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SMITHSONIAN PYRIIELIOIMETRY AND THE 

 STANDARD SCALE OF SOLAR RADIATION 



By L. B. ALDRICH and C. G. ABBOT 

 Smitlisonian Institution 



Since its beginning in 1890, the Astrophysical Observatory of the 

 Smithsonian Institution has devoted much time to the development 

 and improvement of pyrhcliometers for the accurate measurement of 

 total solar radiation. Numerous types have been investigated, and 

 many thousands of individual measurements and intercomparisons of 

 various pyrhcliometers have been made. The two that have been 

 most useful and satisfactory for our purposes are the water-flow 

 pyrheliometer, a standard instrument, and the silver-disk pyrheli- 

 omcter, a secondary instrument. 



During the past 40 years, Andrew Kramer, veteran instrument 

 maker of the Astrophysical Observatory, has constructed in our 

 shop nearly 100 silver-disk pyrhcliometers. Most of these instru- 

 ments as completed have been sold or loaned to interested institutions 

 and are now in use on every continent. The silver-disk instrument 

 was devised and designed by Dr. Abbot. It is mechanically simple 

 and rugged and with reasonable care it continues indefinitely to give 

 reliable readings of solar radiation. Our faith in the permanence of 

 the constant furnished with each instrument has been supported by 

 manv intercomparisons extending over long periods of time.^ Since 

 it is not practicable to use the silver-disk pyrheliometer as a standard 

 instrument, the constant of each one is determined by careful com- 

 parisons against a standard pyrheliometer. One of our silver-disk 

 instruments, A. P.O. No. Sbu has been kept at the Observatory as a 

 substandard since it was built 40 years ago, and a second one, S.I. 

 No. 5bii, has been similarly used in recent years. 



In the years 1910 to 1913, the Observatory conducted an intensive 

 campaign to produce a standard pyrheliometer, and to establish the 

 correct standard scale of solar radiation. The water-flow and water- 

 stir standard pyrhcliometers, both devised by Dr. Abbot, were selected 



^ See detailed tabulations of these comparisons in volumes 3 to 6 of the Annals 

 of the Astrophysical Observatory. 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 110, NO. 5 



