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X. On the Thermo-dynamic Theory of Steam-engines with dry saturated Steam, and 
its application to practice. By William John Macquoen Eankine, C.E., LL.I)., 
F.R.SS.L. & E., Pres. Inst. Eng. Scot., Regius Professor of Civil Engineering and 
Mechanics in the University and College of Glasgoiv. 
Eeceived December 27, 1858,— Bead January 27, 1859. 
Introduction. 
It was demonstrated independently from the laws of Thermo-dynamics, by Professor 
Clausius and the author of this paper, in 1849*, that when steam or other saturated 
vapour in expanding performs work by driving a piston, and receives no heat fiom with- 
out during that expansion, a portion of it must be liquefied. 
That theoretical conclusion has since been amply confirmed by experience in actual 
steam-engines ; for it has been ascertained that the greater part of the liquid water 
which collects in unjacketed cylinders, and which was once supposed to be wholly 
carried over in the liquid state from the boiler (a phenomenon called “ priming ”), is 
produced by liquefaction of part of the steam during its expansion ; and also that the 
principal effect of the ^^jacTcet,'' or annular casing enveloping the cylinder, filled with 
hot steam from the boiler, which was one of the inventions of Watt, is to prevent that 
liquefaction of the steam in the cylinder. 
That liquefaction does not, when it first takes place, directly constitute a waste of 
heat or of energy ; for it is accompanied by a corresponding performance of work. It 
does, however, afterwards by an indmect process, diminish the efficiency of the engine ; 
for the water which becomes liquid in the cyhnder, probably in the form of mist and 
spray, acts as a distributer of heat and equalizer of temperature, abstracting heat from 
the hot and dense steam during its admission into the cylinder, communicating that heat 
to the cool and rarefied steam which is on the point of being discharged, and thus lower- 
ing the initial pressure and increasing the final pressure of the steam ; but loweiing the 
initial pressure much more than the final pressure is increased. Accordingly, in all 
cases in which steam is expanded from a high down to a low pressure, it has in practice 
been found necessary to envelope the cylinder in a steam-jacket 'j'. The liquefaction 
which would otherwise have taken place in the cylinder, takes place in the jacket 
instead, where the presence of the liquid water produces no bad effects, and that watei 
is returned to the boiler. 
In double-cylinder engines, where the expansion of the steam begins in a smallei 
* Poggendoeit’s ‘ Amialen,’ 1850; Edinburgb Transactions, vol. xx. 
t Unless the steam is superheated (Sept. 1859). 
2 B 
MDCCCLIX. 
