488 



Messrs. Stewart and Loewy on the true 



[June 17, 



V. " An Account of Experiments made at the Kew Observatory for 

 determining the true Vacuum- and Temperature-Corrections to 

 Pendulum Observations." By Balfour Stewart, Esq., P.R.S., 

 and Benjamin Loewy, Esq., F.R.A.S. Received May 27, 1869. 



1. Pendulum-observations, whether undertaken for the purpose of ob- 

 taining 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 almost entirely withdrawn ; and in order to render such observations, made 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 prin- 

 ciples 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 vibra- 

 tions 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 withdrawn ; from the difference in the number of vibrations thus found 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 con- 

 nexion 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 ob- 

 tained 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 \ an 

 inch to \\ 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 approxi- 

 mate 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. 



3. "With a view to collect for the theory of the subject a great many 



