TERHEST.RIAL MAGNETISM. 3 67 



the globe was dependent on the high or low temperature of 

 the part of space through which the solar system has moved. 

 This hypothesis., imagined by one of the profoundest ma- 

 thematicians of our time, has been satisfactory to few, if 

 indeed to any one except himself, and has certainly not been 

 received by physicists and geologists. 



But whatever may be the cause of the internal heat of our 

 planet, and its limited or unlimited increase at increasing 

 depths, it conducts us, in this general contemplation of 

 nature, through the intimate relation of all the primary phse- 

 nomena of matter, and through the common bond which 

 unites the molecular forces, into the obscure domain of Mag- 

 netism. Changes of temperature elicit magnetic and electric 

 currents. Terrestrial magnetism, of which, in its threefold 

 manifestation, incessant periodical variation is the leading 

 characteristic, is ascribed either to inequalities in the tempe- 

 rature of the globe, ( 141 ) or to those galvanic currents which 

 we regard as electricity moving in a closed circuit. ( 142 ) The 

 mysterious march of the magnetic needle is dependent both 

 on time and space, on the course of the Sun, and on its 

 own change of place on the surface of the Earth. The hour of 

 the day may be known between the tropics by the direction 

 of the needle, as well as by the height of the mercury in the 

 barometer. It is affected instantly, though only transitorily, 

 by the distant Aurora by the streams of richly coloured 

 light which shoot in bright flashes across the polar sky. When 

 the ordinary horary movement of the needle is interrupted 

 by a magnetic storm, the perturbation manifests itself, often 

 simultaneously in the strictest sense of the word, over land 

 and sea, over hundreds and thousands of miles; or propa- 



