216 
Captain Hall's details of experiments 
which the clock is going during the particular period of ob- 
serving ; or, in other words, that number of beats, and parts 
of a beat which, were the clock to go on uniformly from that 
period, would be indicated by its dial plate, in twenty-four 
hours, or 86400 seconds of mean time. As the method of 
transits of stars, however, gives no more than average rates, I 
sought, by the arrangements above stated, to obtain, in like 
manner, average results from the mean of observations made 
at the extreme temperatures. 
A thermometer was suspended so that its bulb stood 
one inch in front of the middle part of the pendulum, and 
another was hung between the clock case and the pendulum, 
lower down. The average temperature at night was 74 0 , 
and in the day time 86° and 90 ° ; the latter, as I have said, 
depending principally on the state of the sky. The allowance 
for expansion was made from the deductions which resulted 
from your experiments on a similar pendulum ; but I propose 
instituting a series of experiments with the pendulum which 
I used, in order to investigate this important branch of the 
subject more directly. 
An astronomical circle, by Troughton, was used as a 
transit instrument, and was so placed in a small octagonal 
observatory of light pannels communicating by a door with 
the tent, that the clock could be seen, and its beats heard by 
the observer at the instrument ; thus, with the exception of 
the first day’s transits, the time was recorded directly from the 
clock, without the intervention of a chronometer. The meri- 
dian mark was placed near the sea, at the distance of 806 feet : 
a strong post having been driven into a cleft of the rock and 
firmly secured, there was nailed to it a screen made of copper, 
and perforated with a system of holes from one-fourth to one 
