TO write an Encomiastique of the Mag­net is not my intention; the bold & confident attempts of Seamen, tho­rough those hidden paths of the vast Ocean, to all navigable parts of the earth, will spare me the labour; nei­ther will these few lines permit me to speake of all Magneticall qualities, they that list may have recourse for their satisfaction herein, to that most learned worke (admired by all Foreiners) long agoe penned by Mr. Dr. Gilbert our countryman, sometimes Physitian to that Renowned Lady our late Queene. That which I here principally ayme at, is the deflection of the Needle from the Terrestriall Meridian, together with that ab­struse and admirable variation of the Variation, lately discovered to the world.

The Variation then is the deflection of the Magneti­call Meridian from the Terrestriall, the Horizon deter­mining its Quantitie and Qualitie. These Meridians sometimes are coincident, and in such places there can be no Variation▪ Sometimes againe they are diffe­rent, [Page 2] and then that Hemispere wherein either extremity of the Needle lyes, denominates its Quality, for if it be the Easterne Hemispere, it is called Easterly variation, if the westerne, westerly. And the Horizon supposed (as all great Circles are) to be divided into 360 parts, & each part subdivided into Centesmes or Millesmes, de­fines the Quantity as 10, 20, 30, &c. parts of variation.

For our more distinct proceeding (and to give some satisfaction to those that are not altogether Mathemati­call) it will not be impertinent to our present purpose to give some probable Reason for this variation or swarving of the Needle from the meridians of most places.

Take a vigorous Magnet, convert it into a Sphericall Body, and having found its Poles, delineat the same with Meridians, the Aequinoctiall, and Parallells.

Then take a fine sowing Needle, and applying it to the in­tersection of a Meridian and the Aequinoctiall, it will be found precisely to point out the Poles of the stone; and if the Needle be moved towards either Pole, accor­ding to its owne direction, it will trace out a Circle o­ver the said Magnetique Pole. But if this Sphaericall Mag­net shall have a part of it excavated, as admitt about the forty fift Paralell, and forty fift Meridian or degree of longitude, the Needle moued forwards from the fore­said intersection, assoone as it shall draw neere to the limb of this excavation, will forthwith leave its former position, as lying in the plane of the prime meridian, and incline to the other solide side, not respecting at all the aforesaid Poles: and this deflection we call the vari­ation of the Needle: And the greater will this variati­on be, the neerer it approacheth to the Poles. For [Page 3] whereas before the excavation each side being a like potent, Which may be some cause of those suddaine and exorbitant variations in the Nortwest and other Nor­therne & Southerne parts of the Earth. the Needle like an upright and indifferent Arbiter inclined to neither part; after the excava­tion it will convert it selfe to the more solide side, rejecting the imperfect and defective, and so much the more powerfully, by how much the other side shall be found to have lost more of its Magnetique vigorous substance.

The like will fall out if the Needle be so placed, as that it shall bisect the excavated part, for then will it acurately point out the true Poles of the stone former­ly found, but suddainely swarue from them after any little dislocation, strongly applying it selfe to the more solide side.

As it is with this little, so likewise with the greater Magnet the Earth, whose solide magnetique parts are great Continents, the defective and excavated, the deepe and vast channell of the Sea. For if there were observations made in the midd way betweene the Ea­sterly Coast of China and this westerne of Europe, it is very probable there would be found no variation at all, the Terrestriall and Magneticall Meridians being congru­all; but if we shall incline to either side, as admit to the westward, the Needle will in like manner move it selfe to the Easterne Continent, as the more prevalent side, the channell of the Sea being a part deficient of the Sphericall Body of the Earth, and the Needle finding no incouragement to apply it selfe there to.

And so likewise for the Ocean, which palliates the imperfect parts of the Earth, it is evident, that if the [Page 4] Needle shall equally divide the Marine distance between two continents, there will be found no variation, as having both the Meridians united, the Needle equally disposing itselfe to either Continent. But if we shall swerue from this mediety, it will preponderate with the next continent.

