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Page:Popular Science Monthly Volume 57.djvu/505

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THE HUMAN BODY AS AN ENGINE.

tender into the boiler and in pumping air into the reservoir for the use of the air brakes. This may be called the internal work of the engine. A second portion of the heat is therefore expended in internal and external work.

3. The steam after expanding in the cylinders of the engine escapes into the atmosphere. Although it has been cooled somewhat by expansion, it is still hot, and carries a large amount of heat away with it. Moreover, the smoke and hot air which pass out through the smokestack carry away a large quantity of heat. Hot ashes Likewise carry away heal. Hence a third portion of heat is lost through smoke and steam and ashes. And this is the largest portion of the total quantity of heat generated by the burning coal.

When coal is burned, oxygen of the air unites chemically with the carbon and hydrogen of the coal to form carbonic acid, or carbon dioxid, as it is technically called, and water vapor. The incombustible mineral matter of the coal remains as ashes. Hence smoke contains carbonic acid gas and water vapor in addition to fine particles of unburned coal carried away in the draft of air.

When the grade is steep a great deal of work must be done by the locomotive, much steam is required, and the quantity of fuel burned is large in proportion. When the road is level fuel burns less rapidly, and when the train stops, still more slowly. At night the locomotive rests, fires are hanked and combustion is very slow. This process so briefly and incompletely sketched, is more interesting as one examines it closer, and a locomotive seems almost living when one considers minutely its wonderful performance.

But interesting and instructive though the operation of the locomotive may he, it is not for its own sake that E have mentioned it. It is rather in order to point out a remarkable parallel between its operation and that of a human body. A parallel, indeed, between the operation of a complex inanimate engine of iron and steel, and a still more complex living engine of flesh and hone and blood: both obeying the law of the conservation of energy, as well as the other laws of physics and chemistry.

Consider now a human body as a living engine. That man is more than matter is. of course, conceded. But we here regard only the animal body, guided by the brain as its engineer. The day begins, as with the locomotive, by taking a store of fuel and water, namely, food and drink. Food is not. however, burned in the body in a confined receptacle, like coal in the fire box of an engine, hut is digested, assimilated and distributed through the body by means of the circulating blood. And while some of it goes to repair bodily waste, becoming tissue, other portions are oxidized or burned to produce heat. Non-digestible parts of the food pass away from the body as refuse, like ashes