142 ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 1915. 



62^ pounds, and hence an average cubic foot of the sun weighs 86^ 

 pounds, while an average cubic foot of the earth weighs 345 pounds. 

 For comparison it may be mentioned that a cubic foot of granite 

 weighs 165 pounds. The density of the sun being so small, it is 

 concluded that it can still go on contracting, and hence that it is 

 probably getting hotter instead of cooler, as is popularly supposed. 

 If this be so, it is a hopeful feature for future workers in the field of 

 solar energy. 



The diameter of the sun is 863,600 miles, or about one hundred 

 times the diameter of the earth, and an earthly pound weight at its 

 surface would Aveigh 27^ pounds. The glowing surface which the 

 sun presents to us, even considering him as a flat disk, has the enor- 

 mous area of 585,750,000,000 square miles, each square foot of which 

 emits the enormous amount of about 12,500 horsepower, and the 

 radiant energy received at the outer surface of the earth's atmosphere 

 is equivalent to 7,300 horsepower per acre. Of this about 70 per 

 cent (say, 5,000 horsepower per acre) is transmitted to the land sur- 

 face of the earth at noon on a clear day, and less in the morning and 

 evening, owing to the greater thickness of atmosphere through which 

 the radiation has to pass. 



The quantity of solar heat per unit area which arrives in unit time 

 at the outer surface of our atmosphere is called the solar constant, 

 and its value, as determined in 1913 by C. G. Abbot, of the Smith- 

 sonian Institution, after making 696 experiments in different parts 

 of the globe, is 1.93 calories per square centimeter per minute (equal to 

 7.12 B. t. u. per square foot per minute). Its value given by various 

 experimenters between 1881 and 1909 was considerably higher, and 

 this makes it all the more remarkable that John Ericsson, the engineer 

 and inventor, who spent some £20,000 on experiments with solar 

 energy, when writing in 1876 a record of his life's work, gave the 

 value of the solar constant as 7.11 B. t. u. per square foot per minute 

 and said, " In view of the completeness of the means adopted in 

 measuring the energy developed and the ample time which has been 

 devoted to the determination of the maximum intensity, it is not 

 probable that future labors will change the result of our determina- 

 tion," and, as-shown above, his confidence was justified. 



Perhaps the most remarkable things about solar radiation are that 

 it passes through the 93,000,000 miles (1,000,000 is 2,740 a day for a 

 year) of space between the sun and the earth, the temperature of 

 which is nearly absolute zero (i. e., it is about —263° C), and that 

 only three-fifths of it produces any impression on the eye. It is not 

 till the radiant energy impinges on some material body that it is con- 

 verted into heat. The best body for causing such conversion is a dead- 

 black one. 



