SECRETARY'S REPORT 117 



tions of the solar constant was made on each day that skies were 

 sufficiently clear. And throughout this whole period every effort 

 was made to maintain the solar-constant values on the same scale. 

 It seems worth while, therefore, to examine whether in these years 

 of observation there is evidence of a progressive change in total radia- 

 tion given off by the sun. The following table gives the mean solar 

 constant for three decades: 



Total number of solar- 



constant deter mivations Mean value of the 

 Period (Montezuma, Chile) solar constant 



1921-30 (inclusive) 5,820 1.9431 



1931-40 (inclusive) 5,520 1.9463 



1941-48 (inclusive) 5,004 1.9478 



This increase of one-fourth of 1 percent in the radiation emitted by 

 the sun in two decades (1925-44) is of particular significance in view 

 of evidence that has accumulated of climatic changes in various 

 localities over the earth. On the whole, these changes indicate that 

 average temperatures have increased somewhat during the past 100 

 years, with an accelerated increase during the past several decades. 

 A very interesting discussion of these evidences is given by Prof. 

 Julian Huxley (Natural history in Iceland, Discovery, vol, 11, No. 3, 

 March 1950; reprinted in general appendix of this Report, p. 327). 



As stated in last year's report, funds have been requested to re- 

 establish a third field station. Clark Mountain, in southern CaH- 

 fornia near the Nevada border, has been chosen as the most satisfac- 

 tory available location. 



Work at Washington. — Statistical studies of, and final corrections 

 for, the observational data from our two field stations (Montezuma, 

 Chile, and Table Mountain, Calif.), were carried on under the super- 

 vision of W. H. Hoover, chief of the division. Mr. Hoover also 

 supervised the preparation of instruments and equipment for the 

 solar-constant observations, for special research problems in progress 

 at Table Mountain, and for requests for certain instruments received 

 from other institutions. Of these latter, three silver-disk pyrheliom- 

 eters, each calibrated against the Observatory's standard pyrheli- 

 ometer, were furnished at cost during the year, as follows: 



1. S. I. No. 82 to Dominion Physical Laboratory, New Zealand. 



2. S. I. A. P. 0. No. 17 to Observatorio Cagigal, Caracas, Venezuela. 



3. S. I. A. P. O. No. 18 to Central Meteorological Institute, Bucharest, 

 Rumania. 



o 



In addition, two modified Angstrom pyi'heliometers and one special 

 water-vapor spectroscope were furnished to the Central Meteorologi- 

 cal Institute at Uccle, Belgium. 



A summary of silver-disk pyrheliometry, in the form of a revision 

 of Dr. Abbot's paper of 1922 on "The Silver Disk Pyrheliometer," 



