RECENT PROGRESS IN ASTRONOMICAL RESEARCH. 165 



Measurements of solar radiation have been made by J. Herschel, 

 Pouillet, Forbes, Violle, Langley, Crova, K. Angstrom, and others, 

 in varions situations as regards altitude and sky conditions, and these 

 investigators and others have devised and used many kinds of appa- 

 ratus for the purpose. Such instruments are known as pyrhelio- 

 meters or actinometers, and the best known of them are Pouillet's 

 water pyrheliometer, Violle's actinometer, Crova's alcohol acti- 

 nometer, and Angstrom's compensation pyrheliometer. Of these 

 four instruments, all excepting Crova's purports to be capable of 

 giving absolute measurements when the constants depending on form 

 and dimensions of the particular instrument are determined. As 

 a matter of fact, however, differences and theoretical objections are 

 still found, so that the absolute amount of solar radiation, as deter- 

 mined and published by different observers, is doubtful to the extent 

 of 10 per cent, at least. Futhermore, there is no common ground 

 of comparison, so that the amount of radiation found 1)V one observer 

 at one locality can not be compared with that found by another 

 observer in quite another part of the world. 



The Union ventured to attempt to alter this state of a If airs by 

 resolving to adopt the readings of the Angstrom compensation 

 pyrheliometer as standard, notwithstanding that it is known that 

 this instrument necessarily reads too low and possibly by amounts 

 varying Avith the locality. Still, as this type of instrument is some- 

 what generally distributed in the world, it seems possible that some 

 abatement of the confusion of radiation measurements may be 

 brought about by this action of the Union. Recognizing the need 

 of a more certain standard, a committee was appointed to investigate 

 this subject. 



Measurements of solar radiation at the earth's surface do not 

 suffice to indicate the amount of radiation available to the earth, or 

 whether this amount varies sufficiently to affect climates. For the 

 atmosphere of the earth scatters, reflects, and absorbs the sun's rays, 

 in w^ays which are intricate and difficult to determine, and the amount 

 of hindrance to radiation which the atmosphere offers varies so 

 rapidly and differs so widely from place to place and from day to 

 day that nothing can be concluded as to the possible variability of 

 the sun frcmi simple measurements of solar radiation at the earth's 

 surface. 



As stated in former Smithsonian reports, the Astrophysical Observ- 

 atory of the Smithsonian Institution has been investigating the 

 amount of solar radiation at the earth's surface and the amount 

 absorbed in the atmosphere, for several years, by methods involving 

 not only measurements of the total radiation, but also measurements 

 of the intensity of each ray of the spectrum by itself. The jDroba- 

 bility that the solar radiation outside the earth's atmosphere actually 



