82 



ON MAGNETISM. 



while the horizontal force, always drawing that end to 

 the magnetic north, now draws it to a part of the 

 supporting frame opposite to the former, the edge of 



Fig. 31. Fig. 32. Fig. 33. Fig. 34. 



the needle which was below will now be above. It is 

 evident here that we shall have the means of eliminating 

 the effect of that ordinate of the centre of gravity which 

 is transversal to the needle. 



But for eliminating the effect of that ordinate which 

 is longitudinal to the needle, we must have some method 

 of altering the direction of magnetism with respect to 

 that ordinate. And there is but one way of doing this ; 

 namely, by reversing the poles of the magnet. This, 

 which may be done by the power of a galvanic current, 

 or in other ways, is done in practice most conveniently 

 by the method of double-touch, described in Article 8. 

 Inasmuch as the application of a pair of magnets will 

 produce certain magnetism in a needle, it is easily con- 

 ceivable, and is accurate in fact, that the use of the 

 opposite ends of those magnets (which possess magnet- 



