CORRECTIONS FOR THE EFFECT OF INDUCTION. 165 



d / _sin0 1 -sm0 1== tan&(0 t --0J 



A ~" sin t + sm0, ~~ tan & (6 1 + 2 ) ' 



The value of I thus obtained is the amount of induced 

 magnetic force produced by the local terrestrial vertical 

 force. The local terrestrial horizontal force is found by 

 multiplying the local vertical force by cotan. local dip : 

 hence the induced magnet power produced by local 

 terrestrial horizontal force E will be 



A x cotan. local dip x 



It is most carefully to be observed that the terrestrial 

 force E and the dip here spoken of are those peculiar to 

 the place of this experiment ; that d l and 2 are angles 

 peculiar to this experiment; and that A is constant so 

 long as the power of the magnet is not changed. 



74. Correction of the formula}, used in the deter- 

 mination of the Earths horizontal magnet-power, for 

 effects of induction. 



In examining the process in Articles 26 to 28, it 

 will be seen that we have to consider the effect of a 

 magnet A in disturbing a needle when A is inclined to 

 the meridian : and in Article 32, we have to treat of the 

 earth's horizontal force upon A when A is in the meri- 

 dian. These must be considered separately. Let E' be 

 the earth's horizontal power at a second station. 



First, for the disturbance of a needle by A. If $ be 



