2 ON MAGNETISM. 



of heat at the place of union of different substances, 

 magnetic effects can be elicited. On the larger scale, 

 it is certain that the whole Earth acts as a combination 

 of magnets, and there is reason to think that the Sun 

 and the Moon also act as magnets. 



2. Our accurate knowledge of magnetism is limited 

 to the magnetism of iron, steel, and the Earth, to which 

 this work will be confined: with allusion finally to 

 galvanism and thermo-electricity. 



The laws of magnetic force, however, have been 

 experimentally examined with philosophical accuracy, 

 only in its connexion with iron and steel; and, by 

 inferences bearing considerable probability, in the in- 

 fluences excited by the Earth as a whole. The accurate 

 portions of the following work will therefore be con- 

 fined to the investigations connected with these metals 

 and the Earth. But it will be advantageous, before 

 terminating the treatise, to allude in a more general 

 way to the laws of the connexion between magnetism 

 on the one hand and galvanism and thermo-electricity 

 on the other hand. 



