THEORY OF THE DIPPING-NEEDLE. 83 



ism of the kind opposite to that of the ends first used) 

 will first destroy the magnetism planted in the needle, 

 and will then plant in it new magnetism of the opposite 

 kind. It is only necessary to caution the operator that 

 the touch-magnets used must have much greater mag- 

 netic power than the needle : otherwise it might happen 

 that the needle, fully charged with magnetic power, 

 would reverse the poles of the touch-magnets. With 

 touch-magnets of adequate power, this never happens : 

 the poles of the needle are reversed, without injury to 

 the powers of these magnets : and the magnetic power 

 of the needle in its state of reversed magnetism is 

 sensibly equal to that before reversion, as is ascertained 

 by calculations to be mentioned below. 



38. Mathematical theory of the Dipping-Needle : 

 first, on the supposition that the magnetic intensity after 

 reversion is equal to that before reversion ; simplification 

 when the needle is very nearly balanced. - 



In Figures 31, 32, 33, 34, the same part of the edge 

 of the dipping-needle is represented by the strong line 

 in a portion of the outline. The magnetic north is 

 supposed to be to the right. Commencing with Figure 

 31, the needle is so turned in Figure 32 that the edge 

 which was downwards is now upwards, but no change is 

 made in the magnetism of the needle. After this, the 

 magnetism is reversed ; and, as is seen in the shading 

 of the Figure, the end, which was charged with blue 

 magnetism and was uppermost, is now charged with red 

 magnetism and is lowest. Between Figures 33 and 34 



62 



