OF THE VIBRATIONS OP 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 of 32° : it was then covered with a receiver, and the air withdrawn : the ther- 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 4- 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 when the air was withdrawn.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 5^0 dth 

 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. 



