VARIATION. 



: Thus, if the commonalty of a town make a compoGtion 

 ■with a lord, and afterwards baihfFs be granted by the king 

 ; to the fame town, there, if the lord commence any fuit for 

 breach of the compofition, he muft vary from the word com- 

 monalty, ufed in the compofition, and ufe bailiffs and commonalty. 

 Variance is alfo ufed for an alteration of fomething 

 i formerly laid in a plea': or where the declaration in a 

 , caufe differs from the writ, or from the deed upon which it 

 I is grounded. 



VARIATION of Quantities, in Algebra. See Changes 

 !■ and Combination. 



Variation, in AJlronomy. The variation of the moon, 

 ' called by BuUialdus the rejleaion of her light, is the third 

 ; inequality obferved in the moon's motion ; by which, when 

 I out of the quadratures, her true place differs from her place 

 I twice equated. See Place, Equation, &c. 

 ' Sir Ifaac Newton takes the moon's variation to arife 

 ' partly from the form of her orbit, which is an ellipfis ; and 

 < partly from the inequality of the parts of fpace, which the 

 I moon defcribes in equal times, by a radius drawn to the 

 I earth. 



; To find the greatejl variation, obferve the moon's longfitude 

 ' in the oftants ; and, for the time of obfervation, compute 

 I the moon's place twice equated : the difference between the 

 [ computed and the obferved place is the greatefl variation. 

 ' Tycho makes the greateft variation 40' 30" ; Kepler 

 makes it 51' 49" ; fir Ifaac Newton makes the greateft varia- 

 tion, at a mean diftance, between the fun and the earth, to 

 i be 35' 10" : at the other diftances, the greatefl variation is 

 I in a ratio compounded of the duplicate ratio of the time of the 

 . moon's fynodical revolution direftly, and the triplicate ratio 

 ; of jhe diftance of the fun from the earth inverfely. And, 

 therefore, in the fun's apogee, the greateft variation is 33' 14", 

 and in his perigee, 37' ii"; provided that the excentricity 

 of the fun be to the tranfverfe femidiameter of the orfeis 

 magnus, as i64|^to 1000. Or, taking the mean motions of 

 the moon from the fun, as they are flated in Dr. Halley's 

 tables, and the greateft variation at the mean diftance of the 

 earth from the fun will be 35' 7", in the apogee of the ftin 

 33' 27", and in his perigee 36' 51". Phil. Nat. Princ. Math, 

 prop. 29. lib. iii. apud Horfley's Newtoni Opera, vol. iii. 

 p. 71. 



Variation, in Geography, Navigation, &c. a term ap- 

 plied to the deviation of the magnetic needle, or compafs, 

 from the true north point, towards either eaft or weft ; called 

 alfo the declination. 



The variation, or declination, of the needle, is properly de- 

 fined, the angle which a magnetic needle, fufpended at liberty, 

 makes with the meridian line on a horizontal plane ; or an 

 arc of the horizon, comprehended between the true and the 

 magnetical meridian. 



In fea-language, the variation is ufually called north- 

 eajllng, or north-ivejling. 



All magnetic bodies, we find, range themfelves, in fome 

 degree, to the meridian ; but it is rare that they fall in pre- 

 cifely with it : in one place they decline from the north to 

 the eaft, and from the fouth to the weft ; and in another 

 place, on the contrary, from the north to the weft, and from 

 the fouth to the eaft ; and that too differently at different 

 times. 



The variation of the compafs could not be long a fecret, 

 after the invention of the compafs itielf : accordingly, Fer- 

 dinand, the fon of Columbus, in his life written in Spanifii, 

 and printed in Italian at Venice in 157 1, afferts, that his 

 father obferved it on the 14th of September, 1492 : though 

 others feem to attribute the difcovery of it to Sebaftian 

 Cabot, a Venetian, employed in tUe fervice of our king 



Henry VII. about the year 1500. And as this variation 

 differs in different places, Gonzales d'Oviedi found there 

 was none at the Azores ; whence fome geographers have 

 thought fit in their maps to make their firft meridian pafs 

 through one of thefe iflands ; it not being then known that 

 the variation altered in time. See Gilbert de Magnete, 

 Lond. 1600, p. 4, 5: or Purchas's Pilgrims, Lond. 1625, 

 book ii. feft. i. See Variation of the Magnet. 



Various are the hypothefes framed to account for this ex- 

 traordinary phenomenon ; of which we fhall mention fome of 

 the later, and more probable, only premifing, that Mr. Robert 

 Norman, the inventor of the dipping-needle (which fee), dif- 

 putes againft Cortes's notion, that the variation was caufed 

 by a point in the heavens, contending that it fhould be 

 fought for in the earth, and propofes how to difcover fts 

 place. 



The firft is that of Gilbert (De Magnete, lib. iv. p. 151, 

 &cj which is followed by Cabeus, &c. 



This notion is, that it is the earth, or land, that draws 

 the needle out of its meridian direftion ; and hence they 

 argue, that the needle varied more or lefs, as it was more or 

 lefs diftant from any great continent ; confequently, that if 

 it were placed in the middle of an ocean, equally diftant 

 from equal trafts of land on each fide, eaftward and weft- 

 ward, it would not decline either to the one or the other, 

 but point juftly north and foutb. Thus they fay, in the 

 Azores iflands, which are equally diftant from Africa on the 

 eaft, and America on the weft, there is, in effeft, found no 

 variation : but as from the Azores you fail toward* Africa, 

 the needle begins to dechne from the north to the eaft ; and 

 that ftill more and more, till you reach the (hore. 



If you ftill proceed eaftward, the declination gradually 

 diminifhes again, by reafon of the land left behind on the 

 weft, which continues to draw the needle. 



The fame holds till you arrive at a place where there are 

 equal trafts of lands on each fide ; and there again there 

 is no variation. 



The obfervations of our mariners, in their firft Eaft India 

 voyages, feemed to confirm this fyftem ; as they proceed to- 

 wards the Cape of Good Hope the variation is flill eaftward ; 

 at length arriving ajt the Cape De las Aguillas, q. d. of the 

 Needles, the meridian line, then dividing Africa into two 

 equal parts, there is no variation at all ; but as they proceed 

 farther, and leave the African coaft on the weft, the variation 

 becomes weftward. 



But the misfortune is, the law does not hold univerfally ; 

 in effeft, a great number of obfervations of the variations, 

 in various parts, made and eollefted by Dr. Halley, overturn 

 the whole theory. 



Some, therefore, have recourfe to the frame and corn- 

 pages of the earth, confidered as interwoven with rocks and 

 fhelves, which being generally found to run towards the 

 poles, the needle has been obferved to have a general ten- 

 dency that way ; but which feldom going perfeftly in the 

 direftion of the meridian, the needle, of confequence, has 

 commonly a variation. 



Others hold various parts of the earth to have various 

 degrees of the magnetic virtue, as fome are more intermixed 

 with heterogeneous matters, which prevent the free aftion 

 or effeft of it, than others. 



Others afcribe all to magnetic rocks and iron mines, which, 

 affording more of the magnetic matter than other parts, 

 draw the needle more- 



Laftly, others imagine earthquakes, or high tides, to have 

 difturbed and diflocated feveral conliderable parts of the 

 earth, and fo changed the magnetic axis of the globe, which 

 originally was the fame with the axis of the globe itfelf. 



4H 2 But 



