V. An account of experiments made at the kew observatory for determining the true vacuum- and temperature-corrections to pendulum observations
Balfour Stewart, Benjamin Loewy · Proceedings of the Royal Society of London · 1869
Abstract 1. Pendulum-observations, whether undertaken for the purpose of obtaining unalterable standards of length or for physical and geodetic objects, are usually made in air or in a receiver, from which the air is partially or almost entirely withdrawn; and in order to render such observations, made at different places and by different observers, capable of intercomparison, they are, by means of a “correction for buoyancy,” reduced to a vacuum. It is well known that the most illustrious physicists and mathematicians have given a great deal of attention to a correct determination of the principles on which this reduction to a vacuum ought to be based, and of the actual resistance which such a body as a pendulum meets during its vibrations in a fluid body. Until some years ago, especially since the researches of General Sabine and Bessel, it was thought best to determine for every pendulum a certain constant by finding its vibrations in air at the usual pressure, and also in a receiver from which the air is as much as possible withdrawn; from the difference in the number of vibrations thus found the correction was then calculated on the assumption that this difference is proportional to the difference of density of the air. 2. In the pendulum-observations made at the Kew Observatory in connexion with the Great Trigonometrical Survey of India (vide Proceedings of the Royal Society for 1865, No. 78) we adopted, for determining the necessary constant, the method first carried out by General Sabine, and of which a detailed account is given in the Philosophical Transactions for 1829, Part I. page 207 &c. But since our account has been published, two eminent physicists, Professor Clerk: Maxwell and Professor O. E. Meyer in Breslau, have independently investigated the internal friction in gases, and its effect upon bodies moving in them; and among the prominent results obtained by them is this, that the influence of the internal friction of a fluid on a moving body is not proportional to its density. However, for small differences of pressure, such as those experienced by General Sabine in his researches, the old method for determining the correction is sufficiently accurate; or again, if a series of such experiments as our own fundamental Kew observations for India be made at a very low pressure, say from ½ an inch to 1½ inch, the correction is itself a very small quantity; and the application of a more correct principle of reduction will not sensibly affect the ultimate results, because the difference between the true and approximate correction is in such a case extremely small. But if, as is the case in the Indian observations, experiments are made at higher and varying pressures, it is very desirable to apply experimental methods which will give the true correction.