215 
employed in the Observatory. 
moment of inertia, then, being known, the moment of 
rotation, produced by the earth’s magnetism on the mag¬ 
netism of the needle, may be concluded from the observed 
time of the vibrations. Hence we have arrived at one 
of the conditions necessary; viz.—the “product of the 
two forces.” 
Let us now suppose the magnetic bar suspended in 
the Declination Magnetometer, and its positions ob¬ 
served ; first, when hanging freely in the meridian, and 
subject to the influence of the earth’s magnetism alone ; 
and, secondly, after the first magnet (whose moment of 
inertia has been ascertained) has been placed at a con¬ 
siderable distance at right angles to it. The angle of 
deflection produced, shows what fraction of the force of 
the earth’s magnetism the magnetic force of the first 
magnet corresponds to at the selected distance ; and an 
equal fraction of the moment of rotation, found in ascer¬ 
taining the moment of inertia of the first magnet, gives 
the moment of rotation which the same magnet, at that 
distance, would impart to a similar one. This result 
multiplied by the cube of the distance of one bar from the 
other gives the reduced moment of rotation, and the 
square root of this gives the force of the first magnet in 
absolute measure. Finally, the moment of rotation of 
the first magnet divided by this square root gives the 
expression for the absolute measure of the earth s mag¬ 
netism. 
The above description is intended to give a geneial 
idea of the method of an observation for absolute inten¬ 
sity, without entering into the very elaborate mathema¬ 
tical investigations by which such results have been 
arrived at. Its chief excellence consists in the possibility 
of expressing the force of the earth’s magnetism by a 
number which shall be perfectly independent of the 
individuality of the magnetic bars employed ; and I would 