For the evidence of the truth hereof, let a man exa­maine generally the variations of our most expert Sea­men, (a Catalogue wherof we have annexed to that ac­curate worke of Mr. Wright in his detection and co­nection of errors in Navigation) (although by reason of the diversities of the setts of their Compasses and oftentimes unapt handling of their instruments, they very seldome concurre) obserued in the Atlantique O­cean, from the Equator unto the parts of Norway, all along the East Coast from the Meridian of the Azores, as farre North as hath beene discovered, and he shall finde the ordinary practise to testifie the truth hereof. As also after the same manner from the Equator south­ward, unto the streights of Magellan, and all along the backe parts of America in the South Sea, and on the East Coast unto the Cape of Bona Speranza, and he shall perceive the like agreement; But in fayling from the aforesaid Cape further Eastward, that sometime they doe finde it otherwise, the cause proceedeth from the different situation of the South Continent hitherto undiscovered. And whereas in the Atlantique Ocean about 135 leagues from the Ilands of Corues and Flores to the westward, Mr. Iohn Wells. there hath beene found by an able Ar­tist no variation at all, it is a manifest argument, that the Place is equidistant from the Europaean or Affrican con­tinents and the American: And as we approach to ei­ther [Page 5] Continent, so shall we find the Needle praepotent in its conversion; from whence the variation shall re­ceive its denomination.

Therefore its absurd for Seamen to touch their Compasses with the allowance of the variation of any particular place, considering this variation is perpetu­ally variable according to the shipps motion.

And moreover its absolutely necessary for that Sea­man who sailes by his Compasse, continually to search the variatiō, that so by the Prosthaphaereticall application thereof, the true point of the compasse (which is his principall Mercurialist) may be rectified.

And this artifice may be performed by the assistance of the Planets or fixed Starres, and that either without or with their Altitudes. I shall not need to exemplify all varieties, as having elswhere copiously and demon­stratively delivered the same. I thinke it only conve­nient to touch lightly at one or two, and such as best conduce to our present purpose.

And first without an Altitude. Finde two fixed Starres of equall right Ascention, and holding vp a thread and plummet, observe when the thread bisects them (for that is the exact Meridian of the place) and at that instant apply the Compasse to them; if the Needle precisely respect them, there is no varia­tion; if not, then is the difference, the variation inqui­red; and that part of the Horizon to which it inclines, (alwayes accounting from the same intersection of the Horizon and Meridian) claimes it.

It may likewise be performed by the Amplitude Or­tive or Occiduall of the Sunne, and as well by night as by day, by the rest of the Planets or fixed Starres, ha­ving [Page 6] onely their declinations and the Latitude of the places, either Geometrically by an Analemme, or Arithmetically by the helpe of the Canon of Trian­gles, for the ordinary termes of the proportion runne thus.

As the Sine complement of the Latitude, is to the Radius, So the Sine of the declination, to the Sine of the Amplitude. Turning therefore the Compasse to the Sunne or Starres, the difference of the Arcs is the vari­ation inquired. Or by knowing the time of the Sunnes incidence into the Plane of the Prime verticall, after the like manner we may obtaine our desires.

But in these and some other wayes, we are confined to a certaine time: If therefore to the former requisits we shall add an Altitude, this Artifice may be perfor­med at any convenient appearance of a Planet or Starre, as we shall by and by shew.

And for the more easie and certayne obtaining of the variation at Sea, I would advise the Sea-man to extend or prolong the capitall of his Needle, or fix some stile on it, so that it may cut the Flye at right Angles, for then the Compasse coverted to the Sun, this stile will readily shadow out the Azimuth Magneticall, and comparing it with that of the Sunne, he hath the variation neere enough without any further labour or prepa­ration.

Thus hitherto (according to the Tenents of all our Magneticall Philosophers) we have supposed the vari­ations of all particular places to continue one and the same: So that when a Seaman shall happly returne to a place where formerly he found the same variation, he may hence conclude, he is in the same former Lon­gitude. [Page 7] For it is the Assertion of Mr. Dr. Gilberts. Varia­tio v [...]icuius (que) Loci constans est, that is to say, the same place doth alwayes retaine the same variation. Nei­ther hath this Assertion (for ought I ever heard) been questioned by any man. But most diligent magneticall observations have plainely offred violence to the same, and proved the contrary, namely that the variation is accompanied with a variation. For whereas in the yeare 1580 Mr. Burrows (a man of unquestionable abi­lities in the Mathematiques) found the variation at Lim­house neere London to be 11 gr. 15 min. or neere one point of the Compasse; In the yeare 1622 Mr. Gunter sometimes professor of Astronomie in Gresham Colledge, found the variation in the same place to be but 6 gr. 13 min. And my selfe this present yeare 1634 with some friends had recourse to Dedpford (where Mr. Gunter had heretofore made the same observations with those at Limehouse) and found it not much to exceed 4 de­grees.

