew as red as blood, whereby you may know a diving Woman from all other Women. I know, it may be said, that these dis­eases [Page 96]may proceed from the coldness and moisture or other qualities of the Sea; nor would I confidently reject such a surmise: But it may also be possible, that the compression, they suffer'd under water, might have at least a share in the production of these ill effects. For how are we yet cer­tain, that the pressure of the water against their bodies, though it does not manifestly dislocate any solid or firm part, but only somewhat press inwards, as in the above mentioned Tadpole the outward skin and the fibres, (both which will easily yield a little way without being painfully stretch'd,) may not, by straitning the Vessels, and otherwise inconveni­ently, alter the circulation of the blood and the motion of the humors, spirits, and other fluid parts of the body? And I am not sure, that much of the cold, that Divers are [Page 97]wont to complain of, when under water, may not be a disaffection pro­duc'd in the nervous and membra­nous parts, occasioned by the com­pression of the ambient water, there being divers things, and pressure among others, besides actual cold, that will make men complain of being cold; and in our case this sensation may be excited or assisted by the hin­dering of the usual perspiration at the constipated pores of the skin. And it seems not impossible, that one, not so ignorant and heedless as Divers are wont to be, may refer a new sensation, that really proceeds from pressure, to other Causes; since Lear­ned and Intelligent men, when pre­possest (as these Common Divers usually are) with the vulgar opi­nion about the Non-gravitation of Water and Air in their natural places, [Page 98]do almost always refer The reason of which experiment may be gathered from the 4th. Chap­ter of the Author's long since publish'd Defence against Li­nus. an experiment of my Engine to Suction, which is indeed the effect of the pressure of the Ambient, (as I have In a Paradox about Suction. elsewhere clear­ly shewn,) and affirm, that the pulp of the finger or hand is drawn up into a hollow Pipe, into which it is indeed thrust by the weight of the Ambient air. But all these things I have mentioned, not as if I laid any great weight upon each of them, but to let you see, that 'twas not altogether without cause, that I complain'd of the incompetency of the History of what Divers feel under water; espe­cially at great depths, where this want of information may be more consi­derable: For, as far as I have yet learnt by perusing Voyages and en­quiring [Page 99]of Travellers of my acquain­tance, the places, where they are wont to dive for Pearl, are but moderately deep, and indeed shallow in compa­rison of the great depths of the Sea; so that if we were furnished with as many Relations of these profound places, as we have of the others, pos­sibly the accounts would be different enough to render doubtful or to cor­rect the received opinions about the conditions of Divers at the bottom of the Sea. For, I remember that a cre­dible eye-witness, who, (if I mistake not) was the Intelligent Oviedo, speaking of the Pearl-fishing on the American Island of Cubagna, has a­mong many other notable observations such a passage as this; But whereas the place is very deep, a man cannot naturally rest at the bottom by reason of the abundance of aery substance, which is in him, as I have oftentimes [Page 100]proved. For although he may by vio­lence and force descend to the bottom, yet are his feet lifted up again, so that he can continue no time there. And therefore where the Sea is very deep, these Indian Fishers use to tye two great stones about them with a coard, on each side one, by the weight whereof they descend to the bottom, and remain there until them listeth to rise again, at which time they unloose the stones and rise up at their pleasure.

And now to come closer to the ex­plication of our difficult Problem; there yet occurrs to me nothing more likely in order to it, than what I have already mentioned in the Paper you will meet with about the Differing pressures, &c. And therefore it shall here suffice me to enlarge, and by fur­ther Considerations and Experiments confirm, what is there more summarily discoursed; namely, That the Phaeno­menon [Page 101]may depend (chiefly) upon these two things, the uniform pressure of the fluid Ambient, and the robust texture of a humane body expos'd to this Pressure.

