Page:Popular Science Monthly Volume 76.djvu/245

Rh 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 thermodynamic 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 (b). 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 temperature, 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 temperature of the exhaust to compensate for the conversion of the remainder into work.

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 involved in the operation of an engine. The boiler is at high temperature $$T_{1}$$ 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