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 , 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 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 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 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 it s absolutely necessary for that Seaman who sailes by his Compasse , continually to search the variation , 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 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 . For it is the Assertion of Mr. Dr. Gilberts . Variatio v●…icuiusque 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 .