2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



tice to them to write a summary of the whole research. I hope to 

 make it so thoroughly supported by varied evidences as to convince 

 the professional scientists that it can no longer be ignored and allowed 

 to sink into oblivion. But it is quite impossible for me to give half the 

 evidence which saturates my mind with the certainty that the family 

 of regular harmonics of 273 months, in solar radiation and terrestrial 

 weather, is a controlling geophysical fact, 



1. THE "SOLAR CONSTANT" 



Pouillet invented his pyrheliometer, and about 1876, after measur- 

 ing the heat of sun rays at different solar altitudes, he estimated that 

 the instrument would have indicated 1.76 calories per square centi- 

 meter per minute if exposed outside the atmosphere at the earth's 

 mean solar distance. Langley strongly argued that since the atmos- 

 phere transmits different wavelengths unequally, spectroscopic meas- 

 ures are necessary additionally to pyrheliometry to estimate the solar 

 constant. He invented the bolometer for this purpose and used it at 

 Allegheny Observatory and on and near Mount Whitney. Errone- 

 ous theory caused him to prefer 3.0 calories as the solar constant. 

 K. Angstrom, from solar measures on the Island of Tenerift'e, attrib- 

 uting excessive influence to atmospheric carbon dioxide, preferred a 

 value of 4.0 calories. 



In volume 2 of Annals, A.P.O.,^ is demonstrated the true theory 

 for the spectrobolometric determination of the solar constant. An 

 improved pyrheliometer similar to Pouillet's is described. Measure- 

 ments at Washington, D. C, during the years covered by volume 2 

 indicated an average solar constant of 2.20 calories. A hint of solar 

 variation appeared to be indicated by results of 1903 and 1904. By 

 invitation of Director George E. Hale, we made measurements of the 

 solar constant on Mount Wilson, Calif., in 1905 and 1906. From 

 1908 to 1920 the Smithsonian sent expeditions to Mount Wilson. 

 A long- focus vertical telescope was installed in addition to solar- 

 constant apparatus. Every day that solar constants were observed, 

 the distribution of brightness over the diameter of the sun's disk 

 was observed by allowing the 8-inch solar image from the telescope 

 to drift without a clock over the slit of the spectrobolometer, in rays 

 at various wavelengths. (See fig. 52, p. 62.) 



In volume 3, Annals, A.P.O., pages 21-29, a full description of 

 solar-constant measurement is given. The silver-disk pyrheliometer 



2 We thus abbreviate Smithsonian Astrophysical Observatory. 



