THE RELATIVE FORCE OF GRAVITY AT KEW AND GREENWICH. 
543 
immediately after the initial observations of coincidence, to obtain the interval 
between successive coincidences. 
In the Indian operations the agate planes were always carefully levelled before the 
pendulum was set up on them, and the level readings, as taken before and after the 
swings by each pendulum, were recorded and published. As the pendulum is neces¬ 
sarily made to vibrate with its tail-piece a short distance in front of the scale for 
measuring the arc of vibration, this distance was read off on a scale fixed at right 
angles to the arc-scale, to enable the observed arc reading to be duly corrected ; and 
it was measured in the two positions of the instruments, both with the marked face 
towards the observer and towards the clock. Thus, as the knife-edge was always 
lowered by the Y’s down to the same line on the agate planes, and as the planes were 
always horizontal, half the difference between the distances of the tail-piece from the 
arc-scale, in the two positions, indicates the magnitude of any deviation of the bar of 
the pendulum, at the tail-end, from perpendicularity to the knife-edge. These dis¬ 
tances were always recorded, and they show that both the pendulums were bent, but 
that the bending was practically constant throughout the whole of the operations; 
the marked face of Pendulum No. 4 was deflected ‘5 inch outwards, and that of No. 6 
(1821) ’3 inch backwards, at the tail-end. As, however, a genej-al constancy was 
preserved throughout, the whole of the results were truly differential. 
Colonel Herschel commenced his operations at Kew by swinging No. 4 pendulum 
in the condition in which he found it. He soon noticed bends in both the two 
pendulums, and also found that the knife-edges were somewhat rusted. There was 
reason to suspect that the pendulums might have received some injuries when set up 
at one of the great Annual Exhibitions in South Kensipgton which was held a few 
years after their return from India. Consequently both the pendulums were 
straightened and their knife-edges were re-ground. This, of course, causes a break of 
continuity with the antecedent operations with these pendulums, and destroys the 
differentiality of the vibration numbers obtained before and afterwards. 
In the revisionary operations at Kew and Greenwich the pendulums were swung in 
the same condition as when employed by Colonel Herschel and the officers of the 
United States. 
In the Indian operations each set of swings was usnallj of about 9 hours’ duration, 
from 8 A.M. to 5 P.M., intermediate readings of the tliermometers and observations of 
coincidence being taken at intervals of about hour apart. When all the observa¬ 
tions were finally reduced it was seen that, whenever the daily variation of tempera¬ 
ture was considerable, the clock rate at different hours of the day varied sensibly 
from the mean daily rate; thus it was evident that the vibration-number, which 
depends on the actual clock rate during the set of swings, but is deduced from the 
mean daily rate which is derived from successive transits of the same stars, might be 
much influenced by variations of rate occurring during the part of the day when the 
pendulum is under vibration. 
