474 On the Effect of Fluid Friction in drying Steam. 



Fui'thpr, by applying the same procedure to heat proceeding 

 from different sources, we shall have a new method of ascertain- 

 ing whether the differences in the reflexion of those rays from 

 the same metallic mirror are, or are not, a consequence of a cor- 

 responding difference in the lengths of the imdulations. 



The authors have also proved, that the relative quantities of 

 solar heat reflected from different mirrors, both glass and me- 

 tallic, correspond with the relative quantities of solar light re- 

 flected from the same mirrors ; and that the formul?e developed 

 by Fresuel for glass, and by M. Cauchy for metallic mirrors> 

 apply equally to the calorific and to the luminous phsenomena. 



LXV. Note on the Effect of Fluid Friction in drying Steam which 

 issues from a High-pressure Boiler through a small Orifice into 

 the open Air. By Prof. W. Thomson*. 



IN a letter to Mr. Joule wi-itten last October, and since pub- 

 lished in the Philosophical Magazine t, I pointed out that 

 the remarkable discovery (made independently by Messrs. Ran- 

 kine and Clausius), that steam allowed to expand requires heat 

 to be added to it to prevent any part of it from becoming lique- 

 fied, can only be reconciled with the known fact of the dryness 

 of steam issuing into the open air from a high-pressure boiler 

 through a small aperture, by taking into account the heat deve- 

 loped by the fluid friction in the neighbourhood of the aperture. 

 I may add, that the immediate mechanical effect of the work 

 done by the steam in pressing out, is the generation of vis viva 

 in the fluid ; and all of this vis viva, except the very small pro- 

 portion of it retained by the steam after leaving the rapids, is 

 lost in friction before the fluid reaches the locality where its 

 pressure is equal sensibly to that of the atmosphere ; that is, is 

 converted into thermal vis viva or heat. M. Clausius |, in an in- 

 vestigation of the circumstances of this case of expanding steam, 

 points out distinctly that work is done, but overlooks the me- 

 chanical effect produced ; and in consequence arrives at the con- 

 clusion that my proposition, that the steam when it has ex- 

 panded till its pressure is equal to that of the atmosphere could 

 not be dry without the heat, or some of the heat, it gets from 

 fluid friction, is false. These remarks are, I trust, sufficient to 

 show that M.Clausius's objections to my reasoning are groundless. 

 London, May 10, 1851. 



* Communicated by the Author. 

 t Vol. xxxvii. p. 387, December 1850. 



X In an article recently published in Poggendorff's Annalen, and repub- 

 lished in the last Number (May) of the Philosophical Magazine. 



