592 A century's progress of the steam engine. 



The problem of the engineer engaged in the perfection of the steam 

 engine may perhaps he accurately and concisely stated thus: 



The conditions of the case as affecting the ideal, purely thermo- 

 dynamic machine l)eing known and exactly specified, to produce a 

 real engine of similar cycle, free, to the greatest extent practical )le, 

 from the defects of cycle and from the extra thermodynamic wastes 

 which characterize all real engines in higher or lower degree. 



The ideal engine would be a purcdy thermodynamic machine, in the 

 sense that its only wastes would be such as would occur in a steam 

 cylinder constructed of a perfectly nonconducting material; it would 

 not waste heat by conduction or radiation or hy transformation into 

 useless work. The solution of the problem thus obviousl}^ involves 

 simply the adjustment of a valve-gear fei such manner as to secure the 

 proper form of cycle, as a geometric figure, and the provision of either 

 a nonconducting cylinder, of a nonconducting working fiuid, or both; 

 or, in case neither of these equivalents can be secured, such approxi- 

 mation to these ideal conditions through such othcn" expedients as will 

 insure the best possible approximation to the ideal. Superheating, 

 compounding, and the employment of high-speed engines are simply 

 such expedients, while the increasing of steam pressures and ratios of 

 expansion, and the adoption of condensation and of other plans for 

 reduction of back pressure, are expedients for increasing the ideal 

 efficiency of the engine. 



It will be interesting to look back over the century just closing and 

 to observe to what extent the adoption of now familiar plans for 

 improving the performance of the steam engine during the period of 

 its existence — practicallj^ coincident in its working life with the nine- 

 teenth century — have had the desired result, and how far efficiencies, 

 duties, and thermodynamic opei'ations have been approximated to the 

 figures for the ideal, thermodynamic, machine. The principal direc- 

 tions of general progress have been toward higher engine speed, 

 toward higher steam pressures and correspondingly increased ratios 

 of total expansion, decreased back pressures, superheating and coin- 

 pounding, and the use of improved forms of valves and valve gearing. 



Increasing engine speed secures greater immunity from losses by 

 conduction and radiation, Avithin and without, by simply securing a 

 larger amount of work and the use of more steam in the unit of time 

 with a given C3dinder volume, thus reducing the waste per unit of 

 weight of steam and of useful work to a lower magnitude. Dou))ling 

 the speed of engine approximately reduces the waste percentages in 

 proportion to the difference in the squares of the two speeds, while it 

 gives, other things being equal, double the power, thus also reducing 

 costs of construction for stated powers of engine. In the engines of 

 Watt, the steam pumping engine — or the Cornish engine, as it came to 

 be called — was more economical than the same size of rotative engine 



