274 On the Effect of Fluid Friction in di-ying Steam. 



of any fluid through a small orifice, is given in a paper commu- 

 nicated last April to the Royal Society of Edinburgh, and since 

 published in the Ti'ansactions (vol. xx. part II.) under the title 

 " On a Method of discovering Experimentally the Relation be- 

 tween the Mechanical Work spent and the Heat produced by 

 the compression of a gaseous fluid/^ 



I take the present opportunity of correcting a mistaken ex- 

 pression in my first communication regarding steam issuing 

 from a high-pressure boiler, by which I gave a false, or an in- 

 adequate, representation of the connexion of that application of 

 Mr. Joule^s general principles which I was bringing forward, 

 with one which he had himself made in one of his published 

 papers. The following is the passage of my communication 

 (addressed as a letter to Mr. Joule), which requires correction : — 



'^ The pretended explanation of a corresponding circumstance 

 connected with the rushing of air from one vessel to another in 

 Gay-Lussac's experiment, on which you have commented, is cer- 

 tainly not applicable in this case, since, instead of receiving 

 heat from without, the steam must lose a little in passing through 

 the stop-cock or steam-pipe by external radiation and convec- 

 tion*." I wrote this under the impression that Mr. Joule had, 

 in his paper '* On the Changes of Temperature produced by the 

 Condensation and Rai*efaction of Airf/^ pointed out the incor- 

 rectness of an explanation often given of Gay-Lussac's experi- 

 ment J, and shown that the phsenomenon could be truly explained 

 only by taking into account the heat developed in the air by 

 friction in its passage from one vessel to the other through the 

 stop -cock. I find, however, on looking to the paper, which I 

 had not by me when I wrote, that it contains no reference to 

 Gay-Lussac's experiment, but the following passage, referring 

 to Mr. Joule's own experiments on the heat developed by the 

 compression of air, and the heat absorbed by air allowed to 

 expand from a vessel into which it has been compressed, through 

 a small orifice, into the atmosphere, from which I obtained the 

 idea of considering the heat developed by the friction of steam 

 issuing from a high-pressure boiler. 



" It is quite evident that the reason why the cold in the ex- 

 periments of Table IV. was so much inferior in quantity to the 

 heat evolved in those of Table I., is, that all the force of the air, 

 over and above that employed in lifting the atmosphere, was 

 applied in overcoming the resistance of the stop-cock, and was 

 there converted back again into its equivalent of heat§." '"^^ 



Ardmillan, Ayrshire, Sept. 4, 1851. 



* Phil. Mag. S. 3. vol5:^!!Cdirp. 388. 



t Ibid. vol. xxvi. p. 369 (May 1845). 



X See Lara^, Cours de Physique, vol. i. § 352. 



§ Phil. Mag., S. 3. vol. xxvi. p. 381. 