In one of the follow­ing. New Experiments about the differing Pressure of heavy Solids and Fluids. Papers, you will find examples of the great pressure that may be sustain'd unharm'd by such frail bodies as Eggs and thin Glasses, that one would ex­pect should be broken in pieces thereby, provided the pressure be exercised by the intervention of an Ambient liquor; as water. And by the account elsewhere refer'd to, of the Tadpole, it seems highly probable, that even that tender animal, when it seem'd by some small diminution of the bulk to be every way a little com­press'd inwards, was put to no consi­derable (or perhaps to any sensible) pain or inconvenience, since it seem'd [Page 102]to swim without any irregular mo­tions, which would in likelihood have insued, if it had been much harm'd or incommodated. Which example, with those formerly pointed at, may teach us, that there may be a vast difference betwixt the resistance that a body can make when compress'd immediately by Solid bodies, & when in the compression every way ambi­ent Fluids intervene. Which you will the less admire, if you consider, that by reason of the grossness, hardness, or rigidness of visible Solid bodies the pressure can never be made every where so equally as by the parts of Liquors, whose smalness, which ren­ders them singly invisible, fits them to accommodate themselves far more closely and conveniently to all the superficial parts of the body immers'd in them, and to have the force of the compressing body more uniformly [Page 103]distributed to them. But because the Instances referr'd to, are taken from bodies surrounded with water, I will take two or three about the resistance of bodies to violently compress'd Air; partly, because those made in our Engine are wont to be perform'd with Air (not condens'd, but) rarified or expanded beyond its usual con­sistence; and partly, because it will not be deny'd, that the corpuscles of Air may be really comprest or thrust against one another, since 'tis clear, that they may be crouded into far less room, than they possess'd before, and bear so strongly against the Glasses that imprison them, as not seldom, if too much compress'd, to burst them in pieces.

Consider then, that among bodies not fluid the Swims of smaller fishes are likely to be judged none of the most able to resist compression, since [Page 104]they consist of bladders so thin and delicate, that a piece of fine Venice-Paper is very thick in comparison, and that they contain nothing in them but soft Air not-compress'd by any outward force. I caused one of these bladders of above an inch in length and proportionably great, to be taken out of a Roach, and anoin­ted it with Oyl to keep it supple, and preserve it from being pierced or softened by the water; and having by a weight of Lead, fastend to the neck of it, let it down to the bottom of a hollow Cylindrical tube, seal'd at one end, and made purposely large, and about 56 inches long, for some Hy­drostatical Experiments; we could not perceive, that by the weight of all the incumbent water it was manifestly compress'd, or that it did discover the least wrinkle or other depression of the very thin membrane, [Page 105]though stuffed but with Air. And this tryal was made more than once with the same sucess; and yet, that this proceeded rather from the robustness of the bladder, that was able to resist the weight of a taller pillar of water, than from the Non-gravitation of water in the upper part of the tube on that in the lower, we shew'd, by presently letting down such a Mercu­rial-Gage as is describ'd, & often men­tioned in the Continuation of our New Experiments. For letting down this by a string to the bottom of a tube, the weight of the incumbent water forced up some of the Mercury out of the open leg of the Syphon into the seal'd one, and consequently com­press'd the air included there, which though it were not very much, yet it was very manifest. For the un­compress'd Air being 3 inches and ⅝ in length, we judg'd it at the [Page 106]bottom of the tube about ⅝ by the in­trusion of the Mercury that was im­pell'd up; and to satisfie my self and others, that, if the incumbent water had been heavy enough, it would have visibly depress'd the bladder in spite of any Principium Hylarchicum, since I could not have a tube long e­nough, the bladder was sunk into a Chrystal-Glass that had a long and Cylindrical neck, and was so well stuffed with a stopple that was Cy­lindrical too, that 'twas very difficult for any thing to get out betwixt it and the orifice of the Glass; then, a competent Quantity of air being left above the water, the stopple was wa­rily and by degrees thrust down, and so, lessening the capacity of the Glass, compress'd the air that was next it, and, by the intervention of that, the water that was under it. And though there did not upon a slight compres­sion [Page 107]of the outward air appear any sensible operation upon the bladder, that was at the bottom of the water; yet, upon a farther intrusion of the stopple the pressure being encreas'd, the immers'd bladder discover'd not only one but two considerably deep wrinkles, which presently disappear'd upon the drawing up of the stopple. Upon whose being thrust in again, de­pressions were again to be seen on the Swim. And we having been careful to conveigh into the same Glass such a Mercurial Gage as has been lately spoken of, we estimated by the con­densation of the air in the seal'd leg of that Gage, that the bladder had been expos'd to a pressure, that might be equivalent to that of a pillar of about 40 foot of water.

