PROGRESS OF MAGNETIC THEORY. 61 



solely at the surface, as Mr. Barlow found it, it fol- 

 lowed from the theory, that the sensible magnetic/ 

 intensity assumed the same distribution as if the 

 fluids could permeate the whole body, instead of 

 the "magnetic elements" only. Coulomb, indeed, 

 had not expressly noticed the result, that the sensi- 

 ble magnetism would be confined to the surface of 

 bodies; but he had found that, in a long needle, 

 the magnetic fluid might be supposed to be concen- 

 trated very near the extremities, just as it is in a 

 long electric body. The theoretical confirmation of 

 this rule among the other consequences of the 

 theory, that the sensible magnetism would be dis- 

 persed at the surface, was one of the results of 

 Poisson's analysis. For it appeared that if the sum 

 of the electric elements was equal to the whole 

 body, there would be no difference between the 

 action of a solid sphere and a very thin shell. 



We may, then, consider the Coulombian theory 

 to be fully established and verified, as a represen- 

 tation of the laws of magnetical phenomena. We 

 may add, as a remarkable and valuable example 

 of an ulterior step in the course of sciences, the 

 application of the laws of the distribution of mag- 

 netism to the purposes of navigation. It had been 

 found that the mass of iron which exists in a 

 ship produces a deviation in the direction of the 

 compass-needle, which was termed "local attrac- 

 tion," and which rendered the compass an erroneous 

 guide. Mr. Barlow proposed to correct this by a 



