42 Declination of the Compafs. [Book I. 



irregular magnets : for a good loadftone is always of 

 an uniform texture, and has only two poles, which lie 

 in oppofite points of the furface, in fuch a manner, 

 that a line drawn from the one to the other would 

 pafs through the center of the magnet*. When a 

 magnet has more than two poles of equal ftrength, 

 the fupernumerary poles may be fo fituated that the 

 magn&t will not, in the technical language, traverfe j 

 in other words, it will not, when fufpended by a 

 thread, &c. or when floating on water, afTume the 

 ufual direction to the poles of the earth f. 



The magnetic meridian feldom coincides with the 

 real meridian, but generally is found to vary a few de- 

 grees from the true direction of north and fouth. This 

 is called the declination of the compafs. The mag- 

 netic needle varies fometimes to the eaft, and fomc- 

 times to the weft ; and it varies not only in different 

 places, but even in the fame place at different times. 

 The declination at London is not the fame now as it 

 was a few years ago J. Nay, fome very nice obfer- 



vations 



* Cav. p. 40. f Ib. p. 43. 



t The following table fhews the mean declination of the needle 

 at different times in Paris and London. 



Year. 'Paris. Year. Lsujcn. 



O / Of 



1580 n 30 E, 1576 11 15 E. 



1610 8 o E. 1634 4 - 5 E. 



, 1640 . 3 o E. '657 o o. 



1666 o o. 1665 i 22 W. 



1670 i 30 W, 1692 6 o W. 



1700 8 12 W. 1730 10 15 W. 



1728 14 o W. 1,756 15 15 W. 



1771 91 45 W. 1774 i\ 16 W. 



1776 21 47 W. 



Near the equator, in long. 40 Eaft, the higheft variation, from 

 the year 1700 to 1756, was 17 i5'We(l; and theleafl i63o / W, 



Jn, 