This I hope will lessen the wonder, that Bodies of so firm a texture as those of lusty men, should support [Page 108]the pressure of the water at such depths, as Divers are wont to stay at; since we see, what resistance can be made by so exceeding thin and de­licate a membrane stuff'd only with air, in comparison of the strong mem­brans and fibres of a man, stuff'd besides Air with more firm parts. I will not here urge, that great weights may be sustain'd in the Air by such tendons (or cords of fibres,) and by other fibres, as it were, inter­woven into membrans, in compari­son of what an ordinary man would expect: But I shall invite you to consider with me, that not only upon the account of the stable parts of the humane Body, but of the Spirits too, it may resist very violent pressures (and such as perhaps have not yet been considered) of a fluid Body, not only without any manifest con­rusion or dislocation of parts, but [Page 109]without any sense of pain; which I suppose you will grant me, if, consi­dering what great effects Gusts of Wind have upon Dores, Trees, nay Masts of Ships, blowing them down, nay breaking them; and that yet a man without being extraordinary strong will stand against the impe­tuosity of such a strong Wind, and walk directly against it by vertue of the vigour of his muscles and spirits, without being thrown down or bruis'd by so violent a Current of Air as beats upon him, but without so much as complaining that he feels any pain; and this, though the Wind that beats against him, however it be a fluid Body, yet because it acts as a stream, does not uniformly compress him, but invade only the fore-part of his Body. Likewise, in the lifting up heavy weights by Porters, Car­riers and other lusty men, we may see [Page 110]the slender tendons of the hands loaded with 100 or 150, or perhaps a far greater number of pounds, with­out having their fibres so far com­press'd or stretch'd as to make the lifters complain of pain, though sometimes they may of difficulty. So that, (as I could, if it were needful, confirm by other Instances) a humane Body is an Engine of a much firmer structure than Scholars are wont to take notice of. And here let me add, that I doubt, whether, if the structure of a man were not con­siderably (though not perhaps e­qually) firm, he would, especially in a deep Sea, be able to bear the pres­sure of the water, though not imme­diately applyed, without pain. For (to give you one Reason more of my not acquiescing in vulgar reports about Diving,) having several times convers'd with a man, apt enough [Page 111]both to enquire and observe, who got his living by taking up Ship­wrack'd goods, he answer'd me, when I ask'd him whether he felt any pecu­line pressure against the Drums of his Ears, which are membranes [...]t so well back'd as those of other parts; that when he stai'd at a consi­derable depth, as 10 or 12 fathoms, under the surface of the Sea, he felt a great pain in both his ears, which often put him to shifts to lessen it; which by his manner of describing it I concluded was from the incompe­tent resistance of the Air, which he acknowledg'd to me he found by ma­nifest tokens to be notably com­press'd by the Superior water. Which Relation from such a person does not only confirm our explication, but like­wise warrant us to doubt, whether the Common Reports that are made concerning Divers be fit to be re­ly'd [Page 112]on, without farther Examen and observation.

In the mean time I shall add two or three Experiments more to confirm the resistance, that Animals may make to a great pressure, when exercis'd by the mediation of a fluid Body And I the rather gave you an ac­count of this way of making tryals, because it may be also helpful to discover the resistances of inanimate Bodies, whose Shape and Consistence we may choose and vary (almost at pleasure) to the pressure of (totally or in great part) ambient fluids. And if I had been furnished with a tube wide enough, and a quantity of Mercury great enough, I might by the way have shewn you, that, what­ever the Learned Doctor More is plea­sed to suppose, that to Butter it self even as considerable a pressure may be so applyed as not to be able [Page 113]to make it yield thereunto. For on this occasion I shall adde, that I well remember, that, among other tryals to the same purpose, I caused a piece of fresh Butter, about the bigness of a small Hen-Egg, to be brought to an irregular shape, that, if the compres­sion were such as many would ex­pect, the long corners or solid angles being at least flatted, the Butter might be reduc'd into a more capa­cious figure and less remote from roundness. But though having put this lump of Butter into a Bladder, almost full of fair water, we proceeded, both in the same brass Cylinder, and much after the same manner that I employed about the Egg mention­ed in the Fourth Experiment of the Tract of the Differing pressure of heavy Solids and Fluids; yet I found, that after the plugg had been loaded with a weight of Lead of above 50 pound, [Page 114]neither I, nor the Operator, perceived, the irregular figure of the Butter to be altered. Nor was this the only tryal of this kind I made with the like success upon Butter, though I dare not charge my memory with the Circumstances; and therefore I shall without delay proceed to what I was about to recite concerning the Resistance of Animals.

