INTENSITY OF THE EARTH'S MAGNETIC FORCE. 260 



2. Needle A is then to be removed, and the loaded needle, B, 

 substituted ; and its inclination to the horizon, TJ, is to be observed 

 in the four positions of the needle and limb. The deviation of 

 this needle from the position due to the earth's magnetic force 

 alone is u = 9 - j, the angle 7j being positive when measured at 

 the same side of the horizontal line with 0, and negative in the 

 contrary case. 



3. Needle B is now to be removed from the agate planes to its 

 supports between the microscopes, and needle C substituted ; and 

 the inclination of the latter to the horizon is then to be observed 

 in one position of the needle and limb. The observation is to be 

 repeated with the north end of needle B turned in the opposite 

 direction, by the revolution of the moveable arms which cany 

 the microscopes ; half the difference of the readings in the two- 

 positions is the angle of deflection, u'. 



The total intensity is given by the formula 



sm u sin u 



which is fitted, without any artifice, for logarithmic computation.* 

 In strictness a correction is required for the effect of the 

 change of temperature of needle B, in the two observations in 

 which it is employed ; but as one of these observations may be 

 made to follow the other quickly, and as the needle may be placed 

 in both in nearly the same circumstances, the correction may 

 generally be disregarded. It will be necessary, however, that this 

 needle, when employed as a deflector, should be protected from 

 the heat of the observer's body by a small case of glass or 

 of metal. 



The method here proposed appears to offer the following 

 advantages to the travelling observer : 



1. It is applicable, with equal accuracy, at all parts of the 

 globe. 



2. It dispenses with the employment of a separate instrument 



* If the weight be attached to the loaded needle at a fixed point, the formula 

 becomes 



