THE SOLAR CONSTANT OF RADIATION.^ 



By C. G. abbot. 

 {Read April 21, 1911.) 



If we had no eyes we should still know of the sun by the feeling 

 of warmth. The intensity of solar rays in any part of the spectrum 

 can be measured by delicate thermometry. Msion and photography 

 are both restricted within comparatively narrow limits of wave- 

 length, and each differs in its sensitiveness from wave-length to 

 wave-length. Ultra-violet, visible and invisible red rays, however, 

 all produce their just and proportional influences on the bolometer, 

 or thermopile. This is not universally known, and there are still 

 many who suppose we should distinguish between so-called actinic, 

 visible and heat rays. Doubt has been expressed, for instance, 

 whether bolometric measurements give true indications of the 

 intensity of those rays which promote plant growth. Such doubts 

 are not justified, and we may expect very valuable results in the 

 future from the application of the spectro-bolometer to the interest- 

 ing questions of radiation and plant physiology. 



We use heat units to express the intensity of solar radiation. 

 The solar constant of radiation may be defined closely enough as 

 the number of degrees by which one gram of water at 15° centi- 

 grade would be raised, if there should be used to heat it all the 

 solar radiation which would pass at right angles in one minute 

 through an opening one centimeter square, located in free space, 

 at the earth's mean solar distance. Experiments were begun about 

 1835 by Pouillet and by Sir John Herschel for the measurement of 

 this great constant of nature. The investigation has been continued 

 by Forbes, Crova, A'iolle, Radau, Langley, K. Angstrom, Chwolson, 

 W. A. Michelson, Rizzo, Hansky, Scheiner and others. It is an 

 indication of the great difficulty of the research that entire uncer- 



^ Published by permission of the Secretary of the Smithsonian Institution. 



235 



