20 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
pletion of work begun three j^ears ago for the purpose of obtaining 
new level spaces for yards and inclosures. 
ASTROPHYSICAL OBSERVATORY. 
Several important investigations relating to the war, begun last 
year, were continued by the staff of the Astrophysical Observatory 
under the general direction of Dr. C. G. Abbot, in addition to the 
regular work of the observatory. These researches are mentioned 
elsewhere in this report under the heading " General considerations." 
At Washington work on solar radiation computations has gone 
on steadily, and progress has been made with the preparation of a 
new medium, potassium iodide, for the investigation of the rays 
beyond where rock salt is transmissible. A new instrument, based 
upon the principle of the perfect radiator, or " absolutely black body," 
was constructed for the purpose of measuring nocturnal radiation, 
such as the earth sends out to space. At the close of the year 
this instrument was reported as operating successfully on Mount 
Wilson. 
In view of the fact that the total eclipse of the sun of May 29, 
1919, would be visible at La Paz, Bolivia, which is not very far from 
the Smithsonian solar constant observing station at Calama, Chile, 
a successful expedition was undertaken by Dr. Abbot, with the 
double purpose of observing the eclipse and visiting the Calama 
station. Good photographs of the phenomenon and also pyrano- 
metric observations by Mr. A. F. Moore of the brightness of the sky 
were obtained during the progress of the eclipse. A conference was 
held with officials of the Argentine Government, which is likely to 
prove of great value in the future, in that it concerned the employ- 
ment of solar-radiation measurements for weather forecasting by 
the Argentine meteorological service. At Calama, Chile, Dr. Abbot, 
in cooperation with the Smithsonian observers there, Messrs. Moore 
and Leonard Abbot, devised a new method of reducing solar radia- 
tion observations, so as to determine the solar constant of radiation 
with at least equal precision to that obtained by the older method, 
the advantages of the new method being (1) its independence of the 
variability of atmospheric transparency; (2) the time required is 
only one-fifth of the former period. 
On Mount Wilson Mr. Aldrich continued the observations of the 
solar constant of radiation, and in September, 1918, made an inter- 
esting observation in cooperation with the Army Balloon School at 
Arcadia, Calif., on the measurement of the reflection of sun and 
sky radiation from layers of fog, which led him to conclude that 
a great horizontal fog bank reflects to space 78 per cent of the radia- 
tion of the sun falling upon it. 
