REPORT OF THE SECRETARY. 93 
shaded by cirrus clouds until after the mercury froze in their ther- 
mometers. The highest elevation at which a radiation record was 
obtained was about 14,000 meters, or nearly 45,000 feet. As stated in 
last year’s report no results indicating that values of solar radiation 
exceeding our solar constant value (1.93 calories) are obtainable by 
pyrheliometric measurements at any elevation, however high, appear 
from these balloon pyrheliometer experiments. In view of the pro- 
posed repetition of the experiments with improved apparatus no 
further statement of these preliminary results is necessary here. 
The tower-telescope work.—As stated in former reports, investiga- 
tions were carried on at Washington during the years 1904 to 1907 
to determine the distribution of the sun’s radiation along the diame- 
ter of the solar disk. It was shown by this work, in accord with 
results of earlier observers, that the edge of the solar disk is much 
less bright than the center, and that this contrast of brightness is 
very great for violet and ultra-violet rays, but diminishes steadily 
with increasing wave lengths, and becomes very slight for red and 
especially for infra-red rays. These phenomena are well shown in 
Biue-GREEN UctRAMIOLET 
A=.S03 AL A=.37 lye 
' Inrra-Reo ‘INFRA-RED Tale me 
Az 1.55 »=-966u A«.670 
Fic. 1.—Brightness distribution along sun’s diameter for different colors. 
the accompanying illustration, from observations of 1913. The 
measurements were continued at Washington on all suitable days in 
the hope that some fluctuation of this contrast of brightness between 
the edge and center of the solar disk would be disclosed. It seemed 
probable that there might be such fluctuations associated with the 
’ irregular variability of the total solar radiation. It proved, however, 
that such fluctuations, if existing, were of so small an order of 
magnitude that it was not certain whether they were really shown 
by the observations at Washington, hampered as these were by vari- 
able transparency of the air. 
When the observing station was erected on Mount Wilson in 1908 
provision was made for a tower telescope designed to continue this 
research. When in 1911 and 1912 the Algerian expeditions confirmed 
the sun’s variability, added interest was felt in the proposed experi- 
ments. Accordingly, the tower, 50 feet in height, was completed in 
1912. Not sufficient funds were available to equip the tower tele- 
scope, but Director Hale, of the Mount Wilson Solar Observatory, 
kindly loaned considerable apparatus, and with this and some appa- 
ratus which remained from eclipse expeditions, and by using any- 
