THE ORIGIN OF ORGANIC MATTER 29 



they have been found at Washington, Mount Wilson and Mount Whitney, 

 for various wave lengths. To compute the transmission for other than 

 zero zenith distances, the coefficients here given must be raised to a povv^er 

 equal to the secant of the zenith distances. This does not hold closely for 

 zenith distances above 75°." 



TABLE 1 

 Abbot's Table of Me^.'in Coefficients of Atmospheric Transmission. 



The intensity and composition of solar radiation varies considerably 

 v^^ith the altitude ; and also, at a given place, the composition of the light 

 varies considerably, almost from minute to minute. This is especially 

 true in the violet end of the spectrum. 



Probably of even greater significance than the gaseous constituents of 

 the atmosphere for the absorption of solar radiation is the dust in the 

 upper atmosphere. The origin of this dust is usually volcanic. Benjamin 

 Franklin already suggested that the hard winters which had been recorded 

 in history may be due to the decrease in solar radiation by the fine volcanic 

 dust and that these abnormally cold periods may be synchronous with 

 volcanic activity in dififerent parts of the world. That such is the case, 

 at least to a considerable degree, now seems highly probable. Thus. 

 Abbot ^ found, following the volcanic eruption of Mount Katmai. Alaska, 

 June 6-7, 1912, that evidence of the dust appeared at Bassour, Algeria, 

 on or before June 19, and at Mount Wilson, California, on or before 

 June 21. The effect reached its maximum in August and reduced the total 

 direct radiation of the sun by nearly 20 per cent. The complicated effects 

 produced by such layers of volcanic dust on solar radiation have been 

 studied also by Kimball '" and by Humphreys.'^ The latter has compiled 



"Abbot, Smithsonian Inst. Ann. Aslroplivsical Obscr., 3, 214 (1913). 



"Kimball, H. H., Monthh Weather Revietv, 46, 355 (1918). 



" Humphreys, W. J., Physics of the Air. Philadelphia, 1920, p. 569. 



