424 THE PRINCIPLES OF SCIENCE. 



and homogeneous body. The slightest irregularity or pro- 

 trusion from the surface will destroy the rigorous cor- 

 rectness of the assumption. The spheroid, on the other 

 hand, has no invariable centre at which its mass may 

 always be regarded as concentrated. The point at which 

 its resultant attraction acts will move about according 

 to the distance and position of the other attracting body, 

 and it will only coincide with the centre as regards an in- \ 

 finitely distant body whose attractive forces may be con- 

 sidered as acting in parallel lines. 



Physicists speak familiarly of the pole of a magnet, ! 

 and the term may be used with convenience. But, if 

 we attach any real and definite meaning to it, the pole 

 is not the end of the magnet, nor is it any one fixed 

 point within, but the variable point from which the 

 resultant of all the forces exerted by the particles in 

 the whole bar upon exterior magnetic particles may be 

 considered as acting. The pole is, in short, a Centre of 

 Magnetic Forces ; but as those forces are really never 

 parallel, this centre will vary in position according to 

 the relative place of the object attracted. Only when 

 we regard the magnet as attracting a very distant, or, 

 strictly speaking, infinitely distant particle, does the 

 centre become a fixed point, situated in short magnets 

 approximately at one sixth of the whole length from 

 each end of the bar. We have in the above instances 

 of centres or poles of force sufficient examples of the mode 

 in which the Fictitious Mean or Average is employed in 

 physical science. 



The Precise Mean Result. 



We now turn to that mode of employing the mean 

 result which is analogous to the method of reversal, but 

 which is brought into practice in a most extensive manner 

 throughout many branches of physical science. We find 