We took then a common Flesh­slie, neither of the biggest sort of all, nor of the least, but of a middle size, and having put it into the shorter leg of a bent Glass, which we caus'd to be Hermetically seal'd at the end, there was put in as much Mercury as fill'd that leg and a part of the other, lea­ving little more than an inch of Air between the Quick-silver and the seal'd end, that there might be room both for the Fly and the Condensa­tion of the Air, and then with a little [Page 115]Rammer, fitted for the purpose, we caus'd the Mercury in the open leg to be thrust against that in the seal'd leg, which thereupon did necessarily croud the Air near the Fly into less room; so that, by our guess, it was condensed into about a third part of the space, which it possess'd before, and which it regain'd, when the Ram­mer was withdrawn: And though this were done more than once, yet not only the Fly was thereby not kill'd, but not so much, that appear'd, as sensibly hurt, and I perceiv'd her, whilst she was pent up, to move her legs and to rub them one against the other, as 'tis usual with that sort of Insects to do of their own accord in the free Air. Nor did I question but that, if the Glass had not been inconveniently shap'd to admit the Rammer farther into it, the Fly would have supported a far greater Pressure.

Another Experiment to the same purpose we try'd with Water instead of Mercury; but, whereas this last named liquor could neither wet nor drown our Fly, (for which reason I chiefly made choice of it,) the other did first wet its wings, and soon after by a mischance drown it. But first we had an opportunity to compress the Air into a third, if not into a fourth part of its former dimensions, and yet the Fly continued to move divers of her parts and especially her legs very vigorously, as if nothing troubled her but her being, as it were, glu'd to the inside of the Glass by part of her wetted wings. And this I hope will keep the Resistance of Divers to the Ambient water from seeming incredible; since such Flyes were able to resist, and (for ought ap­pear'd) without harm or pain, the pressure of the crouded particles of [Page 117]the Air; though we guess'd this to have been as much compress'd by the force of the Rammer, as it would have been by a Gylinder of water of 50 or between 50 and 60 foot high. By which also we may be help'd to conceive, how great a difference there is, whether the same pressure be exercis'd by a solid or by a fluid Body. For, according to our esti­mate, the pressure against the Body of the Fly was as great as if a slender pillar of Marble, having the Fly for its Base, and 18 or 20 foot in height, had lean'd upon the little Animal; which I presume you will easily think was more than enough to crush her to Death.

But because, though the fore-go­ing tryals are not like to be rejected by the skilful, yet they require a somewhat dextrous and nimble Ex­perimenter, and leave something to [Page 118]his estimate, I will subjoyn an Ex­periment more easie to be made, and wherein the weight may be deter­mined by Measure rather than Con­jecture, being made to be perpendi­cularly incumbent on the Fly or other Animal. For the Experiment may be as well made on other Insects, as Worms, though some that I had provided chanc'd to miscarry before they came to be used.

