2)2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



Eighth objection. — As Mr. Very, in a recent article, has shown that 

 Mr. Bigelow's thermodynamic considerations are erroneous, it is not 

 necessary to discuss them further. 



SOUNDING BALLOON OBSERVATIONS 



Now we come to the final piece of experimental evidence which 

 we have secured, which seems to us to show that our solar constant 

 results are undoubtedly very close to the true ones, and that if there 

 be any circumstances which have led to the underestimation of the 

 losses which the solar beam suffers in the atmosphere, they at any 

 rate relate to the part of the atmosphere which lies beyond the alti- 

 tude of 24 kilometers, and where the total pressure of it is less than 

 one twenty-fifth of that which prevails at sea level. 



In January, 191 3, it was determined on the part of the Smithsonian 

 Institution to support an expedition to California, in charge of Mr. 

 A. K. Angstrom, for the purpose of observing the nocturnal radiation 

 at various altitudes. In connection with this work, the Institution 

 invited the cooperation of the United States Weather Bureau for the 

 purpose of sending up sounding balloons and captive balloons, in 

 order to determine the humidity and temperature at various heights 

 in the atmosphere, at the time of Mr. Angstrom's experiments. While 

 discussing the proposed expedition with Mr. Angstrom, he inquired 

 of us whether it might not be possible that an instrument could be 

 devised for measuring the intensity of the radiation of the sun at the 

 highest altitudes to be reached by sounding balloons. After due 

 consideration of the matter, it was deemed by us feasible to do this. 



Accordingly in the months of April, May, and June, 1913, there 

 were constructed at the instrument shop of the Astrophysical Observ- 

 atory, five copies of a special recording pyrheliometer, modified in 

 form from the silver disk pyrheliometer which we ordinarily employ 

 in solar-constant work. 



The five instruments were sent up, in cooperation with the U. S. 

 Weather Bureau, by Mr. Aldrich, at Avalon, Santa Catalina Island, 

 California, in July and August, 1913. All were recovered, and all 

 had readable records of more or less value. In these experiments, 

 the balloon in one instance reached the height of 33,000 meters, 

 but unfortunately, owing to the freezing of the mercury contained in 

 the thermometers, the pyrheliometric records did not extend above an 

 altitude of 14,000 meters in any case. There were, besides, certain 

 sources of error which had not been anticipated at that time, so that 

 the results of the expedition could only be regarded as of a prelimi- 



