62 HISTORY OF MAGNETISM. 



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 equivalent effect to the disturbing 

 cause (c). 



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 12 , "in the first 

 place, that vulgar opinion of recent writers con- 

 cerning magnetic mountains, or a certain magnetic 

 rock, or an imaginary pole at a certain distance 

 from the pole of the earth." For he adds, " we 

 learn by experience, that there is no such fixed 

 pole or term in the earth for the variation." Gil- 

 bert describes the whole earth as a magnetic globe, 

 and attributes the variation to the irregular form 

 of its protuberances, the solid parts only being 

 magnetic. It was not easy to confirm or refute 

 this opinion, but other hypotheses were tried by 

 various writers ; for instance, Halley had imagined, 

 from the forms of the lines of equal variation, that 

 there must be four magnetic poles; but Euler 13 

 showed that the "Halleian lines" would, for the 

 most part, result from the supposition of two mag- 

 netic poles, and assigned their position so as to 

 represent pretty well the state of the variation all 

 over the world in 1744. But the variation was not 



12 Lib. iv. c. i. De Variatione. 13 Ac. Berlin, 1757. 



