NO. 4 SOLAR RADIATION ABBOT, FOWLE, AND ALDRICH 53 



elevation the barometric pressure was 2.98 cm. Hg., or in round 

 numbers, 3.0 cm. Hg. From our examination of the records of 

 various balloon flights at Omaha and Avalon, we suppose this would 

 be regarded as corresponding to an elevation of 24,000 meters, which 

 is in good agreement with the results obtained by theodolite work. 



COMPARATIVE RESULTS OF PYRHELIOMETRY AT REDUCED 

 ATMOSPHERIC PRESSURES 



In a recent publication, Prof. H. H. Kimball gives the highest 

 value of solar radiation ever observed at Washington, for zenith 

 distance 6o°, as 1.5 1 calories per cm. 2 per min., observed on December 

 26, 1914. Reduced to vertical sun and mean solar distance, this 

 result would have been about 1.58 calories. 



The highest values observed on Mt. Wilson are those of November 

 2, 1909, and yield to a similar reduction 1.64 calories, at mean solar 

 distance and vertical sun. 



For Mt. Whitney, for the maximum obtained on September 3, 1909, 

 the reduced value is 1.72 calories at mean solar distance and ver- 

 tical sun. 



In balloon flights of August 31, September 28, and October 19, 

 1 91 3, Dr. A. Peppier of Giessen observed with an Angstrom pyr- 

 heliometer at great altitudes. On September 28 the results were, 

 in his opinion, vitiated by a defect of the apparatus. On August 31, 

 the highest result, as reduced by Peppier to the Smithsonian scale of 

 pyrheliometry, was 1.77 calories, obtained at zenith distance 45 °, 

 altitude 5,900 meters, air pressure 36.5 cm. This result, however, is 

 not a complete Angstrom measurement depending on " left, right, 

 left " readings, and therefore may be vitiated by galvanometer drift. 

 Moreover, it stands very high as compared with others of that date, 

 and, indeed, much higher than others of that date obtained at greater 

 altitudes. On October 19, the highest complete result was 1.67 

 calories, obtained at zenith distance 61 °, altitude 7,500 meters, air 

 pressure 29.8 cm. This result is in good agreement with the others 

 of that date. Peppier regards the results of October 19 as his best. 

 When reduced to zenith sun and mean solar distance, the result of 

 October 19 comes out about 1.755 calories per cm. 2 per minute. 



These direct observations from manned balloons are very meri- 

 torious, and of course entitled to far greater weight than those 

 obtained at similar altitudes in our free balloon work at Avalon, in 

 1913. Hence, although our results there were in complete accord 

 with Peppler's, we have not thought it worth while to give them. 



