HEAT 127 



become so hot that the packing in the boxing becomes 

 ignited. In machinery, oil is used in many places to 

 reduce the friction and prevent excessive heating. 



When a bullet is stopped by a steel plate, it becomes 

 hot. The motion has been changed to another form of 

 energy, namely, heat. It is the same with friction. 

 Friction retards the motion, and this loss in motion shows 

 up as heat. The greater the loss in motion or the greater 

 the friction, the greater the amount of heat developed. It 

 is said that man formerly obtained his fire by rubbing 

 two sticks together. We still obtain our fire by friction, 

 but the process has been very much simplified by the 

 substitution of other substances for one of the sticks, as 

 in the modern match. 



Compression. In the chapter which treated of the 

 making of artificial ice it was noted that the expanding 

 ammonia gas produced a temperature .cold enough to 

 freeze water. If the gas is compressed, an opposite ef- 

 fect is produced, the gas becoming much warmer. This 

 may be tested with a common bicycle pump by closing the 

 tube leading from the pump and repeatedly compressing 

 the air. Note the temperature of the barrel of the pump. 



Chemical Action. For many years man has used fire, 

 and in so doing he uses some of the energy that has been 

 stored up by the plants. The chief fuels are wood, coal, 

 natural gas, petroleum, and alcohol. When man causes 

 these fuels to unite with oxygen and burn, he is using some 

 of the energy of the sun which was stored up by the plants, 

 and in some cases by the animals. 



Sun. Practically all the heat of the earth's surface 

 comes from the sun. When we realize that we receive 

 only one two-billionth of the sun's heat and still have 

 enough to make the earth a very pleasant place in which 



