SECRETARY'S REPORT 111 
for determining the brightness of the sky immediately surrounding 
the sun. Excellent results have been obtained with it at the Harvard 
Station at Climax, Colo. It is of considerable interest to compare its 
readings with simultaneous readings of the Smithsonian pyranometer 
which also measures the brightness of the sky in a zone around the 
sun. In preparation for comparison tests a rigid mounting has been 
prepared for the instrument with slow-motion adjustment in altitude 
and azimuth. 
During the fiscal year, two silver-disk pyrheliometers, Nos. 80 and 
81, were built, calibrated and sold at cost, one to the Hebrew Institute 
of Technology, Haifa, Palestine, and the other to the Dublin Institute 
for Advanced Learning. Inadditiontwomodified Angstrom pyrheliom- 
eters and one special instrument for the spectroscopic determination 
of atmospheric water vapor have been prepared for the Belgium 
Meteorological Institute. These were nearly completed at the end 
of the year. 
Dr. C. G. Abbot, research associate of the Observatory, continued 
his studies of the dependence of weather upon solar changes. This 
work has been published in Smithsonian Miscellaneous Collections, 
vol. 111, Nos. 5,6, and 7. Dr. Arctowski’s studies of solar and terres- 
trial atmospheres were retarded by illness, but his work was resumed 
before the close of the year. 
Work win the field——Daily observations of the solar constant were 
in progress throughout the year, as far as skies permitted, both at 
Montezuma, Chile, and at Table Mountain, Calif. The skies during 
the year were apparently normal at Table Mountain, but at Monte- 
zuma, the observers noted an unusual number of days with light cirrus 
clouds. 
Early in the year Mr. Hoover carried the Observatory’s substandard 
silver-disk pyrheliometer S. I. No. 5 to Miami for direct comparisons 
with the pyrheliometers at that station. In February 1949, in the 
course of changing the personnel at Montezuma, Chile, substandard 
S. I. No. 5 was carried to Montezuma by the new Montezuma ob- 
server, and brought back in April by the retiring observer, after inter- 
comparisons had been made in Chile. The previous year S. I. No. 5 
had been carried to Table Mountain by the director for similar inter- 
comparisons. Thus there are now very recent direct comparisons 
between all field pyrheliometers and substandard S. I. No. 5, which 
in turn was carefully compared in 1947 with the absolute water-flow 
standard. These many intercomparisons show no material changes 
in constants. They satisfactorily confirm the adopted scale of pyr- 
heliometry. A revision of Dr. Abbot’s paper of 1922 on “The Silver- 
disk Pyrheliometer”’ is in preparation, summarizing the constants of 
all silver-disk pyrheliometers, and describing certain changes which 
