TRANSACTIONS 
OF THE 
ROYAL IRISH ACADEMY. 
I.—On the Determination of the Intensity of the Earth’s Magnetic Force 
in absolute Measure. By the Rev. HumPurey Luoyp, D.D., Fellow of 
Trinity College, Dublin, and Professor of Natural Philosophy im the 
University; F.R.S.; V.P.R.I.A.; Honorary Member of the American 
Philosophical Society, and of the Batavian Society of Experimental Phi- 
losophy. 
Read January 9, 1843. 
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 de- 
duce the product of the horizontal component of the earth’s magnetic force into 
the moment of free magnetism of the first magnet,—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. 
B2 
