60 HISTORY OF MAGNETISM. 



the thickness of the fluid in the direction of the 

 radius would be as the distance from the center 10 . 

 But the subject was taken up in a more complete 

 manner in 1824 by M. Poisson, who obtained general 

 expressions for the attractions or repulsions of a 

 body of any form whatever, magnetized by influ- 

 ence, upon a given point; and in the case of 

 spherical bodies was able completely to solve the 

 equations which determine these forces 11 . 



Previously to these theoretical investigations, 

 Mr. Barlow had made a series of experiments on 

 the effect of an iron sphere upon a compass needle ; 

 and had obtained empirical formulae for the amount 

 of the deviation of the needle, according to its 

 dependence upon the position and magnitude of the 

 sphere. He afterwards deduced the same formulae 

 from a theory which was, in fact, identical with that 

 of Coulomb, but which he considered as different, in 

 that it supposed the magnetic fluids to be entirely 

 collected at the surface of the sphere. He had 

 indeed found, by experiment, that the surface was 

 the only part in which there was any sensible mag- 

 netism ; and that a thin shell of iron would pro- 

 duce the same effect as a solid ball of the same 

 diameter. 



But this was, in fact, a most complete verifica- 

 tion of Coulomb's theory. For though that theory 

 did not suppose the magnetism to be collected 



10 Bull des Sc. No. li. 



11 A. P. for 1821 and 2, published 182G. 



