64 HISTORY OF MAGNETISM. 



The various other circumstances which terres- 

 trial magnetism exhibits, the diurnal and annual 

 changes of the position of the compass-needle ; 

 the larger secular change which affects it in the 

 course of years ; the difference of intensity at dif- 

 ferent places, and other facts, have naturally occu- 

 pied philosophers with the attempt to determine, 

 both the laws of the phenomena and their causes. 

 But these attempts necessarily depend, not upon 

 laws of statical magnetism, such as they have been 

 explained above ; but upon the laws by which the 

 production and intensity of magnetism in different 

 cases are regulated; laws which belong to a dif- 

 ferent province, and are related to a different set 

 of principles. Thus, for example, we have not 

 attempted to explain the discovery of the laws by 

 which heat influences magnetism ; and therefore we 

 cannot now give an account of those theories of the 

 facts relating to terrestrial magnetism, which de- 

 pend upon the influence of temperature. The con- 

 ditions of excitation of magnetism are best studied 

 by comparing this force with other cases where the 

 same effects are produced by very different apparent 

 agencies; such as galvanic and thermo-electricity. 

 To the history of these we shall presently proceed. 



Conclusion. The hypothesis of magnetic fluids, 

 as physical realities, was never widely or strongly 

 embraced, as that of electric fluids was. For though 

 the hypothesis accounted, to a remarkable degree 

 of exactness, for large classes of the phenomena, 



