220 
Description of the Instruments 
place of observation is known, and the times of vibration 
of the needle, in the vertical and horizontal planes 
respectively. 
The knife edge is adjusted so as to pass as nearly as 
possible through the centre of gravity of the unloaded 
needle, and the other adjustments are effected by small 
screws, working in fixed nuts, one on each end ; the axis 
of one of these screws being parallel to the magnetic 
axis of the needle, and the other perpendicular to it: by 
the movement of the former the needle is brought to the 
horizontal position, and by that of the latter the centre 
of gravity is made to approach or recede from the 
centre of motion, and the sensibility of the instrument 
augmented or diminished. 
The changes in the position of this needle are observed 
by two micrometer microscopes fixed on pillars of copper, 
and firmly attached to the base of the instrument. 
These microscopes contain two wires, one fixed or im¬ 
moveable, and the other moving , as the micrometer-liead 
is turned; the adjustment of the head being so arranged 
that one complete revolution corresponds to five minutes 
of arc : consequently, as the head is divided into 50 
parts, the changes in the position of the needle can be 
measured within 6" of arc. 
In adjusting this instrument, it is essential to bring 
the fixed wires of the microscopes to the same horizontal 
line. That being obtained, the magnetic needle itself is 
to be balanced to the horizontal position, which is effected 
(as has been already stated) by the screw moving in a 
direction parallel to its magnetic axis. The needle being 
accurately balanced and permanently fixed in the azi¬ 
muth in which it is intended to move, any change in the 
vertical component moves the cross wire at the two ends 
from the horizontal line to which they were first ad¬ 
justed ; and the mode of observation simply consists in 
