56 ON MAGNETISM. 



in Article 23). Note the position taken by the needle. 

 Then turn the suspension-apparatus through a mea- 

 surable angle, say 100, and again note the position 

 taken by the needle. If this position differs from the 

 former, say by 1, it shews that the torsion-power pro- 

 duced by a torsion of 100 is able to neutralize the 

 Earth's action by the quantity corresponding to an incli- 

 nation 1 ; and therefore that the terrestrial action on 

 the needle at any inclination 6 is augmented by the 

 power necessary to overcome -^ part of terrestrial 

 action at that angle ; and that in fact the power 

 which we measure is f of the terrestrial power. 



In this imperfect state the determination of the 

 Earth's magnet-power remained till about the year 1835. 

 And it does appear, at first view, impossible to obtain a 

 numerical value, entirely freed from all dependence on 

 weight of "needle, quality of its steel, intensity of its 

 magnetism, &c., for such an unprehensible element as 

 terrestrial magnetism. A method however was intro- 

 duced (first suggested, the writer believes, in an imprac- 

 ticable form by Poisson, and subsequently changed to 

 an easy and accurate shape by Gauss) which solves the 

 problem perfectly, and is now in daily use : it may well 

 be considered as one of the most beautiful processes in 

 experimental philosophy. To this process, after some 

 preliminary experiments, and a determination of the 

 last Law of Magnetism, we shall turn our attention. 



26. Experimental determination of the proportion of 

 the magnet-power of a magnet to the Earth's horizontal 



