
TRANSACTIONS OF THE SECTIONS. 103 
vious that if the moment of inertia of the pendulum increase as the air’s resistance 
lessens, the are of vibration may be made permanent; and that this is the change pro- 
duced by the descent of the mercury in the compensating tube. It is not easy in the 
ordinary work of an observatory, to determine the precise relation between the are and 
the condensity ; but by placing the clock zm vacuo, as Bessel proposes (and as Sir James 
South has actually done for several years past), the effect of resistance can be deter- 
mined exactly, and the diameter of the tube selected, which will nearly correct it. 
This is not mere speculation, for I have verified it by trial. The diameter which I 
selected for my tubes (0:1 inch) is not far from the truth. In the autumn of last 
year a fall of 1-6 inch produced no appreciable change of arc. The temperature, how- 
ever, was then nearly stationary; but notwithstanding its changes during the interval 
from that time till my leaving Armagh, the arc has been between 1° 36! and 1° 39!, 
Before the tubes were applied, the limits for the same period were 1° 42! and 1° 51’. 
The changes in Bessel’s own clock, though made by Kessel, a first-rate artist, were 
still greater, being from 1° 25! to 1° 39’, an excess owing in part probably to the great 
severity of the German winter. From what I have seen of the vacuum apparatus 
used by Sabine or South, I cannot refrain from expressing a wish that the experiment 
were tried of mounting a transit clock permanently in vacuo: such a clock would have 
many advantages, besides its exemption from changes of barometric pressure. 
