Chap. 6.] Suppofed Caufe of Polarity; 43 



vations feem to determine that the declination varies 

 at different times of the day *. 



The polarity of the magnet has been attempted to 

 be accounted for, by fuppoiing the earth a large load- 

 ftone, or at leaft that a mafs of ferrugineous matter, 

 equivalent to fuch a loadftone,. is contained within the 

 bowels of the earth, to which all finaller magnets muft 

 neceffarily conform. Attempts have alfo been made 

 to explain the variation or declination of the compafs 

 upon firnilar principles. If the mafs of ferrugineous 

 or magnetic matter which the earth contains is fup- 

 pofed to act upon all magnetic fubftances, and if this 

 mafs is almoft conftantly varying its pofition and 

 compofition by fubterraneous fires, it is not very dif- 

 ficult to fuppofe, that the magnetic needle will be fub- 

 ject to considerable variations from thefe important 

 movements f . 



The magnetic center is the point between the two 

 poles, where the magnet porTeffes neither attraction 

 nor repulfion. If, however, a part of a magnetic bar 

 is broken off at either pole, the fragment will ftill be 

 a complete magnet, having two poles and a center, 



In lat. 15 N. and long, 60 W. the variation was conftantly 

 5 E. In lat. 10 South, and long. 6o Q E. the variation decreaf- 

 ed from 17 W. to 7 15' W. In lat. 10 S. and long. 5 W. it 

 increafed from 2 15' to I245'W. In lat. 15 N. and long. 

 20 W. it increafed from 1 W. to 9 W. In the Indian feas, the 

 irregularities were greater, for in 1700, the Welt variations feem 

 to have decreafed regularly from long. 50 E. to long. 100 E. 

 but in 17^6 the variation decreafed fo fait, that there was Eaft va- 

 riation in long. 80, 85, and 90 E. and yet in long. 95 and 

 100 E. there was Weil variation. 



In the year 1775, ^ n ^ at - 5^ i?'S. and long. 348 16' E. it 

 was o \6' W. In lat. 2 24' N. and long. 32 12' W. it was 

 o 14' 45" W. In lat. 50 6' 30" N. and long. 4 o' W. it was 

 ?9 28 W. Enfield's Inft. Phil. 



* Adams on Mag. p. 415, 416, f Nicholfon's Phil. 



9 though 



