86 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



much damaged when landing and remained a great while imdis- 

 covered, so that the record was quite spoiled. Apparently, too, the 

 clockwork had stopped before reaching a very great elevation. The 

 third flight was highly successful. 



RESULTS OP BALLOON PYRHELIOMETRY. 



A complete account of the balloon pyrheliometers, the circum- 

 stances of the flights, and the results obtained has been published in 

 a paper by Messrs. Abbot, Fowle, and Aldrich, entitled, " New Evi- 

 dence on the Intensity of Solar Radiation Outside the Atmosphere " 

 (Smithsonian Miscellaneous Collections, vol. 65, No. 4, 1915). The 

 following is a summary of the principal results : 



In the flight of July 11, 1915, the balloons reached an elevation of 

 approximately 25,000 meters, or 81,000 feet. The pressure of the air 

 remaining above the instrument was approximately 3 centimeters, or 

 1.25 inches of mercury, about one twenty-fifth of the barometric 

 pressure at sea level. Seven readable measurements of solar radia- 

 tion were recorded at various levels. Of these the three near highest 

 elevation were the best. Their mean gives a value of 1.84 calories 

 per square centimeter per minute, as the intensity of solar radiation 

 at mean solar distance, at noon on July 11, at the altitude of about 

 22,000 meters, or 72,000 feet. It appears reasonable to add about 

 2 per cent for the quantity of solar radiation absorbed and scattered 

 by the air above the instrument. This gives 1.88 calories as a value 

 of the solar radiation outside the atmosphere, on this day, according 

 to the balloon pyrheliometry. Unfortunately no solar-radiation 

 measurements were secured on Mount Wilson on July 11, but the 

 result falls well within the range of values for the solar constant of 

 radiation which have been obtained by the bolometric method at 

 various stations, and compares well with the mean of these values, 

 1.93 calories. 



UNIFORMITY OF ATMOSPHERIC TRANSMISSION AT IMOUNT WILSON. 



In solar-constant measurements on Mount Wilson the atmospheric 

 transmission for vertical rays is determined in the following man- 

 ner for numerous spectrum wave lengths : 



Spectrobolographic observations are made at different zenith dis- 

 tances of the sun, usually between 75° and 30°. Between these 

 limits the length of the path of the rays within the atmosphere is 

 proportional to the secant of the zenith distance. Knowing the 

 length of path and the intensity of the transmitted rays, the co- 

 efficient of transmission for any ray is readily computed. In this 

 determination it is assumed that the atmosphere remains unchanged 

 in transparency during the whole period of observation. Several 



