20 W. J. M. Rankine on the Means of 



arrangement of the series of changes of volume and pressure 

 undergone by the working substance during the stroke. The 

 fourth and fifth causes of waste are often connected with each 

 other. 



13. Application to the Steam-Engine . — In the steam- 

 engine the first cause of waste of heat exists when the boiler 

 presents an insufficient surface to the products of combustion, 

 and may be considered to be almost completely removed in 

 tubular boilers of the best construction when properly worked. 

 It is well known that, with such boilers, the consumption of 

 fuel per horse-power per hour is about one-fifth of what it has 

 in some instances been ascertained to be where boilers of in- 

 sufficient surface have been employed. The second cause of 

 waste exists where the condensation is imperfect. The third 

 cause of waste, where the cylinder, steam-passages, and boiler 

 are exposed to the loss of heat by conduction and radiation. 



As the means of indefinitely diminishing the waste of heat 

 in steam-engines from those three causes are already to a great 

 extent practised, it is unnecessary here to refer to them 

 farther. 



14. Action of the Steam in a perfect Steam-Engine. — To 

 understand the mode of operation of the fourth and fifth 

 causes of waste in the steam-engine, let us consider what the 

 action of the steam in a theoretically perfect engine ought to 

 be. We shall commence with the process B of the cycle con- 

 stituting the stroke, described in article 6. An assigned por- 

 tion of water being at the required temperature of evaporation, 

 is converted into steam at that temperature, and at a pressure 

 depending on that temperature, by the expenditure of a cer- 

 tain amount of heat, called the latent heat of evaporation, 

 which also depends on the temperature. The steam being ad- 

 mitted to the cylinder, propels the piston before it; and when 

 the assigned portion of water has been thus admitted in the 

 form of steam, the communication with the boiler is shut. 

 This completes the process B. 



The steam now, without receiving or discharging any heat, 

 expands : during this expansion its temperature falls by the 

 conversion of heat into mechanical power ; the pressure, of 

 course, diminishes at the same time : this expansion ceases 



