HEAT. 



17. The Heat we Receive from the Sun. 



The heat which we receive from the sun is necessary 

 for animal life and for plant life. As you know, it is 

 useless to plant seeds in very cold weather, and animals 

 often die when they do not have a warm place in which 

 to live. (Later we shall learn more about the effect of 

 heat upon plants.) 



Just as the amount of light which we receive from 

 the sun varies with the different seasons, so the amount 

 of heat which we also receive from the sun changes from 

 hour to hour and from season to season. When is the 

 warmest part of the day? When is the coolest part? 

 Look back at Section 10 and try to explain why this is 

 so. When is the warmest part of the year? When is 

 the coldest part? Perhaps you may obtain a suggestion 

 from Section 9. The next experiment will show why 

 the heat from the sun varies as it does. 



Experiment 21. *The Varying Heat from the Sun. 



Apparatus: Scissors, protractor. 



Materials: Cardboard 12"x8", paper staple. 



a. Make the apparatus as shown in the illustration. 

 The radius of the quarter circle should be five inches. 

 The divisions numbered 1 to 44 should be one-fourth 

 inch, and the movable strip should be one inch wide. One 

 inch of the end of the strip should be bent up and a quar- 



* See Section 62 before performing this experiment. 