For the clearer manifestation of this truth, (because all men have not beene furnished with or seene the fore said observations, I hold it necessary to transferre theirs hither, and subnect those which were lately made by my selfe with the assistance of some friends.

[Page 8]

The way of finding the variation after this manner is easy to be conceived. A time being chosen when the Heavens were like to be unmasked for the whole day, the altitudes of the Sunne were taken by him to each degree of a Quadrant both forenoone & Afternoone, from 17 to 25 gr. making nine severall observations. And at every Altitude turning his Needle to the Sunn, observed the degree and parts cut thereby. Then com­paring the Magneticall Azimuthes made in the Fore­noone, with those in the Afternoone answerable to the same Solar altitudes, the halfe of the difference is the variation. Thus at the altitude of 17 gr. in the [Page 9] Forenoone, the Needle cutting 52 gr. 35 min. and at the same altitude in the Afternoone 30 gr. 0 min: the dif­ference is 22 gr. 35 min: the halfe whereof is 11 gr. 17 min: 30 Sec. the variation inquired.

Before we proceed to other Observations it will be requisite we take a review of these, and after correc­tion of some of the apparent Altitudes, call the Solar Azimuthes to the Iudicature of a Triangle.

It is not unknowne that about this time of Mr. Bur­rowes his observations the accurate and corrected ta­bles of the Sunnes motion were not published to the world, Neither the Angles of the Obliquity and Pa­ralaxes of the Sunne so truely determined. And as for the refraction (which insinuates it selfe in every Alti­tude not exceeding 45 degrees) being then in its infan­cy and a stranger to the world it was not called into the account, no not by the Ancients themselues, nor limited before that great Astronomer and honorable Dane Tyche Brahe opened the same way. If therefore we shall examine these Altitudes acording to the late corrected grounds, we shall somewhat alter the vari­ations of Mr. Burrowes, but so as that we shall not much differ from him, or suspect any great error in his worke.

These observations then of his, were made the 16 of October Anno 1580. And by Astronomicall Calculation should begin about 9 ½ in the Morning, and end about 2 ⅓ in the Afternoone. Let us now make choice of the Magneticall Azimuthes answerable to the Altitudes of 20 gr. being the fourth observation. The true place of the Sunne about this time correspondent to this Alti­tude according to the late Rud [...]lphins Tables, (with [Page 10] consideration to the difference of Meridians and E­quation of Time) is thus found after the Centesimall way with the connexion of our British Logarithmes, but with some alteration of the Anomalies

And because we haue made choice of the apparent Altitude of 20 gr. the Parallax ( Kepler admitting lit­tle or none,) according to Tycho Brahe will be

That the insuing work may be the better cōceived, let FZB in the annexed scheme) be the Meridian of Lime­house; Z the zenith or Pole of the Horizon FOB. The elevation of the Pole PB. ZP the complement thereof. AE the Equator. The Southerne Parallell of the

Sunne DSO. ZSH a Quadrant of a verticall Circle intersecting the Sunnes parallell, and passing through his Center in the point S. HS the Sunnes Altitude a­boue the Horizon; and ZS the complement or his di­stance from the zenith. SI the declination. SP the di­stance of the Sunne from the Pole of the world. Also let the obscure periphery MZG be the Magneticall Me­ridian passing through the zenith Z, and deflecting or

varying from the Terrestriall Meridian the Arc MB, the measure of the Angle MZB the variation inquired, which will be determined by comparing the magneti­call Azimuth CZS with the Solar Azimuth PZS, always greater then the Magneticall before Noone, but lesse after Noone, where the deflection is Easterly as here with us in England and the adjacent parts. But the Magneticall azimuth is altogether found by instrumen­tall observation, the Solar by Calculation or the reso­lution of the obliquangular sphaericall Triangle PZS, whose sides are already knowne, and therefore the An­gle PZS may be obtained by the fourth compend of the last chapter of the British Trigonometry after this manner.

The double whereof is 1460008 the Angle inquired PZS. But the Magnet. Azim. is 1350000 namely the Angle CZS. The differ. is 110008 MZB the variation inquired, which converted into sexagesmes is 11 gr. 0 min, 0 Sec. Mr, Burrowes by the helpe of the afternoons observation making it 11. gr. 22 min. 30 sec. the dif­ference being 0 gr. 22 min. 30 sec.