We took then some ordinary black Flies (such as use to haunt Butchers stalls in warm seasons,) of a middle size, (the length of the Body and Head of one Animal, which for trials sake we measured, being about three eights of an inch,) and having pla­ced one of them with the head up­wards, that there was some distance left bewixt her and the sealed end of the Glass-tube 9 or 10 inches long; we poured in Quick-silver very slowly [Page 119]and cautiously, lest the force of so heavy a body, acquired by the acce­leration of its descent, should more than the meer weight it self of the li­quor oppress the Fly. To this effect stooping the Glass very much towards the Horizon and letting the Mercury puss into the tube through a Funnel, whose lower part was very slender, that it might come down but by little and little, we at length got in as much Mercury as the tube would receive, and then holding it upright, we watched, whether the Fly would make any motions; and finding, that she did manifestly stir notwithstanding the incumbent Mercury, we measur'd the height of the Mercurial pillar, reaching from the middle of her body to the top of the liquor, and found it to be about eight inches, and the Quicksilver being poured out, the Fly appear'd to be so lively and vi­gorous, [Page 120]that I doubted not, but if we had had a longer Glass, the Experi­ment had been much more consider­able. But when afterwards I was able to procure a better tube, the season of Flyes being almost quite past, I could scarce get any, and those not brisk, as they are wont to be in Sum­mer. But however we repeated the Experiment with one of the best we could take of the above-mentioned size, and ordering the matter so, that the Mercury incumbent on her, (for there was some beneath her,) appear­ed to be of a greater height than the formerly imployed tube was of, we saw her move one or other of her little leggs divers times, though the tube were held upright; and therefore measuring the height of the Mercury above her, we found it to amount to 16 inches and better, and then free­ing her from this pressure, we obser­ved, [Page 121]that she immediately found her leggs again, and moved up and down briskly enough; but when she was loaden with 23 or 24 inches of the same Quicksilver (though the liquor were soon after poured out) she gave no signs of life, which I suspected might happen, not so much from her having been opprest by the greatness of her weight, as from the great care of the Operator to let down the Mercury very obliquely and warily upon her. And this I was the rather confirm'd in, because having got an other Fly of about the same bigness, though when she was at the bottom of the Quicksilver, she seemed so com­prest as not to have any motion we could take notice of, yet, upon her being taken out of the Glass, she pre­sently appeared to be alive by walk­ing about and beginning to display her wings, though the pillar of Mer­cury, [Page 122]that had leaned upon her, a­mounted to above 27 inches. And I presume, the success would have been much more considerable, if the Experiment had been tryed in the Summer, when these Creatures are brisk and lively, and not as it was in the Winter; besides that probably these little Animals were hurt or wea­ken'd by the violence that would scarce fail to be us'd in catching them, and putting them into such a place and posture in the Glass as was re­quired; the actual coldness of the Quicksilver perhaps also making them somewhat torpid, whilst it touch­ed them so many ways. And it must not be here omitted, that a Fly, that seemed but about half so big as one of those hitherto mentioned, being well placed, with some Mercury un­der it, in a Glass-pipe held upright, sustained a Mercurial pillar of some­what [Page 123]above 25 inches; and though she was not observed to move under so great a weight, yet when once it was taken off, she did not appear hurt, much less crush'd to Death by it, and probably would have escap'd under a much greater weight, if the tube, which was too large, had not already imployed all the stock of Mer­cury we then had at hand. But I do presume, that what we did try will be available to our purpose, since we see clearly, that so small an Animal as a Fly may survive so great a pres­sure, and that she could not only live, but was able to move such long and slender Bodies as her leggs, when she was pressed against by above 16 inches of Mercury, and (consequent­ly) by a weight equivalent to a pil­lar of water of above 18 foot and a half, which being above 590 times her own length, and (according to [Page 124]the estimate our measure suggested) many times more her own height; so that a Diver, 6 foot tall, (which is somewhat more than an ordinary mans stature,) to have as many times his height of water above him, as our Fly might have had and yet have moved under it, must dive (at least in fresh water,) to near a hundred fathom, which is a far greater depth (per­haps by 5 or 6 times) than, for ought I could learn by inquiry, the Divers either for Coral or Pearl are wont to descend.

And now, Sir, having tender'd you the likeliest conjectures that occurr'd to me about the solution of this dif­ficult Problem; I shall return to Do­ctor More, and consider the objection, he frames from the supposed insolu­bleness of it. And on this occasion I shall have two or three things to re­present to you.

The first is, that there would be much more weight in what he ob­jects, if our Assertion of the gravi­tation of water in water were, like the Principium Hylarchicum, a meer Hy­pothesis advanc'd, without any clear positive proof, whereas our Doctrine is not only elsewhere directly proved, by particular Experiments, but by the very controverted one of the Tadpole; to elude whose force so In­genious a person is fain to flye to a Principle, that, (to say here no more,) is not Physical. And from this first of the things I lately mentioned I shall hasten to the second, because it will require to be longer insisted on.

