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XVIII. On the reduction to a vacuum of the vibrations of an invariable pen- 
dulum. By Captain Edward Sabine, of the Royal Artillery, Secretary of the 
Royal Society. Communicated by Dr. Thomas Young, Secretary of the late 
Board of Longitude. 
Read March 12 and 19, 1829. 
The 128th number of Professor Schumacher’s Astronomische Nachriehten, 
published in January 1828, contained an announcement from M. Bessel, that 
he had found the theory incorrect, according to which it has been customary 
to reduce the vibrations of a pendulum in air, to the corresponding vibrations 
in a vacuum : the incorrectness consisting principally, in no provision having 
been made in the theory, for the expenditure of a part of the moving force, 
on the particles of the air set in motion by the pendulum in its vibration. 
On the arrival in London of the number of the Astronomische Nachriehten 
containing this announcement, a proposal was made to the late Board of Lon- 
gitude, to submit the question, of the reduction of the vibrations to a vacuum, to 
the test of the most direct experiment ; by the construction of an apparatus, in 
which a pendulum might lie alternately vibrated in air of full atmospheric- 
pressure, and in rarefied air approaching nearly to a vacuum. The expense 
of the proposed apparatus was estimated at 25 1. ; which sum the Board of Lon- 
gitude, at the recommendation of the President of the Royal Society, and of 
Dr. Young Secretary of the Board, was pleased to allot for that purpose. Mi-. 
Newman, who was employed to make the apparatus, gave great attention 
towards accomplishing it in the best manner ; and to his care in respect to 
expense it is owing, that the cost has but very little exceeded the estimate. 
How well it has answered its intended purpose will be best collected from the 
experiments themselves. 
The apparatus is represented in Plate VI, which may be referred to for the 
particular dimensions. It consists, generally, of six pieces, exclusive of the iron 
