210 Description of the Instruments 
these forces tend to draw the needle into the magnetic 
meridian. The greater the magnetic force, the more 
quickly will the needle recover its former position. It 
is, in short, under the same circumstances as a pendulum 
oscillating by the action of gravity, and the forces are 
as the squares of the number of oscillations made in the 
same time. 
For example, let us suppose that, at a given place, a 
needle oscillating in the plane of the magnetic meridian, 
and round the line of the inclination at that place, per¬ 
forms 12 oscillations in a second of time ; and that the 
same needle at a second place, and observed under the 
same circumstances as before, performs 14 oscillations 
in a second : the intensities of the forces at these two 
places are, therefore, as 12 2 to 14 s , or as 144 to 196, 
or as 1*000 to 1*361. Thus by carrying the same needle 
to various parts of the earth’s surface, providing that its 
magnetism remain unchanged, the intensities of the 
forces may be found by the number of the oscillations. 
There are, however, several practical difficulties 
attending this method of observation in the line of the 
inclination, of which one in particular is, the necessity of 
the needle resting on its axis during its oscillation; 
the friction thereby caused, acting against the force of 
magnetism. This friction does not exist if the needle is 
suspended horizontally by a fine fibre of untwisted silk: 
but a little calculation becomes necessary to deduce the 
total intensity from oscillations thus performed ; because, 
by suspending the needle horizontally, the vertical force 
or the inclination has been counteracted. 
If II represent the total force of the terrestrial mag¬ 
netism at any place, it is easy to decompose this force 
into two others ; viz. HB acting in a vertical direction, 
and which, being overcome by the method of suspension, 
docs not affect the needle, and HA, the other portion of 