The like may we doe with the same Altitude for the Afternoone, if we shall onely rectify the declina­tion answerable to the Sunnes motion for the internall of time. The Sunnes true place was in 3179445 of ♏ the declination correspondent was 1261740. Then in the adiacent diagram, being the same with the former, save onely that the magneticall Azimuth CZS is (as it ought to be) greater then the Solar PZS, we have the same termes given, and therefore by resolving the

Triangle PZS, the angle inquired PZS may be found either by the former or the third variety belonging to the 11 Probleme of the same booke, and agreeing as

well with the Calculation of a Plane Triangle as of a Sphaericall, after this manner.

which converted into sexagesmes, is 11 gr. 32 min. 28: sec. And if with this we compare the morning vari­ation, (which should have been the same with the afternoones) the difference will be 32 min: 28 sec: [Page 15] which may easily arise either from the Insufficiency or Fabrique or grosse handling of the Instruments. So that if we take the Arithmeticall meane, we may pro­bably conclude the variation answerable to his time to be about 11 gr. 16 min: with which very neere con­curreth Dr. Gilbert, finding it 11 gr. 20 min.

From these we proceed to Mr. Gunters Observations, the Table whereof we have here inserted, as we finde them in his mathematicall workes, where we see the variation not to exceede 6 gr. 13 min. His instru­ments were one Quadrant of three foote Radius for the Sunnes Altitudes: His Needles were two, the one of six, the other of 10 Inches in length. The Sunnes Azimuthes were found by calculation, and the Azi­muthes Magneticall, taken off a Horizontall Plane by a line of Chordes, whose Semidiameter was twelue Inches.

Thus haue we prooved that for the Interstice of 42 [Page 16] yeares, there hath beene an evident diminution of five, degrees variation. I deny not the Artifice to be very nice and subtle, and that an error may vnawares easi­ly insinuate it selfe, which together with this great discrepance, moved some of us to be overhasty in casting an aspersion of error on Mr. Burrows observa­tions, Mr. Iohn Marr. (though since upon noe just grounds) till an ac­quaintance of ours, lately applying Mr. Gunters owne Needle to the side of the Cubicall Stone of his Maje­sties Diall in White Hall garden, could not finde the variation so great as 6 gr. 15 min: formerly found; whereupon resolving with some friends to make an ex­periment hereof, we went to Diepford the last yeare 1633 the day of the Sunnes entrance into the summer Solstice, to the very same place where Mr. Gunter heretofore had made observation, and found it much lesse then fiue degrees; And afterwards calling into Que­stion the Insufficiency of our Instruments, that all scruple might be quite taken away, we had recourse this present yeare 1634 Ioh. 12. stilo vet: to the same Garden of our learned and ingenuous friend Mr. Iohn Welles, with a Quadrant of six foote Radius for solar Altitudes, continually rectified with great care, and a Horizontall Quadrant of two foote Radius, for the determining of the Magneticall Azimuthes, exactly set to the Magneticall meridian, and in paralellisme to the Horizon; (otherwise great error might ensue) with two Needles of twelue Inches in length well touch't with good Magnetts; And for the better sa­tisfaction, tooke with us also the very same Needle wherewith Mr. Gunter made the foresaid observations, in length ten Inches; all three must, accurately re­specting [Page 17] the same Magneticall Meridian as we then proved. And least there might arise some diversity in the variation through the touches of severall Stones, I caused the one Needle first touch't by a very good Magnet, to be retouch't by another as good if no bet­ter, and the same wherwith most of our zea compastes are touch't, and yet found no difference betweene them. And so Iune the 14. 1634▪ we made these fol­lowing observations, where we found the variation not to exceed 4 gr. 10 min. as appeareth by the Table annexed.

And because in the Resolutions of these obliquan­gular sphaericall Triangles for the Solar Azimuthes we have necessary use of the Latitude of the Place, the same we determine by the Sunnes meridian Altitude to be 51 gr. 30 min. after this manner.

[Page 18]

These Concordant Observations can not produce a variation greater then 4 gr. 12 min. nor lesse then 3 gr. 55 min. the Arithmeticall meane limiting it to 4 gr. and about 4 minutes.

