the Pendulum vibrating Seconds in the Latitude of London. 99 
temperature, and altitude of the barometer, in an infinitely small 
are, the great weight being alove. 
The frame of the support was now elevated, the pendulum was 
inverted, placed in the Ys, with the great weight lelow ; and the 
knife edges being gently let down as before on the agate plane, 
the same process with respect to the observations was followed, 
which has just been described. And if the mean temperature 
differed from that in the former position of the pendulum, the 
mean number of vibrations was corrected for such difference of 
temperature, the expansion of the pendulum being known by 
experiments hereafter to be detailed, and consequently the gain 
or loss in 24 hours by a given change of temperature. 
The mean number of vibrations thus found, differing from that 
given in the former position of the pendulum, the second weight 
was moved, the number of vibrations again determined; and the 
pendulum being inverted, the process was repeated until the 
vibrations in 24 hours, in either position of the pendulum, were 
brought as near to an equality as could readily be effected by 
means of this weight: it was then firmly secured in its place. 
Whatever alteration may be made in the arrangement of the 
weights, the effect cn the vibrations (except in one particular 
instance) will be the same in both positions of the pendulum, 
always increasing or diminishing their number in both cases, 
though in different degrees; and the vibrations will be least 
affected by such change when the great weight is below, and 
will consequently be nearest to the truth in this position. No 
doubt, therefore, can arise, as to the kind of correction required. 
The number of vibrations -after the adjustment by the second 
weight has been completed, must be left 2 defect, for a reason 
which will be immediately apparent. 
There is a point in the pendulum where the effect of the slider 
in increasing the number of vibrations is a maximum; and it 
appears from Dr. Young’s investigations, that this point in one 
position of the pendulum is different from that in the other. 
Very near either of these points, the pendulum being in its cor- 
responding position, the motion of the slider produces scarcely 
any change in the number of vibrations; but the slider being 
then more distant from the point of maximum belonging to the 
other position of the pendulum, the corresponding increase of 
the number of vibrations arising from such motion of the slider, 
will in that position be very perceptible. 
Inthe present instance, the point of maximum, in either posi- 
tion of the pendulum, is about four-tenths of an inch below the 
middle, and consequently the distance of the two points from 
each other is about eight-tenths of an inch. The slider, which 
had remained stationary during the adjustment of the second 
G2 weight 
