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noiselessly down an almost imperceptible gradient. The 
plan of suspension thus indicated appeared better than the 
use of agate planes, inasmuch as dust and moisture were 
less likely to interfere with the delicacy of the indications. 
Nevertheless it was impossible altogether to avoid the 
effects of these impediments to free rotaton, and the more 
so as the obstruction to rotation is proportional to the square 
root of the height of any small particle between the rolling 
surfaces. In fact, on narrowly watching the periods of 
oscillation they were invariably found to become sensibly 
quicker when the arc was very small, showing that the 
needle was rocking on two points. 
The suspension by inclined silk threads was then tried, 
but soon abandoned, as it was found that the violent torsion 
at the points of attachment could not be certainly allowed 
for, owing to the viscosity of the threads. 
The system brought now before the Society was free 
from the above-named evils. In it each end of the axis 
of the needle is suspended by a fibre of silk, on which it 
rolls. Small washers on the axis serve to keep the fibres in 
a definite position. The ends of the fibres are supported by 
the extremities of a delicate balance beam, placed at the top 
of the instrument. Small pins, &c., are used for adjusting 
the length of the fibres and to regulate the centre of gravity 
of the beam. The needle itself is a piece of softened watch 
main spring, sufficiently long to extend completely across 
the graduated circle. It is seven inches long and weighs 
18 grains. A glass plate fastens before the instrument by a 
notch. By the reflection of the eye of the observer from 
this glass parallax is avoided, #hile the position of each 