I shall then further represent that whatever power he is pleas'd to sup­pose at the bottom of the Sea to sus­pend the impression of the incum­bent Water, I think, that supposition ought to give place, if not to our for­mer [Page 126]Ratiocinations, yet to experi­ence it self, which shews there really is a great pressure exercis'd by the Water at the bottom of the Sea. I remember, that a friend of the Learned Sir R. M. Doctors and mine, who is so eminent a Virtuoso as to have been often President of the Royal So­ciety, related a while since to me, that a Mathematical friend of his, whom he nam'd, having had an op­portunity to try an Experiment, I have in vain endeavoured to get try­ed for me, had the Curiosity to let down in a deep Sea a Pewter-bottle with weight enough to sink it, that he might try, whether any sweet Wa­ter would strain in at the orifice or any other part; but when he had pull'd it up again, he was much surpriz'd to find the sides of his Pewter-bottle very much compress'd, and, as 'twere, squeez'd inward by the Water. I [Page 127]also not long since inquir'd of an observing Acquaintance of mine, that has a considerable estate in America, whether he had not try'd to cool his drink, when he sail'd through the Torrid Zone, by letting down the bottles to a great depth into the Sea, and, if he did, in what Condition he found them when they were drawn up again. To which he answer'd, that he had several times employ'd that Expedient for the Refrigeration of his Drinks, but was at first a­maz'd to find the Corks, with which the strong stone-bottles had been well stopt before, so forcibly and so far thrust in, that they could scarce have been so violently beaten in with a Hammer, and 'twas scarce possible to get them out. And an other In­genious Person, that practises Physick in the Indies, having the like Que­stion put to him, answer'd me, that [Page 128]he had some while since had the Cu­riosity to try in a very deep part of the Sea, whether any fresh Water would strain into Stone-bottles through a thick Cork strongly stopt in, and having let it down with a convenient weight to 100 fathom, was much disappointed, when he drew it up, by finding that the pressure of the Water at so vast a depth had quite thrust down the Cork into the Cavity of the bottle (which else per­haps would have been crushed to pieces;) an effect which he would scarce have expected from the stroaks of a Mallet. And if to all this it be objected, that 'twas not the pressure, but the coldness of the Water that did the recited feats by condensing the included Air, and obliging Nature to do the rest for fear of a Vacuum; I will not lanch into the Contro­versie, whether Nature do any thing [Page 129] ob fugam Vacui, but only answer, that I cannot find by the Relations of the Divers or otherwise, that 'tis ever so cold at the bottom of the Sea, as 'tis frequently above ground in Winter, when great Fishes are com­monly said to return to the deep parts of the Sea for warmth, and yet in the sharpest Winters I never ob­serv'd Corks to be driven in by the cold of the Ambient; nay, I pur­posely tryed with a Frigorifick mix­ture, that very intense degrees of cold, such as would quickly freez many Liquors, would not occasion the breaking of thin bubbles of Glass pur­posely blown at the flame of a Lamp and hermetically sealed.

And to shew ad oculum (as they speak) that Water may press more and more, as it grows deeper, against the stopple of a Bottle, though the Vessel be inverted, I will subjoyn [Page 130]this Experiment. Because we have no Water hereabouts that is near deep enough to force in a Cork, as the Sea­water did in the above recited tryals, I thought of a way of so closing the Glass-vessel, as that the stopple should keep asunder the Air in the Vessel and the outward Water, and hinder all immediate intercourse between them, and also make some resistance against the pressure of the external Water, and yet be capable of freely moving up and down, and so be a good Suc­cedaneum to a solid stopple. Taking then a Glass-Vial, furnished with a (somewhat long) Cylindrical neck, whose Cavity was large in propor­tion to the rest of the Vessel, we put into it as much Quicksilver as would in the neck make a short Mercurial Pillar of between half an inch and an inch; then, a piece of very fine Bladder, dipp'd in Oil, was so tyed [Page 131]over the orifice of the Glass, that no Mercury could fall down or get out, nor Water get in at the orifice, and yet the Bladder, by reason of its great limberness, might be easily thrust up towards the Cavity of the Vial, or depress'd by the weight of the Mer­cury. This little instrument, first furnished with a weight of Lead to sink it, being inverted, the Mercury descended into the neck, and closed the orifice as exactly as a stopple, and yet with its lower part depress'd the Bladder beneath the Horizontal Plane, that might be conceiv'd to pass by the orifice; then the Glass, being a while kept in the Water, (that the in­cluded Air might be brought to the Temperature of the surrounding Li­quor,) and by a string let further down into the same Glass-vessel fill'd to about two foot in height, the pres­sure of the Liquor against the orifice [Page 132]of the Vial did by degrees drive up the Bladder and the Mercurial stopple into the cavity of the Neck, as was manifest by the ascension of the Quicksilver; and when the instru­ment was leisurely drawn up again, the weight of this Mercury made it subside and plump up the Bladder again as before. An Experiment akin to this, and therefore fit to con­firm it, I have deliver'd in another See the Pa­radox about Suction. Discourse.

