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 use- 

 ful 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 sud- 

 denly 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 be- 

 comes brilliantly luminous, and we call it a shooting star. The idea of 

 heat due to frietional resistance is common enough. The exact equiva- 

 lence 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 en- 

 ergy. 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, ^\\w 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 locomo- 

 tive 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 



