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XXXV. A Mathematical Discussion of the properties of the Air 

 Thermometer with respect to its fitness for determining the in- 

 stantaneous changes of Temperature of its contained Air } arising 

 from sudden changes of Density, By Professor Potter, A.M.* 



THE properties of the air thermometer, which it is proposed 

 to discuss in the present paper, are of considerable scien- 

 tific interest. M. Poissonf had found that the theory of sound, 

 upon the hypothesis of the infinite divisibility of matter, required 

 that in the sudden condensation of a gas the temperature should 

 rise 1° Centigrade for every y-f-g-th part condensation, and fall 

 the same quantity for every yy-g-th part rarefaction. This received 

 no experimental support until MM. Clement and Desormes J, in 

 the year 1819, contrived an experiment with a receiver (ballon) 

 furnished with pipes and stopcocks, leading, the one to an air- 

 pump, and the other to the external air, and other pipes forming 

 barometer gauges (manometres) of mercury and water. The 

 air in the receiver, after a slight exhaustion, was allowed to com- 

 municate with the external air, which thus re-entered suddenly, 

 and then the communication with the external air was cut off. 

 By observing the movements of the liquids in the gauges, it was 

 thought that the effect of the heat arising from the sudden con- 

 densation was exhibited by them, and that the apparatus thus 

 became a sensitive air thermometer. 



In the Mecanique Celeste, vol. v. p. 125, published in 1825, 

 M. Laplace describes the method of the experiments of MM. Gay- 

 Lussac and Welter with a like balloon and manometer or gauge, 

 only employing condensations where MM. Clement and Des- 

 ormes had used rarefactions, as well as precautions for increased 

 accuracy. The method is still that of a sensitive air thermometer. 



Mr. Meikle also employed the same method, with a liquid for 

 the gauge much lighter than water, the vapour of which in the 

 receiver might be an objection. 



In my paper in the Philosophical Magazine for January last, 

 I have described some of my own experiments with Newman's 

 air-pump and barometer-gauge on the same principle. There is 

 a remarkable accordance in all the results which have been ob- 

 tained by the different experimenters when the rarefaction or 

 condensation was small; they do not differ more than might 

 have been expected from different forms of apparatus and in 

 different hands, and are not far from what is required in 

 Poisson's theory of sound. The results for greater rarefactions 

 were, however, in my experiments, very different from those cal- 



* Communicated by the Author. 



t Traite de Mecanique, vol. ii. p. 643. 



X Journal de Physique for November 1819. 



