THE ASTRAPHYSICAL OBSERVATORY—ABBOT 
“backlash”? and changes of amplitude 
can be discovered, the periodicity 
may prove valuable for long-range 
forecasting. At least it is an important 
weather element hitherto unrecognized. 
We have shown also that the sun 
varies simultaneously in no less than 
16 periods, ranging from 7 months 
to about 23 years in length, all nearly 
integral fractions of 273 months. 
The 23-year period may be related 
to the magnetic cycle on the sun 
indicated by the reversal of magnetic 
conditions in sunspots during alternate 
11-year sunspot cycles. These solar- 
radiation periodicities are associated 
with fluctuations of temperature and 
rainfall. The correlation at some 
stations is close enough to serve as a 
817869—49——13 
175 
basis for long-range weather 
predictions. 
In recent time, L. B. Aldrich, who 
has succeeded me as Director of the 
Astrophysical Observatory, has been 
engaged, in cooperation with the 
Quartermaster Corps of the Army, 
in a program of measurements of the 
intensity of sunlight and of skylight 
in different parts of the spectrum as it 
reaches the earth. This is of im- 
portance not only to the military, but 
from the medical point of view. It 
is hoped to carry on these studies at 
different stations so as to yield better 
information as to the influence of 
latitude, altitude, humidity, and clouds 
on the sun’s rays which affect all life 
interests. 
