94 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
thing available, as, for instance, a trunk of a tree for a mirror sup- 
port at the top of the tower, Messrs. Abbot and Aldrich succeeded in 
getting arranged on the tower a reflecting telescope of 12 inches 
aperture and 75 feet focus, all ready for observations by September 
9, 1918. Then and thereafter solar constant measurements were sup- 
plemented by determinations of the distribution of radiation along 
the sun’s diameter on each day of observation. These determinations 
are made in seven different wave lengths on each day, ranging from 
0.38u. in the ultra violet to 1.ly in the infra-red. Fortunately, the 
definition of the tower telescope proves to be very good. ‘There is 
slight change of focus during the several hours of observing, and the 
“seeing” seems not to deteriorate much up to 10 o’clock a. m., at 
which time the observations are generally concluded. 
About 45 days of simultaneous observations of the “solar con- 
stant” and of the distribution of radiation over the sun’s disk were 
secured in 1918. The results appear to indicate a variability in both 
phenomena and a distinct correlation of the two in point of time. It 
is indicated that when in course of its short-period irregular varia- 
tion the solar radiation increases, there occurs simultaneously a dimi- 
nution of the contrast between the edge and center of the sun’s disk. 
A change of brightness of about 1.5 per cent was found to occur at 95 
per cent out on the solar radius accompanying a change of 6 per cent 
in the solar radiation. On comparing the mean of all results ob- 
tained in 1913 with the mean of all obtained in Washington in 1906-7, 
it appears that there was distinctly less contrast of brightness be- 
tween the edge and center of the sun’s disk in 1913 than in 1907. We 
have reason, however, to believe that there was distinctly a greater 
total solar radiation in 1907 than in 1913. This result, compared with 
the result stated above, indicates a difference of character between 
the long-period fluctuations of the sun and its short-period irregular 
fluctuations. The changes of contrast found, however, agree in this, 
that whether from day to day in 1913, or as between 1913 and 1907, 
the violet or shorter wave lengths change in contrast more than the 
red or longer wave lengths. 
MOUNT WILSON EXPEDITION OF 1914, 
Mr. Abbot continued the Mount Wilson work, beginning in May, 
1914. Many improvements were made in the tower telescope, lead- 
ing to improved definition and stability of the image of the sun. 
Improved methods of observing were introduced also. 
BALLOON PYRHELIOMETRY. 
Mr. Aldrich, in cooperation with the United States Weather 
Bureau observers, under personal direction of Dr. Blair, arranged to 
‘repeat the balloon pyrheliometer observations, and this time at 
