Solar LJncnjy. 23 



But small as this amount of energy is when compared with 

 that leaving a like area at the sun's surface it is neverthe- 

 less very large. 



It may seem strange that so much energy falling upon 

 the earth does not keep its surface at a higher temperature 

 than is ohserved, but when it is stated that the temperature 

 of the space which surrounds the earth outside its atmos- 

 phere is 273 C. and that only the thin atmosphere 

 shields the surface from this intense cold, it is plain that a 

 large amount of heat must be required to hold the mean 

 temperature even as high as 45 F. which is 



273 + 7 = 280 C above absolute zero. 



If we add to the necessity of holding the earth's surf ace 'at 

 a temperature 280 C. to 300 C. above the space in which 

 it moves, the demand for energy needed to maintain the 

 movements of water and of winds, together with that em- 

 bodied in activities of animal and plant life, then 1.3 horse 

 power per square yard cf surface does not appear so much 

 too large. 



28. Kinds of Ether Waves. The energy reaching the 

 earth from the sun in the form of wave motion is not all 

 alike in that the waves have different lengths, or, what is 

 the same thing, greater numbers of one kind reach the 

 earth in a unit of time. Waves which are so frequent that 

 from 392 to 757 billions reach us per second produce the 

 sensation of light when falling upon the eye; the slower 

 ones producing red light and the more rapid ones the ex- 

 treme violet colors of the rainbow. Associated with these 

 color waves there are many other dark waves to which the 

 human eye is not sensitive. Some of these are much 

 shorter than the color waves and are especially powerful 

 in breaking down the molecular structure of different sub- 

 stances ; that is, in producing chemical changes such as oc- 

 cur on the photographer's plate when the negative is made 

 and such as take place in the green parts of plants when car- 



