120 



perceptible changes for one hundred and forty 

 years past *. 



The intensity of the magnetic forces is another 

 very important phenomenon, to which natural- 

 ists have hitherto very little attended. Graham 

 and Musschenbroeck had attempted to measure 

 the diurnal variations of this force by the velo- 

 city of the horizontal oscillations of a magnetic 

 bar ; but it appears, that Borda was the first, 

 who had the idea of making the same dipping 

 needle oscillate in different places of the Globe. 

 The attempts of this learned navigator did not 

 afford, as he has often assured me, any precise 

 result, on account of the friction, which the an- 

 cient needles underwent on their axis. At this 

 period men were often satisfied with making the 

 needle of the variation compass oscillate : and 

 in the manuscript account of Borda's Voyage to 

 the Canaries it is expressly stated, in speaking 

 of the modifications of the intensity of the mag- 

 netic forces measured by the velocity of the os- 

 cillations, that, at the summit of the Peak, he 

 had counted ten oscillations of the card in 97" of 

 time ; while at Santa Cruz their duration had 

 been 94", at Cadiz 103", and at Brest 113". Mr. 



* Thomson's Hist, of the RoyalSoc.,p. 461. Phil. Trans, 

 vol. 50, p. 330 and 349. (The Oriental Navigator, 1801, 

 p. 650.) 



+ Phil. Trans, vol. xxxiii, p. 332. Thomson's Hist, of the 

 Royal Soc, p. 461. Diss, de Magnete, Exp. 102 and 107 



