30 PHOTOSYNTHESIS 



very interesting data on the relation of volcanic disturbances to solar radia- 

 tion and atmospheric temperatures. It appears that not all eruptions result 

 in a decrease in surface temperatures ; usually only those in which a large 

 quantity of fine dust is thrown into the high isothermal layer of the at- 

 mosphere. The end results depend upon a number of factors, chiefly 

 the absorption by the dust of the direct solar radiation and the effect of 

 the dust on the earth's radiation into space. But there seems to be httle 

 room for doubt that both volcanic dust and solar activity, the latter evi- 

 denced by sun-spots, have decided effects both on the direct solar radiations 

 and on atmospheric temperatures. The effect of such changes is dis- 

 cussed by Humphreys : "For instance, during the summer, or growing 

 season, a change of 0.5° C. produces a latitude shift by fully 80 miles. 

 Hence, if there is but little or no volcanic dust to interfere, during sun- 

 spot minima cereals and other crops may be successfully grown 50 to 150 

 miles farther north (or south in the southern hemisphere) than at times 

 of sun-spot maxima. This alone is of great practical importance, espe- 

 cially to those who live near the thermal limits of crop production. 



"In addition to changing the area over which crop production is pos- 

 sible, a change of average temperature also affects, in some cases greatly, 

 the time of plant development. Thus Walter ^- has shown that a change 

 of only 0.7° C. may alter, and in Mauritius has been observed actually to 

 alter by as much as an entire year, the time required for the maturing of 

 sugar cane. Hence the temperature changes that normally accompany sun- 

 spot variations, though small in absolute magnitude, are of great impor- 

 tance, and, by availing ourselves of the reasonable foreknowledge we have 

 of these changes, may easily be made of still greater importance." 



The astronomer is particularly interested in the amount of solar radia- 

 tion received at the outside of the earth's atmosphere. This quantity he 

 obtains by making determinations of the amount of radiation received at 

 the surface of the earth and calculating the amount of loss sustained by 

 passage through the atmosphere. The value thus obtained is called the 

 "solar constant" of radiation. For this measure Abbot ^^ has given the 

 value of 1.946 calories (15°) per square centimeter per minute for the 

 epoch 1912 to 1920 determined by 1,244 observations. Bigelow ^* arrived 

 at another value by means of different methods ; his results are summarized 

 below, in values of gram calories per square centimeter per hour : 



1. True Solar intensity of radiation 5.85 calories 



2. Effective Solar intensity at the distance of the earth 3.98 " 



3. Effective intensity by the bolometer 3.98 " 



4. Effective intensity by thermodynamics 3.98 " 



5. Extrapolated intensity by the pyreheliometer 1.95 " 



6. Intensity at sea level by the pyreheliometer 1.50 " 



""On the Influence of Forests on Rainfall and Probable Effect of 'Deboisement' 

 on Agriculture in Mauritius" (1908), quoted from Humphreys. 1. c, p. 602. 

 "Abbot, Smithsonian Inst. Ann. Astrophysical Obser., 4, 192 (1922). 

 Bigelow, Treatise on the Sun's Radiation. New York, 1918, p. 211. 



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