218 
Description of the Instruments 
threads from their position in one plane is the greatest, 
the result therefore the most accurate, and small fluctua¬ 
tions in declination have no perceptible effect. The di¬ 
rective force of suspension thus /becomes an invariable 
quantity, whilst every change in the intensity of the ter¬ 
restrial magnetism affects the position of the bar directly. 
In the adjustments of this instrument, the principal 
difficulty consists in ascertaining the moment of the 
directive force, or the angle formed by the lines joining 
the bearing points above and below; and in equalising 
the intervals of the suspension wires at the points of 
contact. It is found that the nearer the ratio of the 
magnetic force to the torsion force (or the force exerted 
by the wires) is to unity, the more delicate will be the 
instrument; and to effect this ratio, it is furnished with a 
number of grooved wheels, round which the suspension 
wire passes, the stirrup that carries the magnet resting 
on the axes of these wheels by inverted Y’s. The dia¬ 
meters of these wheels increase in arithmetical progression, 
for the purpose of varying the intervals of the wires, so as 
to get a correct ratio of the forces ; and the same interval 
is given to the wires at their upper extremity by means of 
two screws, one right-handed and the other left-handed, 
cut in the same cylinder,—the wires being placed in the 
interval of the threads, and the distance between them 
regulated by a micrometer head. Each thread of this 
* screw is ^ths, or ‘0259 of an inch, and the micrometer 
head to the screw is divided into 100 parts; and as it has 
been so adjusted that one revolution of the head corre¬ 
sponds to two threads of the screw, by turning the mi¬ 
crometer only one division, the interval of the wires is 
altered in consequence *0005194, or the I )art 
an inch nearly. This micrometer has been carefully ad¬ 
justed ; so that when the index is at zero, the interval of 
the wires is exactly *5000, or J an inch. For the purpose 
