20 ELEMENTS OF GENERAL SCIENCE 



outside air and is in turn forced upward. The movement of 

 the air is therefore continuous. 



18. Air currents in refrigerators. Some of the most impor- 

 tant devices of modern times relate to preservation of food 

 by use of low temperatures, a subject which is later discussed 

 in connection with dangers of destruction of surplus foods. 

 Some of the principles of refrigeration are shown in figure 18, 

 this topic being more extensively discussed later in this book. 

 If in your shop work you will build an ice box or refrigerator 

 by use of the principles shown in the illustration, you will 

 probably find the experience of value to you during the rest 

 of your life. 



19. Importance of air temperature. The temperature of the 

 air is a matter of importance to us all. It has much influence 

 in determining whether our sports shall be tennis, golf, and 

 baseball or coasting and snowballing ; it decides for us whether 

 the water shall invite us to swim or to skate ; it even settles 

 pretty largely the kind of clothing we shall wear, the food we 

 shall have to eat, and the social activities in which we shall 

 engage. It levies a tax upon us for coal at one time and for 

 ice and electric fans at another. Our whole round of activi- 

 ties in business and pleasure is very largely regulated by the 

 temperature of the air and the seasonal changes. The more 

 fully we understand air currents and the temperature of air 

 and other substances about us, the more fully we shall be 

 able to regulate these things to suit our needs. 



