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Page:Popular Science Monthly Volume 57.djvu/504

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494
POPULAR SCIENCE MONTHLY.

passing through a coil of wire or the filament of a lamp gives up its energy to produce heat and light. The last form of this energy is equal in quantity to the first.

Niagara represents a vast store of energy. Millions of tons of water falling 160 feet could do a vast amount of mechanical work if properly applied through water wheels. More than 50,000 horse power of useful work is actually derived from Niagara's waters, but this is only a small fraction of the total. The energy is, however, given up in falling, even though no useful work is done. In fact, the water is slightly heated by the impact, and the amount of heat produced is exactly equivalent to the mechanical energy lost by the water.

A cannon hall receives a large amount of kinetic energy from the exploded powder as it leaves the muzzle of a great gun. If it be suddenly stopped by a rigid target its mechanical or mass energy is at once converted into heat; that is, into the vibratory motion of the molecules. Ball and target are highly heated. Indeed, lead bullets are often melted by the heat of impact. Meteors living through space come into our atmosphere and their speed is checked by its resistance. Tart or all of their kinetic energy is thus converted into heat. Both air and meteor are heated; heated to so high a temperature that the meteor becomes brilliantly luminous, and we call it a shooting star. The idea of heat due to frictional resistance is common enough. The exact equivalence between the mechanical energy lost and the heat produced is the thing to he especially noticed here.

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 he burned possesses a definite amount of energy. Let us say every pound has one unit of energy, and suppose 5,000 pound 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 he 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 he 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, hut 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