70 ON MAGNETISM. 



be reduced, by applying the correction for temperature, 

 to what it would have been at a fixed temperature of 

 the magnet, as 32. 



32. Accurate determination of the Earths local hori- 

 zontal magnet-power, founded on the method of deflex- 

 ions, used in combination with the measure of the 

 Earth's horizontal action upon the disturbing magnet. 



In the experiment of Article 26, or that of Article 

 27, treated in the manner described in Article 29, we 

 have found A=kxE:k being a number determined 

 by the solution of certain equations given by the 

 observations. In those observations it was necessary 

 to use a needle B: but we have no further occasion to 

 employ that needle. The operation now to be per- 

 formed consists in suspending the magnet A delicately, 

 and observing the time of its vibration under the action 

 of the Earth's horizontal force. The theory of this has 

 been treated in Article 25 ; and the result, substituting 

 A for B, and preserving T to denote the time of a 

 complete double vibration, and M for the moment of 

 inertia of A, is 



T- / M 



^V EA : 



whence EA = -^ x M. 



It will be remarked that the unit, by which E and 

 A are measured, is the same as that described at the 

 end of Article 29, namely, the action of the standard 

 magnet S upon S' at distance 1. 



