sible efficiency of an engine working between these two temperatures
would therefore be 671·4491·4671·4, 27 per cent, nearly.
Heat-engines are often spoken of as very inefficient machines, because they transform into mechanical effect but a small proportion of the heat used. The inefficiency is not so much the fault of the machine as of the conditions under which it is worked. Consider the case of a condensing engine with a boiler pressure of 45 pounds and a vacuum of 25 inches of mercury. The temperature of the source is here about 294° and of the refrigerator 140° Fahr. The possible efficiency under these conditions is about 20 per cent., that is, a perfect engine working between those temperatures could give in mechanical effect no more than one fifth the energy of the heat. The best steam-engines would, under these circumstances, give one-horse power for something less than two pounds of coal per hour. This is an efficiency of 10 or 12 per cent., or more than half the possible efficiency. The engine, as a machine, is not so very imperfect. In speaking of the engine, I of course include the boiler as a part of the machine. Any great improvement must come from an increased range of temperature between the source and refrigerator. The temperature of the refrigerator can not well be lower than the general temperature of surrounding objects, and there are great practical difficulties in the way of a very high temperature of the source. Suppose an engine could be worked with a source at a temperature of 1250° of the absolute scale, or nearly 800° Fahr., and a refrigerator at 500° of the absolute scale, or nearly 40° Fahr., the possible efficiency would be 15001250, or only 60 per cent. It appears, then, that there is not much hope that any large percentage of the energy of heat can, by any practical means, be converted into mechanical effect. But are we, for this reason, to continue wasting the energy of fuel as it is wasted now? Is there no other way in which the energy of chemical separation of carbon from oxygen can be converted into mechanical effect except by first converting it into heat? Why may not the union of carbon with oxygen be made to generate electric currents instead of heat? Electric energies have been made that convert into mechanical effect 60 to 70 per cent, of the energy of the electric current, and a much higher efficiency might, no doubt, be obtained. Already something has been done toward the generation of electric currents by the union of carbon and oxygen; but, so far, no means has been discovered by which such a union can be effected, except at a high temperature, and this involves a great waste of energy in the form of heat. A discovery that would enable us to convert the energy of fuel into electric currents directly and completely would revolutionize, not only the methods of obtaining power, but the methods of obtaining light and distributing heat as well. I have shown elsewhere that, if a Brayton oil-engine is used to
