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XV. Supplement to the “ Account of Pendulum Experiments undertaken in the Harton 
Colliery being an Account of Experiments undertaken to determine the Correction 
for the Temperature of the Pendulum. By G. B. Airy, Esq.^ Astronomer Royal. 
Received February 13, — Read March 6, 1856. 
Section IX . — Introductory and Historical. 
65. In order to remove, as far as possible, the slight uncertainty in the result of 
the Harton Experiment depending on the mean difference of temperature of the 
Pendulums, I again borrowed the two Pendulums and the Clock Shelton from the 
Royal Society, and prepared them for use in the following manner. Two rooms 
were selected at the Royal Observatory, which appeared well adapted for a series of 
pendulum experiments. One is the Laundry of the Dwelling House ; a room on the 
basement story, almost entirely sunk below the level of the Lawn and Front Court, 
but bordered on two sides by a dry area ; with stone floor laid immediately upon 
the solid ground. The chimney-opening and other crevices were carefully stopped 
up, and a hot-air-stove with funnel leading into the chimney was planted in the 
room ; by means of this stove the temperature of the room could be maintained with 
reasonable steadiness nearly to 90° Fahr. The Pendulum 8 with Clock Earnshaw 
were mounted in the usual way in this room. A screen was placed between the 
stove and the pendulum, which effectually prevented the pendulum and its thermo- 
meters from receiving any sensible heat by radiation. The other room is the Lower 
Record Room, a room with stone floor laid (except at its centre) upon the solid 
ground, warmed by a heating apparatus below the centre of the floor. The Pen- 
dulum 1821 with Clock Shelton were mounted in one side of this room. As a hot- 
air-pipe rises in the centre of the room, this pipe also was screened to prevent unfair 
radiation on the pendulum, &c. ; although here the necessity for such caution was 
far less pressing than in the Laundry. 
66. The plan proposed for operations was ; to compare the clocks, by carrying a 
Solar Chronometer from one to the other, and observing their readings at the time 
of coincidence of beats (a method which admits of extreme accuracy when the clocks 
are so near together that the chronometer can be carried without risk of disturbance) ; 
to divide the operations into Four Series, numbered in sequence Fifth, Sixth, Seventh, 
Eighth ; to make no alteration whatever in the pendulums, nor even to dismount 
them, between one series and another; but to make Pendulum 1821 hot and 8 cool 
in the Fifth and Seventh Series, and Pendulum 8 hot and 1821 cool in the Sixth and 
Eighth Series ; and through the extent of each Series to keep up the Swings in 
