132 FIRST YEAR SCIENCE 



these different times. Why are different amounts of surface covered 

 at these different times? 



Place a thermometer in the centers of the surfaces covered by the 

 sunlight passing through the tube at these different times. Note the 

 different readings of the thermometer. Can you suggest a reason why 

 they are not alike ? The opening exposed to the rays has been the 

 same throughout the experiment. Draw diagrams illustrating the 

 action of the sun's rays in the different positions. 



The number of rays of the sun which fall upon a given 

 area depends upon the angle at which they strike the sur- 

 face. Figure 65 shows that the same number of rays fall 



Fig. 65. 



upon a much smaller surface when the direction of the sun 

 is vertical than when it is nearly horizontal. In the 30- 

 degree arcs there are 2 J, 7, and 9 J ray spaces respectively. 

 The sun is here considered to be vertical at the equator, 

 as it is on March 20 and September 23. Thus on these 

 days, other conditions being the same, about one fourth 

 as much heat from the sun falls upon the 30 about the 

 pole as upon the 30 north of the equator. 



The latitude of a place has much to do with the amount 

 of heat that it receives. As the sun becomes vertical to 



