TRANSACTIONS OF THE SECTIONS. 
565 
To obtain exactly the same angular inertia, we have only to de¬ 
termine a series of points for the slider, on each side, the sums 
of the squares of the distances of corresponding pairs of which, 
from the centre, taking one on each side, give a constant quan¬ 
tity. But for all practical purposes, the correction of the in¬ 
clination, hy the more simple method, appears to he sufficient. 
For setting the sliders with accuracy, it is requisite to mea¬ 
sure off the distances by means of a finely graduated scale, and 
a fine pair of compasses. 
The author has found, that a small difference in the flexibility 
and length of the suspension silk, frequently gives rise to small 
differences in the rate of vibration; and therefore recommends 
that the silk for suspension be carefully tested by experiment, 
and be previously prepared, both as to length and size ; so as 
to obtain a series of suspension silks, which may be considered 
as being identical in effect on the bar. 
Mr. Christie’s very interesting experiments “On the Influence 
of the Sun’s Rays on the Magnetic Needle,” (detailed in the 
Transactions of the Royal Society for the year 1826,) are of 
great consequence to researches with the magnetic pendulum. 
If a magnet be allowed to vibrate in air in a closed receiver, 
first in the shade, and subsequently when exposed to the rays 
of the sun, the time in which the arc of vibration becomes re¬ 
duced to a given point, will, in the sunshine, be considerably 
less, whilst the time in which a given number of vibrations is 
performed, is slightly diminished. 
The author endeavours to discover, whether this effect 
arises from any pure magnetic quality in the sun’s rays, or from 
some mechanical influence operating on the bar or medium in 
which it moves. 
In order to avoid the very great inconveniences and inaccu¬ 
racy to which the experimentalist is necessarily exposed under 
the influence of a burning sunshine, the author was led to em¬ 
ploy only a portion of the sun’s rays, thrown by plane mirrors 
upon the needle whilst under a closed receiver; the experiment, 
conducted in this way, became very manageable and satisfac¬ 
tory. The following are the general results which he states 
that he has arrived at. 
1. The influence of the sun’s rays on a magnet, oscillating 
in air, is to reduce more rapidly the arc, and at the same time 
to diminish the time of a given number of vibrations. 
2. The influence of the sun’s rays on a magnet, oscillating 
in void, is to increase the time of a given number of vibrations, 
whilst the arc remains very nearly the same: if the retardation 
of the rate of vibration be small, the arc may be considered as 
