THE 

 LONDON. EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



SUPPLEMENT to VOL. IV. FOURTH SERIES. 



LXXVI. On the Thermal Effects experienced by Air in rushing 

 through small Apertures. By J. P. Joule and W. Thomson*. 



THE hypothesis that the heat evolved from air compressed 

 and kept at a constant temperature is mechanically equi- 

 valent to the work spent in effecting the compression, assumed 

 by Mayer as the foundation for an estimate of the numerical 

 relation between quantities of heat and mechanical work, and 

 adopted by Holtzmann, Clausius, and other wi'iters, was made 

 the subject of an experimental research by Mr. Joulef, and verified 

 as at least approximately true for air at ordinary atmospheric 

 temperatures. A theoretical investigation, founded on a conclu- 

 sion of Caruot's J, which requires no modification § in the dyna- 

 mical theory of heat, also leads to a verification of Mayer's hypo- 

 thesis within limits of accuracy as close as those which can be 

 atti-ibuted to Mr. Joule's experimental tests. But the same 

 investigation establishes the conclusion, that that hypothesis can- 

 not be rigorously true except for one definite temperature within 

 the range of Regnault's experiments on the pressure and latent 

 heat of saturated aqueous vapour, unless the density of the vapour 

 both differs considerably at the temperature 100° Cent, from 

 what it is usually supposed to be, and for other temperatures 

 and pressures presents great discrepancies from the gaseous laws. 

 No experiments, however, which have yet been pubhshed on the 

 density of saturated aqueous vapour are of sufficient accuracy 

 to admit of an unconditional statement of the indications of 

 theory regarding the truth of Mayer's hypothesis, which can- 

 not therefore be considered to have been hitherto sufficiently 



* Communicated by the Authors ; having been read to the British Asso- 

 ciation at Belfast, Sept. 3, 1852. 



t Phil. Mag. May 1845, j). 375, " On the Changes of Temperatiue 

 produced by the Rarefaction and Condensation of Air." 



X Transactions of the Royal Society of Edinbiu-gh (April 1849), vol. xvi. 

 part 5, " Ap])endix to Account of Caniot's Theory," §§ 4()-51. 



§ Trans. Royal Soc. Ediiib. (March 1851) vol. xx. part 2; or Phil. Mag. 

 S. 4. vol. iv. " On the Dynamical Theory of Ileat," § 30. 



Phil. Mag. S. 4. No. 28. Supjd. Vol. 4. 2 1 



