100 ON MAGNETISM. 



SECTION VI. 



THEORIES ON THE PHYSICAL CAUSE OR REPRESENTA- 

 TION OF TERRESTRIAL MAGNETISM. 



43. Eeasons for believing that Terrestrial Mag- 

 netism is not produced, in any important degree, by 

 magnetic forces external to the earth. 



If there were an external cause for magnetism, it 

 seems scarcely conceivable that some large part of it 

 would not act in planes parallel to the geographical 

 equator: and, if so, its effects at any one place would 

 undergo very great changes in the earth's diurnal 

 revolution; every part of the earth being presented, 

 in the course of a day, in different aspects towards forces 

 so acting. Now the fact is that the diurnal changes are 

 very small, perhaps at Greenwich F J 7 part of the whole 

 horizontal force. It would seem therefore certain that 

 external bodies or space do not produce any sensible 

 part of the magnetism in the planes to which the earth's 

 axis is normal. And this carries with it a very strong 

 improbability that they produce any sensible magnetic 

 forces in the direction of the earth's axis also. 



