Let us now take as a final example a locomotive engine. It takes on a store of fuel and water and, directed by its engineer, sets out for a day’s duty. The coal to be burned possesses a definite amount of energy. Let us say every pound has one unit of energy, and suppose 5,000 pounds of coal are taken. What becomes of these 5,000 units of energy, appearing as heat when the coal is burned? 1. A large amount of heat is required to keep the boiler and engine hot, due to the loss of heat to the atmosphere. The engine cylinders, as well as fire box and boiler, must be kept very hot; other parts of the engine become more or less heated. All parts therefore continually give off heat, and a large part of the heat produced by the burning coal is thus expended. 2. A second portion is expended in doing work. If our locomotive hauls a 500-ton train up a one-per cent. grade for 100 miles it would be doing 2,640,000 foot-tons of work in addition to that required to overcome the friction of the rails and the resistance of the atmosphere. This would require nearly 500 units of energy which would come from the heat of the coal. The work is done through the agency of steam, but the energy of the steam comes from the burning coal. A small amount of work is also done in pumping water from the tank on the 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 heat. 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 banked 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.