82 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
degree of certainty could not be secured with the apparatus which 
Mr. Aldrich and the director had designed and which Mr. Kramer, 
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 horizontal 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 AMERICAN EXPEDITION. 
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 of 
the variation of the sun on the temperature and other weather condi- 