And for a further confirmation herof (though mo­ved upon no just reason to suspect these observations, being every way as cautelous in them as possible we could) as soone as the intermission from my publique Exercise would permit, I caused the same instruments to be transported to Paules Cray in Kent, distant from London SE and by S about 12 miles, where the fourth of Iuly following ( Fide Astronimica) I made the subse­quent animadversions, hardly amounting to 4 degrees.

The Latitude of the Place I finde by the same large Quadrant to be 51 gr. 25 min. which thus I prove.

[Page 19]

It were needlesse having so many sufficient testimo­nyes, to produce any more; one onely observation made with circumspection being as good as one hun­dreth. So that its plaine, the observations made in this place do all make the variation to fall neere upon 4 de­grees.

Hence therefore we may conclude that for the space of 54 yeares (the difference of time betweene Mr. Bur­rowes and these last observations of ours) there hath beene a sensible diminution of 7 degrees and better.

If any affected with magneticall Philosophy shall yet desire to see an experiment made for their owne particular satisfaction, where I may prevaile, I would advise them to pitch a faire stone parallell to the Hori­zon [Page 20] there to rest immoveable, and having a Needle of a convenient length strongly touch't by a vigorous Magnet to draw a Magneticall Meridian thereby, and yearely to examine by the application of the same (well preserved from the ayre and rust, its greatest enemies) whether time will produce the like alteration.

I will not here enter into a dispute concerning the cause of this sensible diminution, whether it may be imputed to the Magnet, or the Earth, or both. It is not unknowne to the world, how the Greatest Masters of Astronomie, which this age hath afforded, for the more easy salving the apparent anomalar motions of the fix­ed and erratique caelestiall lights, and avoyding that supervacaneous furniture of the Ancients, do with all alacrity embrace that admirable Copernicean Hypothesis of the diurnal, Annual, & Secular motions of the earth, in so much as conferring with that Great Astronomer D. Phil. Lansberg. in Zealand about Astronomicall mat­ters, did most seriously affirme unto me, he should ne­ver be disswaded from that Truth. This which he was pleased to stile a truth, I should readily receive as an Hy­pothesis, and so be easily led on to the consideration of the imbecillity of Mans apprehension, as not able right­ly to conceive of this admirable opifice of God or frame of the world, without falling foule on so great an absurdity. Yet sure I am, it is a probable inducement to shake a wavering understanding. And which adds somewhat more, I understand by Galilaeus a Florentine, and a most acute and learned Mathematician, that an Ita­lian Gentleman of the Family of the Marsilii hath late­ly found the mutation of the Terrestriall Meridians, and penned a treatise thereof not yet published, which the said Galilaeus hath seene and read constupore, not with­out amazement. His words are these.

[Page 21] Surge di presente una quinta novita, dalle quale si possa arguir mobilita nel globo terrestre, mediante quello, che sottilissimamente va scoprendo l' illustri­ssimo S. Caesare della nobilissima famiglia de i Mar­silij di Bologna, pur' Academico Linceo, il quale in una dottissima scrittura va esponendo, come ha osservato una continua mutazione, benche tardissi­ma, nella linea Meridiana; della quale scrittura, da me ultimamente con stupore veduta, spero che douera fare coppia a tutti gli studio si delle maraviglie della natura.

Which in English runnes thus.

Now there ariseth a fift noveltie, from which we may argue the mobility of the tererestriall Globe, by the meanes of that which most subtllie the most mustru­ous Gentleman Sig: Caesar of the most noble familie of the Marsilij of Bologna a Lyncean Academicke also, hath discovered, who in a most learned tract plainely sheweth how he hath observed a continuall mutation, although very slow, in the Meridian line; which di­scourse, which with wonder I lately saw, I hope he will impart to all those who are Studious of the won­ders of nature.

Surely if it be so, the due consideration of the premisses doth (as I conceive) lay open a faire way for the [Page 22] salving of that Irregular Motion imputed to the Axis of the Earth. I meane the alterable Obliquity of the Ecliptique, which to deny (as some yet doe, not with­out great violence offred to the Ancients) I shall hardly (without very strong argument) be perswaded. I preter­mit likewise a suppofition which might be made of the correspondent motion of the Pole of the Magnet with those of some of the Planets, and divers other conceits, which must be all left to future times to discover, this Invention being but newly presented to the world in its infancy.

Gresham College 5 Cal. Aug. 1634.