SECOND LAW OF THERMODYNAMICS 241 



men, and one of the most humorous of children's verses refers to the 



man whose wondrous wisdom enabled him to circumvent it by direct 



repair : 



There was a man in our town, 



And he was wondrous wise; 

 He jumped into a bramble bush 



And scratched out both his eyes. 

 And when he found his eyes were out, 



With all his might and main, 

 He jumped into another bush 



And scratched them in again. 



Let us return to the fourth statement (d) and consider with the 

 help of an example what is meant by compensation in its thermo- 

 dynamic sense. A gas can be transferred from a region of low pressure 

 to a region of high pressure by means of a pump, and the work that is 

 done in driving the pump, even supposing the pump to be frictionless, 

 is all converted into heat. This conversion of work into heat is the 

 necessary cost or compensation for the transfer of the gas from the low 

 pressure region to a high pressure region. 



Consider the second statement (&). In an artificial ice factory 

 heat is continually abstracted from the freezing-room and transferred 

 to the warm outside air; but to accomplish this result, even by an 

 ideally perfect frictionless mechanism, a certain amount of work is 

 required to drive the ammonia pump and this work is converted into 

 heat. This conversion of work into heat compensates for the transfer of 

 heat from the freezing-room to the warm region outside. 



Consider the third statement (c). In ordinary steam engines, 

 heat is converted into work, but to accomplish this transformation a 

 large quantity of heat must be supplied to the engine at high tem- 

 perature, and some of this heat (about nine tenths of it in the very 

 best of steam engines) must be let down, as it were, to the low tem- 

 perature of the exhaust to compensate for the conversion of the re- 

 mainder into work. 



Heat Engines 



An engine, or to be more specific, a heat engine is a machine for 

 converting heat into mechanical work. The engine is supplied with 

 heat at a high temperature, it transforms a portion of this heat into 

 work, and it delivers the remainder of the heat to a low temperature 

 region. Figure 1 is a diagram for fixing in the reader's mind the 

 various temperatures and quantities of heat and work which are in- 

 volved in the operation of an engine. The boiler is at high tempera- 

 ture T 1} and the condenser is at low temperature T 2 . When the engine 

 is operated for a time, a quantity of heat H r is taken from the boiler, 

 an amount of heat W is developed by the engine (in excess of the work 



VOL. LXXVI. — 17. 