And here I shall sub­joyn what very opportunely occurr'd to me since the writing of the last page. Meeting casually with an In­genious Mechanician, (whom you will find I have In the Tract of the Differ­ing Pressure of heavy Solids and Fluids. elsewhere mentioned) that devised a suit of cloaths and other accommodations, (where­in I once saw him let down into the Water,) by whose help and that of [Page 133]a boat he could (and did) continue there a great while at a considerable depth under water, and there work; I ask'd him afresh (to obtain fuller informations than formerly) whe­ther he felt not the pressure of the water against his breast and belly, to which he answer'd me (more cir­cumstantially than he had before) that when he was about 4 or 5 yards under water, though but in the Ri­ver Thames, his breast and abdomen was so comprest, that there being hardly room enough left for the free motion of his Lungs he could scarce fetch his breath, and was ne­cessitated to make them draw him quickly up, and that (among his later tryals to improve his Engine) having for remedy hereof, caused a kind of Armour for the Chest and back to be made of Copper, though the stiffness of the Metal defended [Page 134]him from receiving any mischief in those parts, yet in the others, where only the Leather, though strong, was interposed, when he came to the depth of about six fathom, though in fresh water, he found a great pres­sure against his legs and armes and all the other parts against which the water was able to thrust the Leathern suit inwards. And this pressure being found by him, as he told me, pretty equal (against all the exposed parts, for from the other, which were more yielding and ob­noxious, the Armour kept it off,) he received no Mischief from it, not yet much Incommodity (and some he might expect from the stiffness and unequal yielding of the Leather;) so that he could stay under water, though not still at so great a depth, about 2 hours or longer. And upon the whole matter he answered me, that he was [Page 135]well satisfied by his tryals, that the am­bient water endeavoured to press him & his Diving suit every way inwards. Whether the coldness of the water had any interest in this Phaenome­non, I particularly enquired of the Engineer; but he replyed, that by reasion of the tightness of his Di­ving suit or instrument, the warm steams of his body that were pent in, and other concurring circum­stances kept him from feeling any cold, and made him sometimes feel a greater Heat than he wished. He has promised me before it be very long to make for me a tryal or two that I propounded to him, from whose success, if he can but reduce them to Experiment, I hope to be able to present you a farther Con­firmation of our Hypothesis. In the mean time, the things already recited, together with the preceed­ing [Page 136]Experiments, may well suffice for our present purpose. For, by what hath been said it appears, that Wa­ter does actually press against bo­dies, whether specifically lighter or heavier than it self, placed under water, and that this pressure in­creases with the height of the water above the immersed Bodies. And this being so, it is not more ne­cessary for me than for men of other Opinions to give a clear reason why Divers can resist so great a pressure of the incumbent water. And the pressure of the water in our recited Experiment having manifest effects upon Inanimate bodies, which are not capable of prepossessions or gi­ving us partial informations, will have much more weight with unpro­judiced persons, than the suspicious and sometimes disagreeing accounts of ignorant Divers, whom preju­dicate [Page 137]opinions may much sway, and whose very sensations, as those of other vulgar men, may be influenced by Predispositions and so many other Circumstances, that they may easily give occasion to mistakes. I know, that Learned men, that never were con­versant in Hydrostaticks, are wont to think it very difficult, if not im­possible, to conceive, how so weak a thing, as they fancy an Animal to be, should avoid the being op­press'd or so much as harmed by so great a weight of Water. But they that shall attentively consider what has been offer'd towards the removal of this difficulty, and re­member, how little they would have believed, that there is so great a difference, as we have by the Tad­pole, the Fly and other instances, shewn there really is between the pressure of Solid and of Fluid bo­dies, [Page 138]will, I presume, be apt to think it fit, that, if for want of a suffici­ent History of matters of fact any scruple remain about the Solution we have offer'd from the nature of the Uniform pressure of Fluids, and the Firm structure of the Humane bo­dy; we should, to remove those re­maining scruples also, rather range about for other Physical helps to solve more compleatly the Problem, about such a thing as Compression, which is an action purely Corporeal and Mechanical, than for want of a ready and compleat Solution to flye to the immediate interposition of an immaterial and intelligent yet Crea­ted Agent, to explain clearly whose manner of working would be a much more difficult Task, than the solu­tion of the Phaenomenon without it.

