72 HEAT 



But whether a material will be a conductor or a non- 

 conductor depends largely upon whether it is woven loose- 

 ly or tightly. For it is the air in the meshes of the cloth 

 that acts as the best non-conductor. Fur and feathers 

 are warm chiefly because they imprison so much air. 

 For the same reason moderately loose winter clothing is 

 warmer than tightly fitting clothing. 



76. Sources of Heat. The chief sources of heat are 

 the sun, body heat, the burning of fuels, friction and 

 collision, and electrical resistance. 



a. The Sun. The earth gets some heat from the 

 moon and stars and its own interior, but the amount is 

 small. Practically all the warmth of the earth's surface 

 and of its atmosphere comes from the sun. The sun is so 

 far away, and our earth is so small, that we get only a 

 very little of the heat sent forth by the sun, perhaps 

 1 part out of 2,000,000,000, yet this is enough to make 

 the earth fit for living things instead of a frozen, unin- 

 habited sphere. The gain and loss of heat in the temperate 

 and frigid zones cause the seasons of these zones, and the 

 cutting off of the sun's rays from any place brings its 

 night. 



6. Fuels. Man is the only animal that uses fire. Other 

 animals and the world of plants store up some of the energy 

 received from the sun, and man uses them for food and 

 fuel. In causing a fuel to unite with the oxygen of the 

 air, man is getting back some of the sun's energy. The 

 chief fuels are wood, coal, charcoal, coke, natural gas, 

 petroleum, and alcohol. Petroleum (cf. 121) is the 

 natural product out of which kerosene, gasoline, and par- 



