154 ANNUAL REPORT SMITHSONIAN" INSTITUTION, 193 3 



tance increases. Our experiments have proved it by observing the 

 change in solar radiation intensity between perihelion and aphelion 

 of the earth. 



TOTAL OUTPUT OF SOLAR RADIATION 



How much energy does the sun lose in this prodigal way ? Every 

 year it loses heat equivalent to 400 billions of trillions (4X10^^) of 

 tons of anthracite coal. Of this prodigious amount the earth inter- 

 cepts the equivalent of 200 trillions of tons. Compared merely with 

 the small part of solar radiation intercepted by the earth, the total 

 3'early consumption of coal for heat, light, and power in the United 

 States is trifling — only 500 millions of tons. 



SOLAR POWER 



This leads to the inquiry why we depend on coal, oil, and waterfalls 

 for power, when the sun is furnishing to the earth so incomparably 

 much more energy free, gratis, for nothing. There are two reasons. 

 First, clouds and night interrupt the continuity of the supply, and 

 cut it off just at those times when we most need power. Even dur- 

 ing the day the rays alter very much in their intensity because they 

 shine through great thicknesses of the atmosphere in the early morn- 

 ing and the late afternoon. Night also varies greatly in length be- 

 tween summer and winter in temperate zones where the most power 

 is used. Unfortunately, night is longest in winter, when we need 

 most power. Second, though sun rays cost nothing to produce, they 

 cost much to capture. If their energy could be fully utilized it would 

 require only about a square yard for 1 horsepower. But the losses 

 unavoidable in using sun rays for power will demand provision of 

 at least 4 and probably 10 square yards per horsepower. In fact, no 

 solar-power plants thus far have reached such efficiency. To install 

 apparatus over 10,000 square yards to obtain 1,000 horsepower, or 

 over 10,000,000 square yards to match Niagara may well give 

 engineers pause. 



Nevertheless, solar power may not be hopeless. Two of the most 

 successful solar-power installations made thus far are those of 

 Boyle and Willsie ^ at Needles, Calif., about 1908, and that of the 

 Sun Power Co. (Eastern Hemisphere) Ltd., at Meadi, near Cairo, in 

 Egypt, in the year 1913. These two installations illustrate two con- 

 trasting types. In the first the inventors avoided expensive equip- 

 ment by contenting themselves with a small temperature rise and 

 therefore with low efficiency. The heater consisted of a shallow 

 wooden basin coated with asphalt and divided by strips into troughs. 

 It was covered hj layers of window glass and insulated at the sides 



2 Engineering News, May 13, 1909. 



