PROFESSOR STOKES, ON THE EFFECT OF THE INTERNAL FRICTION, &c. [9] 



The attention of the scientific world having been called to the subject by the publication 

 of BesseFs memoir, fresh researches both theoretical and experimental soon appeared. In 

 order to examine the effect of the air by a more direct method than that employed by Bessel, 

 a large vacuum apparatus was erected at the expense of the Board of Longitude, and by 

 means of this apparatus Captain (now Colonel) Sabine determined the effect of the air on 

 the time of vibration of a particular invariable pendulum. The results of the experiments 

 are contained in a memoir read before the Royal Society in March 1829, and printed in the 

 Philosophical Transactions for that year. The mean of eight very consistent experiments 

 gave 1*655 as the factor by which for that pendulum the old correction for buoyancy must 

 be multiplied in order to give the whole correction on account of the air. A very remark- 

 able fact was discovered in the course of these experiments. While the effects of air at the 

 atmospheric pressure and under a pressure of about half an atmosphere were found to be 

 as nearly as possible proportional to the densities, it was found that the effect of hydrogen at 

 the atmospheric pressure was much greater, compared with the effect of air, than corresponded 

 with its density. In fact, it appeared that the ratio of the effects of hydrogen and air 

 on the times of vibration was about 1 to 5^, while the ratio of the densities is only about 

 1 to 13. In speaking of this result Colonel Sabine remarks, " The difference of this ratio 

 from that shewn by experiment is greater than can well be ascribed to accidental error in the 

 experiment, particularly as repetition produced results almost identical. May it not indicate 

 an inherent property in the elastic fluids, analogous to that of viscidity in liquids, of resistance 

 to the motion of bodies passing through them, independently of their density ? a property, in 

 such case, possessed by air and hydrogen gas in very different degrees ; since it would appear 

 from the experiments that the ratio of the resistance of hydrogen gas to that of air is more 

 than double the ratio following from their densities. Should the existence of such a distinct 

 property of resistance, varying in the different elastic fluids, be confirmed by experiments 

 now in progress with other gases, an apparatus more suitable than the present to investigate 

 the ratio in which it is possessed by them, could scarcely be devised : and the pendulum, 

 in addition to its many important and useful purposes in general physics, may find an 

 application for its very delicate, but, with due precaution, not more delicate than certain, 

 determinations, in the domain of chemistry." Colonel Sabine has informed me that the 

 experiments here alluded to were interrupted by a cause which need not now be mentioned, 

 but that as far as they went they confirmed the result of the experiments with hydrogen, and 

 pointed out the existence of a specific action in different gases, quite distinct from mere 

 variations of density. 



Our knowledge on the subject of the effect of air on the time of vibration of pendulums 

 has received a most valuable addition from the labours of the late Mr Baily, who erected 

 a vacuum apparatus at his own house, with which he performed many hundreds of careful 

 experiments on a great variety of pendulums. The experiments are described in a paper 

 read before the Royal Society on the 31st of May 1832. The result for each pendulum is 

 expressed by the value of n, the factor by which the old correction for buoyancy must be 

 multiplied in order to give the whole effect of the air as deduced from observation. Four 

 spheres, not quite ll inch in diameter, gave as a mean n = T864, while three spheres, a little 

 Vol. IX. Part II. 26 



