OF THE VIBRATIONS OF AN INVARIABLE PENDULUM. 
215 
was made : — The thermometer being immersed in pounded ice, and placed on 
the brass plate of an air-pump, the mercury coincided exactly with the division 
of 32° : it was then covered with a receiver, and the air withdrawn : the ther- 
mometer fell as the pump was worked ; and when the gauge indicated a pres- 
sure of half an inch, the mercury stood at 31°.25 : on re-admitting the air it 
rose again to 32°. The experiment was repeated with precisely similar results. 
By observing carefully the indications of the thermometer with those of the 
gauge, the following corrections of the thermometer were assigned for different 
pressures : for a near approach to a vacuum + 0°.75 ; for 7 inches and there- 
abouts + 0°.70; for 15 inches and thereabouts + 0.5; and for 20 inches 
+ 0°.4. The propriety of these corrections was subsequently confirmed, in the 
experiments with the vacuum apparatus at Greenwich which will be related 
in the sequel, by registering always the comparative indications of the ther- 
mometer which had been tried in ice, and of two others included in a glass 
cylinder, which had been hermetically closed under the receiver of an air-pump 
when the air was withdrawn. The cylinder including these thermometers 
being suspended by the side of the standard in the vacuum apparatus, the doubly 
inclosed thermometers underwent no change on the exhaustion of the appa- 
ratus ; whilst the standard thermometer fell an amount corresponding to the 
above corrections, and remained permanently lower than the others to the 
same amount, until the air was re-admitted, when the indications of the three 
agreed*. 
The result of the experiment on the 29th of June then was, a difference of 
7.38 vibrations for a difference of pressure of atmospheric air at 72°, corre- 
sponding to 22.765 inches of mercury at 32° : this result is equivalent to the 
reduction to a vacuum, for the vibration in a pressure of 30 inches of air of 72°, 
of 9.725 vibrations per diem. 
The specific gravity of the pendulum being about 8.6 ; and the weight of water 
to that of air, at 29.27 inches of the barometer, and 53° of the thermometer, as 
836 to 1, and the expansion of air for each degree of the thermometer sihjdth 
of its bulk, the correction for the buoyancy of an atmosphere of 30 inches of air 
* On trying a thermometer with a ball of unusually large diameter in the pounded ice, the removal 
of the pressure of the atmosphere made a difference in the height of the mercury at the freezing point, 
amounting fully to 1° of its scale. 
