REPOKT OF THE SECRETAEY. 85 



the instrument maker, had constructed, and so the work was given 

 over for a time. 



At Mount Wilson. — ^Mr. Aldrich continued the observations of the 

 solar constant of radiation until the middle of October, 1918, and 

 returned to continue them early in June, 1919. In September of 

 1918 he made a very interesting observation in cooperation with the 

 Army Balloon School at Arcadia at the foot of Mount Wilson. It 

 consisted in arranging a pyranometer to be hung below the basket 

 of a captive balloon, which could be raised above the level of the 

 great horizontal layer of fog which often covers the San Gabriel 

 and other valleys in the neighborhood of Los Angeles in a sheet 

 many miles in extent. On this occasion the layer of fog extended 

 from 1,000 feet of altitude to 2,500 feet. The balloon was raised to 

 about 200 feet above the layer. An officer of the balloon school 

 exposed the apparatus under the balloon to the radiation from the 

 sheet of fog, while Mr. Aldrich, on the ground, observed the deflec- 

 tions of the galvanometer. The galvanometer was connected to the 

 pyranometer by a pair of wires about a half mile long. Simul- 

 taneously observations were made on Mount Wilson with the pyrheli- 

 ometer to determine the exact character of the day, and on other days 

 of similar character Mr. Aldrich exposed the pyranometer to the 

 radiation of the sun and sky combined. Thus knowing the radiation 

 reflected from the sheet of fog, and knowing the radiation on a similar 

 day coming down from the sun and sky, he was able to determine 

 the reflecting power of a great layer of fog. This observation is 

 very useful for the study of the relations of the temperature of the 

 earth to radiation. The result of the experiments, which were con- 

 tinued for several hours without interruption, was very satisfactory. 

 The final value for the reflecting power of a great hoi'izontal sheet 

 of fog was 78 per cent. 



The weather on Mount Wilson, in the autumn of 1918, was un- 

 commonly poor for the solar constant work, as rain fell frequently 

 and a great many clouds came up. Altogether it was the! most 

 unfavorable weather which has been experienced in any observing 

 season there since it was occupied for solar constant purposes. 



SOUTH AAIEKIdAN 1]XPI]I)ITI;)N. 



Several considerations led to the decision to make a small expedi- 

 tion to South America in the spring of 1919. The Institution had 

 equipped an observatory at Calama, Chile, to measure the solar con- 

 stant of radiation. The Argentine meteorological service, through 

 its chief forecaster, Mr. Clayton, had been determining the effects ol' 

 the variation of the sun on the temperature and other weather condi- 

 tions of the earth, and had been so much impressed by the value of 

 the solar variation observations for forecasting purposes that they 



