SOLAK EADIATION ABBOT 109 



The measurini^ instruments of Pouillet, Herschel, Forbes, and 

 others exposed surfaces blackened with lampblack. This substance 

 is not quite " absolutely black," for it reflects away about 3 per cent 

 of ordinary sun rays. Earchhoff, about 70 years ago, proved that 

 a closed chamber, whether blackened or not, must be a perfect ab- 

 sorber. However small the percentage of rays absorbed at one 

 impact, there is no escape, and the rays must be reflected hither and 

 thither until by innumerable impacts tlieir intensity is reduced below 

 any assignable minimum. This principle of the " absolutely black " 

 chamber has been incorporated into the Smithsonian water-flow 

 pyrheliometer. This instrument thus far is the world's standard for 

 measuring solar radiation. 



Assuming, therefore, that we have accomplished the complete 

 absorption of the solar ray, and its entire transformation into heat, 

 and have devised means for the exact measurement of the heat thus 

 produced, we then may express the intensity of solar radiation at the 

 earth's surface. We are accustomed to express it in terms of the 

 amount of radiation absorbed upon a square centimeter of surface 

 in a minute of time. We measure the heat produced in calories. 

 In these terms we find the heat of solar radiation near noon on clear 

 days to be approximately 1.4 calories per square centimeter per 

 minute. To state the matter in more common terms, the solar 

 radiation, if transformed completely into work, would produce 

 roughly a horsepower per square yard whenever the sun is high in 

 tlie sky. Thus even at 93,000,000 miles, the sun rays are tremen- 

 dously powerful. The sun itself sends them out in every direction 

 eontinuall}^ This output equals the heat of the burning of 400,000,- 

 000,000,000,000,000,000 tons of anthracite coal per year. 



Measurements of solar radiation at the earth's surface are subject 

 to losses by absorption and scattering of the rays in our atmosphere. 

 High up, at an altitude of nearly 40 miles, there exists a small quan- 

 tity of ozone which is that form of oxygen whose molecules contain 

 3 atoms instead of the usual 2. Ozone is a complete absorber of all 

 rays in the extreme ultra-violet from wave length 2,900 A. onward 

 for a considerable range. This is very fortunate. Otherwise our 

 skin would be blistered and our eyes blinded, for these short-wave 

 rays which are totally absorbed by ozone are highly destructive to 

 animal tissues. On the other hand, it is not less fortunate that ozone 

 allows some rays on the border of its absorption band to pass, for 

 these rays between wave lengths 2,900 A. and 3,100 A. are indispen- 

 sable to prevent rickets. The total thickness of gas of the atmos- 

 pheric ozone layer, if it could be brought down to sea level, would 

 be less than one-eighth of an inch. It is astonishing and even terrify- 

 ing to contemjDlate the narrow margin of safety on which our lives 



