36 TEMPERATURE AND HEAT 



ocean is called an ocean breeze. When the air over the ocean 

 is warmer than that over the land the same kind of air move- 

 ment is caused, but in the opposite direction. Since the air 

 now moves over the surface of the land toward the ocean, it is 

 called a land breeze (Figs. 21 and 22). 



Sources of heat. All of the heat which we enjoy and 

 use we owe to the sun. The wood which blazes on the hearth, 

 the coal which glows in the furnace, and the oil which burns 

 in the stove owe their existence to the sun. 



Without the warmth of the sun, seeds could not sprout and 

 develop into the mighty trees which yield firewood. Even 

 coal, which lies buried thousands of feet below the earth's 

 surface, owes its existence to the sun. Coal is simply buried 

 vegetation! Ages ago trees and bushes grew " thick and fast," 

 and the ground was always covered with a deep layer of decay- 

 ing vegetable matter. In time some of this vast supply sank 

 into the moist soil and became deeply covered with mud. 

 Constant pressure, moisture, and heat affected the underground 

 vegetable mass, and slowly changed it into coal. 



The buried forest and thickets were not all changed into 

 coal. Some were changed into oil and gas. Decaying ani- 

 mal matter was often mixed with the vegetable mass. When 

 the mingled animal and vegetable matter sank into moist 

 earth and came under the influence of pressure, it was slowly 

 changed into oil and gas. 



The heat of our bodies comes from the foods which we eat. 

 Fruits, grain, etc., could not grow without the warmth and 

 the light of the sun. The animals which supply our meats 

 likewise depend upon the sun for light and warmth. 



Therefore, whether heat comes directly from the sun and 

 warms the atmosphere or whether it comes from burning coal ; 

 wood, and oil, the sun is its great source. 



