156 ON MAGNETISM. 



degree represent a sheet of metal, and that a very pro- 

 late spheriod may represent a rod. And these are the 

 nearest approaches to practical application made by 

 Poisson's theory. 



The final statement by Poisson of general theoretical 

 results is merely the following: If a, /9, 7, are three 

 rectangular components of terrestrial magnetic force, 

 and if a system of iron masses receive magnetic induc- 

 tion from their action, the forces which they will exert 

 on the pole of a compass-needle, in the direction of the 

 three axes, will be 



POL + Q0 + Ey, 

 POL + Q'{3 



P, Q, &c., being constants peculiar to the masses and 

 positions of the iron. The reader who has seen in the 

 preceding investigations how the resolved parts of ter- 

 restrial induction produce in the first instance three 

 systems of molecular magnets, and who remarks that 

 the mutual action of those in each system will still pro- 

 duce a series of magnets peculiar to that system, and 

 who further remarks how the magnitude and direction 

 of the resulting forces depend on the distance and posi- 

 tion of each molecular magnet, and how the forces can 

 be resolved into rectangular directions, will perceive 

 that such a form of result is necessarily obtained without 

 any abstruse investigation whatever. 



71. Inadmissibility of Poisson s fundamental sup- 



