220 ON MAGNETISM. 



range of possibility that such a combination^ of heat 

 with dissimilar substances, may be the cause of terres- 

 trial magnetism. But there is no evidence for this, 

 beyond mere conjecture. 



It is worthy of remark that the isothermal lines 

 on the earth's surface bear a striking resemblance to 

 the lines of equal magnetic intensity shown in Figures 

 35 and 36. 



On the whole, we must express our opinion, that 

 the general cause of the earth's magnetism still remains 

 one of the mysteries of cosmical physics. 



CAMBRIDGE: PRINTED BY C. J. CLAY, M.A. AT THE UMVERSITY PRESS. 



