576 report— 1884. 



The thermodynamic theory of the steam-engine stands, to-day, sub- 

 stantially as it was left by Clausius and Rankine at the close of their work 

 in this field, in the decade 1850 to 1860. Many treatises have been 

 published, some of them by men of exceptional ability ; but all have 

 followed the general line first drawn by these masters, and have only now 

 and then found some minor point to develope. Rankine's ' Steam-engine 

 and other Prime Movers,' written a quarter of a century ago, is still a 

 standard work on the theory of the heat-engines, and is still used as a 

 text-book in engineering schools in this country and Europe. 



The limitations of the thermodynamic theory of the heat-engines, 

 and of its application in the design and operation of such engines, were 

 first discovered by James Watt a hundred years ago and more. They 

 were systematically and experimentally investigated by Isherwood in 

 1855 to 1865, were observed and correctly interpreted by Clark in 1855 

 and earlier, and were revealed again by the experiments of Hirn, and by 

 those of Emery and many other recent investigators on both sides of the 

 Atlantic. These limitations are due to the fact that losses occur in the 

 operation of steam-engines which are not taken into account by the 

 hitherto accepted theory of the engine, and have no place in the ther- 

 modynamic treatment of the case. 



It is generally assumed, in the usual theoiy of the engine, that the 

 expansion of the working fluid takes place in a cylinder having walls 

 impermeable to heat, and in which no losses by conduction or radiation, 

 or by leakage, can occur. Of those losses which actually take place 

 in the real engine, that due to leakage may be prevented, or, if occur- 

 ring, can be checked ; but it is impossible, so far as is now known, to 

 secure a working cylinder of perfectly non-conducting material. The 

 consequence is that, since the steam or other working fluid enters at a 

 high temperature and is discharged at a comparatively low tempera- 

 ture, the surfaces of cylinder, cylinder heads, and piston are at one in- 

 stant charged with heat of high temperature, and at the next moment, 

 exposed to lower temperatures, are drained of their surplus heat, which 

 heat is then rejected from the cylinder and wasted. Thus, at each stroke, 

 the metal surfaces, exposed to the action of the expanding substance, 

 alternately absorb heat from it, and surrender that heat to the ' exhaust.' 

 In the case of the gas-engines, this waste is rendered enormously greater 

 by the action of the water-jacket, which is there needed to keep the 

 cylinder down to a safe temperature, and which takes away, in the circu- 

 lating stream of cooling water, an immense amount — usually about one- 

 half — of the heat received from the burning gas. In the steam-engine, 

 the loss by the method here referred to is rarely less than one-fourth in 

 unjacketed cylinders, and is often more than equal to the whole quantity 

 of heat transformed into mechanical erergy. The amount of this loss 

 increases with wet steam, and is diminished by any expedient, as steam- 

 jacketing or superheating, which prevents the introduction or the pro- 

 duction of moisture in the midst of the mass of steam in the cylinder. 

 As the range of temperature worked through in the engine increases as 

 the quantity of steam worked per stroke diminishes, and as the time 

 allowed for transfer of heat to and from the sides and ends of the cylinder 

 and the piston is increased, the magnitude of this loss increases. Hence 

 the use of high steam, of a high ratio of expansion, and of low piston 

 speed tend to increase the amount of this waste ; while low steam, a low 

 ratio of expansion, and high engine speed, tend to diminish it. These 



