SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



& o a- a D 



Fig. 3. — Angstrom-Smithsonian pyrheliometer. 



elusion; for they indicate a value outside the atmosphere well within the pre- 

 viously ascertained limits of solar variation. 



The solar-constant method of Langley, which v^e used exclusively 

 until 1920, is fundamental and sound. But it requires several hours 

 of observation through unchanged transparency while the sun is 

 ascending or descending, so that the thickness of air its rays traverse 

 alters enough to give accurate transmission coefficients for all wave- 

 lengths observed. If during a morning series of measurements the 

 atmosphere grows more transparent, the value obtained is too high, 

 and vice versa. The opposite, of course, holds in the afternoon. We 



