34 HEAT. 



tains, it prevents the escape of the heat which is stored 

 in the earth from the previous summer. But for this 

 indirect warming effect of the snow, the cold of a sin- 

 gle winter would be sufficient to kill whole races of 

 plants. Thus, the cold of the winter weaves a garment 

 to protect the earth from its own influence. 

 How do the ^' BUILDING. In building, the same 

 principles of principles apply as in the case of clothing. 

 C piyinthtcas~e Bad conductors, when suitable in other 

 of buildings? respects, are the best materials for walls, 

 making a house cooler in summer and warmer in 

 winter. Wood and brick, for example, are in this 

 respect better than iron. They keep out the heat 

 in summer, and, though they have the same effect 

 to exclude the heat of the sun's rays in winter, they 

 more than make up for this by preventing the escape 

 of the larger quantity of heat produced by the fires in- 

 side. The inhabitants of the Arctic regions build their 

 winter huts of snow, and thus make practical use of 

 its low conducting power. Double doors and windows 

 have more than a double effect in preventing the escape 

 of heat in winter, because of the non-conducting wall 

 of air between them. 

 ,, . . , 66. REFRIGERATORS. These are double- 



What is the 



principle in- walled wooden box- 



es > sed to 



refrigerators ? articles of food from 



the heat of the summer. The 

 space between the double walls 

 and top is filled with pulverized 

 charcoal, which has in itself very little conducting 



