626 



about 10 feet ; for which reason I have employed in the synopsis A, 

 now forwarded, the same values as before, correcting only the ob- 

 vious errors to which I have drawn attention in the second para- 

 graph of this letter ; at the same time I have endeavoured to put 

 within the reach of those who interest themselves in the problem of 

 the figure of our planet, all the data at my command, and if any 

 fresh light should hereafter be thrown on the subject, shall be very 



happy to communicate it. 



GEORGE EVEREST. 



IV. " On the Thermodynamic Theory of Steam-engines with 

 dry saturated Steam, and its application to practice." By 

 W. J.MACQUORN RANKINE, C.E., LL.D., F.R.S.S.L. &E., 

 Pres. Inst. Eng. Scot., Regius Professor of Civil Engineering 

 and Mechanics in the University and College of Glasgow. 

 Received December 27, 1858. 



(Abstract.) 



In 1849 it was demonstrated, contemporaneously and independ- 

 ently, by Professor Clausius and the author of this paper, from the 

 laws of thermodynamics, that when steam or other saturated vapour 

 in expanding performs work, and receives no heat from without, a 

 portion of it must be liquefied. 



That theoretical conclusion has since been confirmed by practical 

 experience. 



The principal effect of the " steam-jacket" invented by Watt is to 

 prevent that liquefaction. 



The presence of liquid water in any considerable quantity in the 

 cylinder of a steam-engine acts injuriously, by taking heat from the 

 steam while it is being admitted, and giving out that heat to the 

 steam which is about to be discharged. Most of the heat so trans- 

 ferred is wasted. 



The only exact thermodynamic formulae for the work of steam 

 hitherto published (by the author in the Phil. Trans. 1854, and by 

 Professor Clausius in Poggendorff's Annalen,' 1856), are adapted 

 to steam which receives no heat in expanding. 



The present paper, after recapitulating the general equation of 

 thermodynamics, and the special formulae for the pressure, volume, 



