21G Description of the Instruments 
refer those persons who may wish to follow out the 
intricate demonstrations connected with it to Professor 
Gauss’s first Annual Report of the Magnetic Association, 
given in Vol. 2, Part 5, of Taylor’s Scientific Memoirs. 
The next instrument in importance is the Bifilar, or 
Horizontal borce Magnetometer, employed to measure the 
changes that occur in the “ horizontal component” of the 
earth’s magnetic force. The essential difference from the 
other consists in its mode of suspension by two equi¬ 
distant threads or wires, by which means a new directive 
force is obtained, witli which the magnetic force is com¬ 
mensurable; and the principle of its construction may be 
briefly explained as follows :— 
The usual method of observing the horizontal intensity 
has been before described ; but as that operation from its 
nature requires a considerable time, the intensity during 
the period of observation may not be constant, and the 
method is otherwise inadequate to ascertain the changes 
in the force that are constantly occurring from hour to 
hour. To supply a correct knowledge of these changes 
is the principal use of this instrument. 
A body of whatever form suspended by two threads, 
and subject to gravity alone, has (when in a state of 
equilibrium) the vertical line passing through its centre of 
gravity and the lines coinciding with the threads that 
suspend it, in one plane ; and they are either parallel with 
each other, or intersect in a fixed point. In all cases, 
then, in the position of equilibrium, the two threads and 
the centre of gravity are in one vertical plane. If the 
two threads are of the same length, and the same distance 
apart above and below, they will, when in a state of 
equilibrium, hang vertically ; and a third vertical line 
midway between them will pass through the centre of 
gravity of the body. If the body is removed from this 
position by means of a rotation around the last-named 
