310 ABBOT, FOWLE, ALDRICH! VARIATION OF THE SUN 



the standard scale of radiation, but they possess the vahiable 

 quahties of simpUcity and of being constant from year to year. 

 Beginning with the year 1903 and extending until the end of the 

 year 1912 we have repeatedly devised and exper'mented with 

 instruments to fix the standard scale of radiation. Three of these 

 instruments (called Water-flow Pyrheliometers Nos. 2 and 3, and 

 Water-stir Pyrheliometer No. 4) have been tested with satis- 

 factory results which are stated in a publication by two of us.^ 

 We are now satisfied that the measurements made since 1903 

 can be reduced to the standard scale of radiation to within 1 

 per cent. 



Measurements of the solar constant of radiation were begun 

 at Washington, practically at sea-level, and were continued when 

 favorable opportunities presented themselves from October, 1902, 

 until May, 1907. Measurements were begun on Mount Wilson 

 in California (elevation 1730 meters) in 1905, and have been con- 

 tinued with the exception of 1907 during about six months in 

 the year in each of the succeeding years. Expeditions to Mount 

 Whitney in California, altitude 4420 meters, were made in 1908, 

 1909, and 1910. Expeditions to Bassour, Algeria, altitude 1160 

 meters, were conducted in the autumn of 1911 and the summer 

 of 1912. In all 696 complete determinations of the solar constant 

 of radiation have been made, and still others are unreduced. 

 The differences found between the results at different elevations 

 are very small, and seem attributable rather to experimental error 

 or slight atmospheric irregularities than to any difference of ele- 

 vation. The mean of all these 696 determinations made princi- 

 pally between the years 1905 and 1912 is 



1.932 calories per square centimeter per minute. 



Subject to the possibility that there may exist ultra-violet rays 

 of appreciable intensity beyond the wave-length 0.29^, which are 

 cut off by the absorption of ozone from reaching the earth's 

 surface, we believe that this value represents the intensity of the 

 radiation of the sun as it would be found in space at the earth's 

 mean solar distance for the epoch 1905 to 1912. 



2 See "Smithsonian Pyrheliometry Revised;" Smithsonian Miscellaneous Col- 

 lections 60: no. 18. 1913. 



