HOW LIGHT AND HEAT TRAVEL 



145 



While the amount of energy received daily from the sun 

 by the earth is actually enormous, it is small in comparison 

 with the whole amount given out by the sun to the numerous 

 heavenly bodies which make up the universe. In fact, of the 

 entire outflow of heat and light, the earth receives only one 

 part in two thousand million, and this is a very small portion 

 indeed. 



139. How Light and Heat Travel from the Sun to Us. 

 Astronomers tell us that the sun the chief source of heat and 

 light -is 93,000,000 miles away from us ; that is, so far distant 

 that the fastest express train would require about 1 76 years to 

 reach the sun. How do heat and light travel through this 

 vast abyss of space ? 



A quiet pool and a pebble will help to make it clear to us. 

 If we throw a pebble into a quiet pool (Fig. 90), waves orrip- 



FIG. 90. Waves formed by a pebble. 



pies form and spread out in all directions, gradually dying out 

 as they become more and more distant from the pebble. It 

 is a strange fact that while we see the ripple moving farther 

 and farther away, the particles of water are themselves not 

 moving outward and away, but are merely bobbing up and 

 down, or are vibrating. If you wish to be sure of this, throw 

 the pebble near a spot where a chip lies quiet on the smooth 

 pond. After the waves form, the chip rides up and down 

 with the water, but does not move outward ; if the water it- 



CL. GEN. SCI. 10 



