80 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
description of the apparatus and methods employed for the various kinds of 
research, and the sources and magnitude of the errors attending their use. 
The work thus summarized seems definitely to fix the approximate average 
value of the intensity of solar radiation at about 2.1 calories per square 
centimeter per minute, and to show decisively that there is a marked fluctuation 
about this mean value, sufficient in magnitude to influence very perceptibly the 
climate, at least of inland stations, upon the earth. This being so, there is 
good reason for making the series of measurements of solar radiation as com- 
plete and continuous as possible for some years to come, in order to determine 
more thoroughly the causes and limits of the solar changes, and their precise 
effects upon climate. The former part of the study will involve further solar 
measurements, and the latter part a more complete study of meteorologica} 
records in connection with the solar measurements. Thus far no other observa- 
tory has been so well equipped as this one for the special kinds of measurement 
involved, and it will naturally be our task for some time to come to continue 
the work along the lines stated. 
Respectfully submitted. 
C. G. ABBOT, 
Director of the Astrophysical Observatory 
Dr. CHARLES D. WALCOTT, 
Secretary of the Smithsonian Institution. 