And now, Sir, having presented [Page 139]to you the Reflections I thought requisite to write upon the Learned Doctors discourses against my Hy­pothesis and Explications, relating to the gravitation and pressure of Fluids, I have little more to trouble you with in this Paper. For, though in the latter part of the 13th. Chap­ter the Doctor is pleased to spend divers pages in the Explication of divers of my Hydrostatical Phae­nomena by the Agency of that in­corporeal Director, that he calls Prin­cipium Hylarchicum; yet since these Explications of his are rather at­tempts to accommodate the Phae­nomena to the Hypothesis, than ob­jections directly levell'd against my Solutions, I shall altogether for­bear to examine them; the main thing that I intended in this Paper, according to what I told you at the beginning, being to shew, that the [Page 140]Arguments urg'd against the Me­chanical solutions of the Experi­ments by me recited, do not evince any of them to be erroneous. And I have neither the design nor the leasure solicitously to examine the Doctors Hylarchical Principle. Of which I shall only say, that though he tells us, it is Page 175. para­tum ad movendum quo­quoversum materiam pro data occasione; yet since he also tells us, Page 167. Quod parti­culae molis corporeae sive stabilis sive fluidae à Prin­cipio Hylarchico in unam aliquam par­tem omnes junctim urgeri possunt & premi, quamvis singulae singulas in nul­lam partem premant, quodque pro mag­nitudine molis major minorve totius fit pressio; and that the force by which it endeavours to keep the E­lements in their true and natural [Page 141]Consistence, though it be very great, is not invincible Pag. 167.: I see no need we have to flye to it, since such Mechanical Affections of matter, as the Spring and Weight of the Air, the Gravity and Fluidity of the water and other Liquors, may suffice to produce and account for the Phaenomena without recourse to an Incorporeal Crea­ture, which 'tis like the Peripate­ticks and divers other Philosophers may think less qualified for the Pro­vince assign'd it, than their fuga Vacui, whereto they ascribe an Un­limited power to execute its Fun­ctions. I leave it therefore to you, Sir, to judge which of the two ways, of explicating an Hydrostatical Phae­nomenon, the Learned Doctors, or that which I have made use of, re­lishes most of the Naturalist. And I shall only tell you, that if I had [Page 142]been with those Jesuites, that are said to have presented the first watch to the King of China, who took it to be a living Creature, I should have thought I had fairly account­ed for it, if, by the shape, size, mo­tion, &c. of the Spring-wheels, ba­lance and other parts of the watch I had shewn, that an Engine of such a structure would necessarily mark the hours, though I could not have brought an argument to con­vince the Chinese-Monarch, that it was not endowed with Life. From which comparison you will easily gather, that what I have thought my self concern'd to doe in this place, was not to demonstrate in general, that there can be no such thing as the Learned Doctors Prin­cipium Hylarchicum, but only to inti­mate, that, whether there be or not, our Hydrostaticks do not need it. [Page 143]Nor do I think it necessary to the Doctors grand and laudable design, (wherein I heartily wish him much success) of proving the existence of an Incorporeal substance. For as I think, Truth ought to be pleaded for only by Truth; so I take that, which the Doctor contends for, to be evincible in the rightest way of proceeding by a person of far less learning than He, without introdu­cing any precarious Principle; espe­cially experience having shewn, that the generality of Heathen Philoso­phers were convinc'd of the being of a divine Architect of the World, by the contemplation of so vast and admirably contriv'd a Fabrick, wherein yet taking no notice of an immaterial Principium Hylarchicum, they believed things to be managed in a meer Physical way according [Page 144]to the General Laws setled among things Corporeal, acting upon one another. And after this I have no­thing more to say, but that I would not have any thing that I have said misconstrued to the Learned Doctors prejudice. For 'tis nor necessary, that a great Scholar should be a good Hydrostatician. And a few hallucinations about a subject, to which the greatest Clerks have been generally such strangers, may war­rant us to dissent from his opinion, without obliging us to be enemies to his Reputation. And therefore if you have found any thing in this Paper inconsistent with a just ten­derness of that, you have not only my consent, but my desire to alter it, as an Expression, that doth not well comply with my Intentions of not appearing any farther his [Page 145]Adversary in our Debate, than the desire of shewing my self a Friend to the Truth I was to defend, should exact of,