214 Description of the Instruments 
fore, necessary to take three readings of the scale, at 
three successive limits of the arc of vibration; and the 
readings being denoted by fl, 6, and c, the mean point of 
scale at the middle of the observation is— 
J (a + 2 6 + c). 
The next manner in which this instrument is employed 
is in experiments to determine the 44 absolute intensity, 
and they are of two kinds—experiments of deflection, 
and experiments of vibration : the former show the ratio 
of the horizontal part of the earth’s magnetic force to the 
moment of free magnetism of the deflecting bar; the 
latter determine the product of the same. "W e have 
already explained, that the square of the number of 
vibrations made by a magnetic needle, in a given time, 
becomes a measure of the intensity of the earth’s mag¬ 
netism, dependent upon the needle employed; and it is 
now necessary to state that the properties of this needle 
have a double influence : first, according to the amount 
of magnetism which it possesses; and, secondly, by the 
effect of its form and weight on the time of vibration. 
When a magnetic needle is turned out of the meri- 
dian, the earth’s magnetism exerts upon it a certain force: 
this force becomes the greater the farther the magnet is 
from the meridian, and is greatest when it is at right 
angles to it, and is termed the 44 moment of rotation. 
This moment is, therefore, dependent upon two things, 
—the magnetism of the earth , and the magnetism of 
the bar . Now, this moment of rotation, and the time 
of vibration, are very simply connected by another 
quality, dependent upon the form and weight of the 
needle, which is called its 44 moment of inertia, easily 
calculated by a well-known expression in mechanics ; and 
it is for the purpose of simplifying this calculation, that 
the magnetic bars are constructed of the perfect regulai 
form described in the Declination Magnetometer. The 
