THE SOLAR CONSTANT OF RADIATION. 1 



By C. G. Abbot, 

 Director of the Astrophysical Observatory of the Smithsonian Institution. 



Langley once wrote : 



If the observation of the amount of heat the sun sends the earth is among 

 the most important and difficult in astronomical physics, it may also be termed 

 the fundamental problem of meteorology, nearly all whose phenomena would 

 become predictable if we knew both the original quantity and kind of this 

 heat; how it affects the constituents of the atmosphere on its passage earth- 

 ward ; how much of it reaches the soil ; how, through the aid of the atmosphere, 

 it maintains the surface temperature of this planet ; and how, in diminished 

 quantity and altered kind, it is finally returned to outer space. 



The first great advance in the study of this matter was made by 

 Pouillet more than 70 years ago. He constructed an instrument 

 which he called a " Pyrheliometer." It comprised a shallow circular 

 metallic box blackened to absorb sun rays, having a thermometer 

 inserted in the center of one circular face, and being arranged so as 

 to expose the other circular face broadside toward the sun. The in- 

 strument was first shaded for a time, as, for instance, five minutes, 

 then exposed to the sun an equal time, then shaded again. By reading 

 the thermometer before and after each of the intervals just men- 

 tioned, the rise of temperature due to the sun, exclusive of the losses 

 and gains of heat due to the surroundings, was thought to be de- 

 termined. Knowing the water equivalent of the pyrheliometer and 

 the area exposed to the sun, the result could be converted to calories 

 per square centimeter per minute. 



But it is not sufficient to know the amount of heat available in the 

 solar beam at the earth's surface, for this is reduced by the amount 

 of haze, dust, and water vapor in the earth's atmosphere, and even, 

 as Rayleigh afterwards showed, diminished by the diffuse reflection 

 of the molecules of air themselves. Hence the intensity of the solar 

 beam not only differs from day to day, but increases between sunrise 

 and noon, and decreases between noon and sunset, depending on the 

 length of path of the beam in the atmosphere. Bouguer and Lam- 

 bert, independently, about 1760, had derived an exponential formula 

 connecting the intensities of the entering and outgoing beams with 



1 Address by C. G. Abbot to the Solar Union Conference at the Mount Wilson Solar 

 ' Observatory, California, Wednesday evening, Aug. 31, 1910. 



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