88 ANNUAL REPORT SMITHSONIAN INSTITUTION^ 1913. 



measurement were constructed and tried at this observatory, as men- 

 tioned in former reports. The results obtained agreed closely to- 

 gether and were checked by observations with known quantities of 

 heat. In October, 1912, another type of standard pyrheliometer, 

 which we called the water-stir pyrheliometer, was devised, con- 

 structed, and used. It proves to give values for the standard scale of 

 radiation almost identical with those which we had before obtained, 

 and in this instrument, as in the others, known test quantities of heat 

 were introduced and measured within less than 1 per cent. In view 

 of all these experiments with standard pyrheliometers, it is now felt 

 that the standard scale of radiation is at length fully established. 

 Accordingly, a publication entitled " Smithsonian Pyrheliometry Re- 

 vised " was issued February 1, 1913, giving the results of all the 

 definitive experiments on the standard scale of radiation and also 

 the experiments made to fix the scales of all the secondary pyrheli- 

 ometers in use at the Astrophysical Observatory or furnished by 

 the Smithsonian Institution to observers in this country and abroad. 



A small correction in the determinations of the solar constant of 

 radiation made at Mount Wilson and elsewhere was found to be 

 required owing to a residual eifect of water vapor in the atmosphere 

 which had not been entirely eliminated. This correction sometimes 

 reaches as great a magnitude as 2 per cent. It has now been applied 

 to all the measurements made at the various stations which have been 

 occupied since 1902, and all the solar-constant measurements, about 

 700 in number, have been reduced to the new standard scale of pyrhe- 

 liometry. 



The mean value of the solar constant of radiation at the earth's 

 mean distance from the sun from about 700 measurements, some at 

 Washington, others at Mount Wilson, others at Bassour, Algeria, and 

 still others at Mount Whitney, Cal., and covering the years from 1902 

 to 1912, has now been taken. It is 1.932 calories per square centi- 

 meter per rtvinute. 



2. THE VARIABILITY OF THE SUN. 



{a) Attending sun spots. 



In connection with the reduction of the measurements of the solar 

 constant of radiation mentioned above, mean values were taken for 

 each month during which observations had been made at Mount 

 Wilson. These monthly mean values, extending from the year 

 1905 to the year 1912, have been compared with the so-called Wolff 

 sun-spot numbers for the same months. The result shows, as indi- 

 cated in the accompanying illustration, that increased solar-constant 

 values attend increased sun-spot numbers. An increase of radiation 

 at the earth's mean distance from the sun of 0.07 calorie per square 



