X. ON THE DETERMINATION OF THE HORIZONTAL 

 INTENSITY OF THE EARTH'S MAGNETIC FORCE 

 IN ABSOLUTE MEASURE. 



Transactions of the Royal Irish Academy. Vol. XXL 



THE attention of mathematicians and experimentalists has been, 

 for some time past, directed to the means of determining the 

 intensity of the earth's magnetic force in absolute measure. 

 These means consist, it is well known, in observing the time of 

 vibration of a freely-suspended horizontal magnet, under the 

 influence of the earth alone, and then employing the same magnet 

 to act upon another, which is also freely suspended, and noting 

 the effects of its action combined with that of the earth. From 

 the former of these observations we deduce the product of the 

 horizontal component of the earth's magnetic force into the 

 moment of free magnetism of the first magnet, and from the latter, 

 the ratio of the same quantities ; and, the product and the ratio 

 being thus known, the two factors are absolutely determined. 

 The former part of this process involving no difficulty which may 

 not be overcome by due care in observing, we shall confine our 

 attention, in the present communication, to the latter. 



Two methods have been proposed for this second observation, 

 one by Poisson, and the other by Gauss. The method of Poisson 

 consisted in observing the time of vibration of the second magnet, 

 under the combined action of the first and of the earth, the acting 

 magnet having its axis in the magnetic meridian passing through 

 the centre of the suspended magnet. In the method of Gauss, 

 which is now universally adopted, we observe the position of 

 equilibrium of the second magnet, resulting from the action of the 

 same forces. The acting magnet being placed transversely with 



