110 abbot: solar constant of radiation 



placed horizontally, and the thermometer was contrived to re- 

 cord its temperature by photography upon a moving drum. 

 The receiving disk was alternately exposed to the sun and shaded 

 by the intervention of a shutter, operated intermittently by the 

 clock work which rotated the drum under the stem of the ther- 

 mometer. Five instruments of this kind were sent up on suc- 

 cessive days. While it was well known that the temperature of 

 the higher air would go as low as — 55°C., it was believed that 

 a blackened disk, exposed half the time to the direct sun rays, 

 would certainly remain above the temperature of —40°, which 

 is the freezing point of mercury. This expectation was disap- 

 pointed. Accordingly, owing to the freezing of the mercury in 

 the thermometer, the highest solar radiation records obtained 

 during the expedition were at the altitude of 13,000 meters, 

 although the balloons in some instances reached the altitude of 

 33,000 meters. 



The results obtained, while they have not the same degree of 

 accuracy as those obtained by direct reading of the silver disk 

 pyrheliometer, are yet of considerable weight. All the measure- 

 ments unite in indicating values of the solar radiation at alti- 

 tudes of 10,000 meters and higher, which fall below the value of 

 the solar constant of radiation as obtained by other methods, 

 and above the value of the radiation at the summit of Mount 

 Whitney as obtained by different observers with pyrheliometers. 

 It is expected in the coming year to repeat the observations with 

 balloons under much improved circumstances. By aid of elec- 

 trical heating apparatus it is expected to keep the surroundings 

 of the disks at approximately the freezing temperature, even 

 though exposed to the air at temperatures as low as — 55°C. 

 In this way it is hoped to obtain good pyrheliometer measure- 

 ments as high as it is possible for sounding balloons to go, and 

 possibly to an altitude of 40,000 meters. As the atmospheric 

 pressure at such altitudes is less than 1 per cent of that prevailing 

 at sea-level, the experiments, if successful, may be expected to 

 remove reasonable doubt of the value of the solar constant of 

 radiation. 



