various Properties of Terrestrial Magnetism. 87 



of the magnetic meridian, may be considered as independent 

 of any modification arising from the earth's magnetism. 



Let the north pole of the needle be deflected above the ho- 

 rizontal line, the deflector being fixed at some given angle 

 from it: suspend minute weights from the northern arm at 

 any given point, say, for instance, midway between the centre 

 and extremity of the needle, till it is reduced to the horizontal 

 position. The weights required to effect this will represent 

 the force of the deflection minus the directive influence of the 

 earth's magnetism on the needle so circumstanced. Remove 

 the weights, and raise the north pole of the deflector as many 

 degrees above the horizontal line as it was before below it, 

 when the deflecting force will cooperate with that of the ter- 

 restrial magnetism, and the weights required to render the 

 needle horizontal, if applied to the corresponding side on the 

 southern arm, will be more than in the former case at every 

 place north of the magnetic equator. One half the excess or 

 difference in the weights will represent the directive force of 

 the terrestrial magnetism, acting on the needle in question 

 when horizontal, and half the sum of the weights employed 

 on both arms of the needle will indicate the force with which 

 it is deflected under the actual circumstances of the case. 

 The latter will be a constant quantity, (i. e. the mean of the 

 results obtained with the deflector turned in opposite direc- 

 tions and the tubes closed, and afterwards drawn out, as be- 

 fore described,) whilst the forces of the needle and deflector 

 remain unchanged ; and if they should have undergone any 

 modification, its extent may be immediately detected by taking 

 one half the weights, however they may be divided, between 

 the two arms. The former quantity or difference of weights 

 on the two arms will be ever varying with the obliquity of the 

 earth's magnetism, and its increased or diminished intensity. 

 The deflecting force, therefore, becomes, I conceive, a stand- 

 ard of comparison for every change which may occur, and 

 the causes of which may hereby be distinguished from each 

 other. Suppose, for instance, the deflecting forces to remain 

 unchanged, and the directive influence of the terrestrial mag- 

 netism on the needle to be increased, the southern arm will 

 require an increase, and the northern a diminution of weight; 

 but if the terrestrial force should be diminished, the case will 

 of course be reversed as it respects the weights, their sum re- 

 maining the same. If the deflecting forces only, and not the 

 terrestrial magnetism, be increased or diminished, the weights 

 must be increased or diminished on each arm, the proportions 

 of which would be equal, were it not that the directive force 



