COMPARISON OF TERRESTRIAL DIRECTIVE FORCES. 55 



Greenwich) ; and if the needle be carried to other 

 stations in rapid course, so that there is little reason to 

 fear any change in its magnet-power ; and if, for con- 

 firmation, the needle be again brought to the standard 

 place : then we may obtain a certain result thus. Di- 

 vide the square of the first equation above by the square 

 of the second, and we have 



_T _.#' 

 T'*~ E' 



Thus we obtain the proportion of the Earth's magnet- 

 power at one station to its magnet-power at another 

 station, at the same time. But we get no positive in- 

 formation on the measure of the Earth's magnet-power 

 at either station. And as we cannot suppose that the 

 magnet-power of the needle will be unaltered through 

 an unlimited time, we cannot use the experiment to 

 determine whether the Earth's magnet-power at any 

 station has altered with the lapse of time. 



The principal value of this method is for very re- 

 stricted local experiments. 



In all applications of the method, we ought in strict- 

 ness to take account of the torsion-power of the suspen- 

 sion-thread : as, on changing the suspension-thread, or 

 in comparing observations where the difference of 

 external magnetic action is great, the omission of that 

 consideration may introduce important error. The 

 torsion-power will be measured thus. Suppose the 

 suspension-piece to be furnished with apparatus which 

 admits of being turned round horizontally (as described 



