219 
employed in the Observatory . 
of ascertaining the angle before mentioned, the instru¬ 
ment is furnished with a well-graduated circle, and a 
vernier attached to the suspension apparatus. 
The magnetic bar employed is of the same form as in 
the declination magnetometer, and is protected in the 
same manner by a circular box. The mode of observation 
is also similar, noting three successive limits of the arc of 
vibration through a reading telescope, as in the declination 
magnetometer ; and the angular value of the scale 
employed being known, the changes in the horizontal 
intensity are converted into parts of radius ; the variation 
of the angle being that quantity which it differs at any 
moment from its mean value. The moment of free mag¬ 
netism of the bar varies with the temperature; and it 
becomes necessary to apply a correction for the same 
before the true changes of the earth’s force can be ascer¬ 
tained. To deduce this correction, experiments are made 
with artificial heat upon the magnetic bars, and the rela¬ 
tive change of their magnetic moment corresponding to 
one degree of Fahrenheit carefully ascertained, and the 
reduction applied to the monthly mean values. This in¬ 
strument is therefore furnished with a thermometer, the 
bulb of which is within the box, for the purpose of ascer¬ 
taining the interior temperature. 
For observing the charges in the vertical component, 
or that portion of the absolute force which the earth may 
be supposed to exert in a vertical direction, the Observa¬ 
tories are furnished with an instrument called the A eitical 
Force Magnetometer: the essential part consists of a 
magnetic needle having a cross of wires at each extremity 
attached to the needle by small pieces of copper. The 
axis of the needle being formed into a knife edge resting 
upon agate planes, and the needle brought to the hori¬ 
zontal position by weights, the changes of the vertical 
force may be inferred when the mean inclination at the 
