IQO ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



more intense radiation than on the other, the brief method of 1923 

 will indeed distinguish the day of more intense radiation, but will 

 show less than the true 1 percent change. Moreover, the deficiency 

 of amplitude in solar variation, due to the method of 1923, is greater 

 the more hazy the days, and the lower in altitude the station from 

 which the sun is observed. 



Consequently, although the published record of solar variation 

 since 1923 shows solar changes at their right times and in their right 

 directions, the amplitudes of variation found are too small. Also, 

 the hazier stations are at a disadvantage, not only because of their 

 less favorable sky conditions, which naturally give inferior results, 

 but because the method of reduction of 1923 inevitably diminishes the 

 resulting amplitude of the variation of the sun, which they were 

 established to determine, even more than it affects clearer stations. 



Our first care was to devise a correct method of reduction, retaining 

 as far as possible the brevity of computation which was the merit 

 of that of 1923. Several months were occupied by the staff at Wash- 

 ington in comparing different proposed methods, checking their 

 results, and at length in computing tables for the one finally selected. 

 This new brief method, although somewhat sliorter than the short 

 method used from 1919 to 1923, is far longer than that of 1923. It 

 requires, what was unnecessary for the method of 1923, the complete 

 measurement of the photographic records of observation, just as 

 complete, indeed, as the long method used prior to 1919. 



Accordingly, ordere were sent to all field stations to have measured, 

 if possible, tliree holographs for eacli day since 1923 when the sun 

 was observed. This heavy task has been to a large degree accom- 

 plished by the field observers. 



In the meanwhile, by financial aid of John A. Roebling, and by 

 the assistance of W. P. A., the computing staff at Washington has 

 been much enlarged. Great progress has been made in the rereduc- 

 tion of the solar-constant observations. Mount Montezuma observa- 

 tions since 1932 have been fully recomputed, and several years' 

 observations at Table Mountain are done. However, it will require 

 many months before the recomputations are fully completed. 



SIL.VE3B-DISK PYRHELIOMETEBS 



As in former years, orders have come fi'om foreign lands for 

 silver-disk pyrheliometers, either new or to be repaired and re- 

 standardized. These instruments are in use at nearly a hundred sta- 

 tions in many countries, in all of the continents of the world, to 

 measure the solar radiation. But nowhere are they used in coopera- 

 tion with the spectroscope, as with us, to make complete determina- 

 tions of the solar constant of radiation. 



