370 



spheric air. The 9th, 10th, and llth experiments were made to ex- 

 amine whether the vibration of the pendulum in air within the ap- 

 paratus was the same as in the free air of the apartment ; this was 

 accomplished by vibrating the pendulum alternately in free and in 

 confined air, by removing and again replacing such parts of the ap- 

 paratus as were necessary for that purpose. It was found, by this 

 means, that the confinement of the medium by the glasses produced 

 no sensible effect on the time of vibration. 



From the mean of the eight experiments on the retardation in 

 common air, the author obtains 10*36 vibrations per diem, as the re- 

 duction to a vacuum of the invariable pendulum, vibrating in air of 

 45 under a pressure of 30 inches of mercury : and by computing 

 the retardation severally for the circumstances of each experiment, 

 and comparing the computed and observed retardations, he shows, 

 that were the amount of the reduction to a vacuum separately de- 

 rived from each of the eight experiments, it would in no case differ 

 more than 0'14 of a vibration per diem, from the conclusion derived 

 from their mean, or one 74th part of the conclusion itself. 



The reduction to a vacuum which would have been previously 

 computed for the vibration in air of 45, and 30 inches pressure, is 

 6*26 vibrations per diem. The retardation in air of that temperature 

 and density is therefore 4 - l vibrations greater than has been hitherto 

 supposed ; and the proportion which the experimental reduction bears 

 to that which is now shown to have been erroneous, is, for the inva- 

 riable pendulum of the ordinary form used in this country, as 1'655 

 to 1. 



From the experiments in hydrogen gas, under a pressure respec- 

 tively of 30 inches, and of less than one inch, the retardation of a 

 pendulum vibrating in hydrogen gas of 40 under a barometric pres- 

 sure of 30 inches, is two vibrations per diem. The hydrogen gas 

 employed was obtained by the action of zinc upon dilute sulphuric 

 acid, and was passed into the apparatus through a cylinder contain- 

 ing muriate of lime. A portion withdrawn after the experiments 

 were concluded, was examined by Mr. Faraday, and found to contain 

 no appreciable mixture of air. 



The two experiments on the comparative retardation in air and in 

 hydrogen gas give the ratio as 10' 55 to 2, and as 10' 41 to 2; or, 

 generally, as 5^ to 1. But the ratio of the respective densities of 

 atmospheric air and hydrogen gas is about as 13 to 1. Whence 

 the author takes occasion to remark, that if the resistance of the 

 elastic fluids to bodies falling through them were simply as the re- 

 spective densities of the fluids, the retardation occasioned by air 

 should be 13 times as great as that occasioned by hydrogen gas, 

 that the difference of this ratio from that shown by experiment is 

 much too great to be ascribed to error of experiment, particularly as 

 repetition produced results almost identical ; that it may rather be 

 regarded as indicating an inherent property in the elastic fluids ana- 

 logous to that of viscidity in liquids, and of resistance to the motions 

 of bodies passing through them independently of their density ; and 



