369 



January 1828, announcing that he had found the theory usually 

 employed for reducing the vibrations of a pendulum in air to the cor- 

 responding vibrations in vacuo, was incorrect, inasmuch as it omitted 

 the expenditure of a portion of the moving force on the particles of 

 the air which are set in motion by the pendulum in its vibration. 

 In order to ascertain by the most direct mode of experiment the re- 

 tardation which a pendulum experiences by vibrating in the medium 

 of the atmosphere, the author constructed, at the expense of the late 

 Board of Longitude, an apparatus in which an invariable pendulum 

 could be vibrated alternately in air of the full atmospheric pressure, 

 and in rarefied air approaching nearly to a vacuum. The apparatus 

 was set up and employed in a room assigned for that purpose in the 

 Royal Observatory at Greenwich : its description is given in this 

 paper with reference to plates, and those processes are particularly 

 dwelt upon which were ultimately successful in ensuring the immo- 

 bility of the suspension of the pendulum, and in rendering the appa- 

 ratus impermeable when the air was withdrawn from the interior. 

 The arrangements for observing the coincidences of the pendulum 

 with a clock, and for ascertaining the exact pressure of the air, both 

 in its ordinary and rarefied state, are minutely described. When the 

 air was either partially or wholly withdrawn, a correction was found 

 to be required for the indications of .the thermometer giving the tem- 

 perature of the pendulum, to compensate the removal of the pres- 

 sure of the atmosphere on the exterior of the ball and tube of the 

 thermometer. The value of this correction for different states of ex- 

 haustion was ascertained by placing the thermometer in pounded ice 

 under the receiver of an air-pump, and noting the height of the mer- 

 cury corresponding to different heights in the gauge. 



The number of distinct experiments made with the above described 

 apparatus is eleven ; of these, the six first were designed exclusively 

 for the purpose of comparing the vibration in air of full pressure, and 

 in rarefied air. Each experiment consisted of three distinct series of 

 vibrations made in succession, and occupying usually the greater 

 part of two days : the first and third series were in air of full pressure, 

 and the second series in rarefied air; the mean of the results of the first 

 and third series gave the vibration in the ordinary atmosphere, which 

 was compared with the result of the intermediate series in rarefied 

 air. The comparison was thus rendered wholly independent of the 

 daily rate of the clock, and in some measure also of its deviations 

 from an uniform rate in intervals of less than 24 hours. The 7th 

 and 8th experiments had a double object : first, to compare the 

 retardations of a pendulum in common air and in hydrogen gas, both 

 under atmospheric pressure ; and secondly, to obtain the amount of 

 retardation in both cases, by comparing the vibration in hydrogen 

 gas under thirty inches pressure, with the vibration in the same gas 

 in a highly rarefied state. There are thus eight experiments on the 

 retardation occasioned by the ordinary atmosphere, and two on the 

 retardation in hydrogen gas under atmospheric pressure ; the latter 

 furnishing also the comparative influence of hydrogen gas and atmo- 



VOL. II. 2 B 



