HISTORY OF MAGNETISM. 



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 concentrated very near the extremities, just as it is in a lon^ 

 electric body. The theoretical confirmation of this rule among the 

 other consequences of the theory, that the sensible magnetism would 

 be collected at the surface, was one of the results of Poisson's analysis: 

 For it appeared that if the sum of the electric elements of the body 

 Avas equal to the whole body, there would be no difference between the 

 action of a solid sphere and very thin shell. 



We may, then, consider the Coulombian theory to be fully esta- 

 blished and verified, as a representation of the laws of magnetical phe- 

 nomena. 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 magnetism 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 attraction," and which rendered the compass an erroneous guide. 

 Mr. Barlow proposed to correct this by a plate of iron placed near the 

 compass ; the plate being of comparatively small mass, but, in conse- 

 quence of its expanded form, and its proximity to the needle, of equiva- 

 lent effect to the disturbing cause. 



[2nd Ed.] [This proposed arrangement was not successful, because 

 as the ship turns into different positions, it may be considered as 

 revolving round a vertical axis ; and as this does not coincide with the 

 magnetic axis, the relative magnetic position of the disturbing parts 

 of the ship, and of the correcting plate, will be altered, so that they 

 will not continue to counteract each other. In hio-li magnetic latitudes 



O O 



the correcting plate was used with success. 



But when iron ships became common, a correction of the effect of 

 the iron upon the ship's compass in the general case became necessary. 

 Mr. Airy devised the means of making this correction. By placing a 

 magnet and a mass of iron in certain positions relative to the compass, 

 the effect of the rest of the iron in the ship is completely counteracted 

 in all positions. 13 ] 



But we have still to trace the progress of the theory of terrestrial 

 magnetism. 



Theory of Terrestrial Magnetism. Gilbert had begun a plausible 

 course of speculation on this point. " We must reject," he says," " in 



12 See Phil. Trans. 1S3G. " Lib. iv. c. 1. De Variatione. 



