28 



OUR OWN WORLD 



of the year. The more nearly vertical the rays, the greater 

 the number that fall upon a given area, and the greater the 

 amount of heat received by that area. In January we 

 are closest to the sun, but its rays strike our hemisphere 

 more aslant and therefore fewer heat rays fall upon a given 

 area than in July. 



Experiment 5. Cut a hole 4 in. square in the center of a board 12 

 in. square. Fit tightly into this hole one end of a wooden tube 4 in. 

 square and 1 ft. long. Paint the inside and outside of the tube a dull 

 black. Hinge the opposite end of this tube 10 in. from the end of a 



baseboard 2 ft. long 

 and 16 in. wide, 

 having 6 in. of the 

 board on either side 

 of the tube. (Fig- 

 ure 9.) 



On a clear day 

 place this appara- 

 tus out of doors on 

 a table freely ex- 

 posed to the sun, 

 with a piece of 



paper on the baseboard under the end of the tube. Point the tube 

 directly at the sun in the early morning, in the middle of the fore- 

 noon, at noon, in the middle of the afternoon and about sunset. 

 Mark on the paper the amount of surface illuminated by the sun- 

 light passing through the tube at each of 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- 



FIGURE 9. APPARATUS FOR SHOWING THE 

 HEATING EFFECTS OF SUN'S RAYS 



