THE HUMAN BODY AS AN ENGINE. 495 



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 ex- 

 ternal work. 



"». The steam after expanding in the cylinders of the engine es- 

 capes 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 un- 

 burned 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 locomo- 

 tive 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 

 ami that of a human body. A parallel, indeed, between the operation 

 of a complex inanimate engine of iron and steel, and a still more com- 

 plex 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 ani- 

 mal 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 re- 

 ceptacle, like coal in the fire box of an engine, hut is digested, assimi- 

 lated 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 



