SOLAR EADIATION ABBOT 



179 



stir pyrheliometers. Our findings with these instruments are ex- 

 pressed as the so-called *■' Smithsonian Pyrheliometry of 1913," 

 which has been accepted quite generally as the standard pyrhelio- 

 metric scale of the world. It differs by about 3 per cent from the 

 Angstr()m scale. Experiments by independent methods are now in 

 j)rogress in Germany to further establish the true pyrheliometric 

 scale. 



THE STANDARD WATER-FLOW PYRHELIOMETER 



In the standard water-flow pyrheliometer, the solar rays admitted 

 by a measured aperture are chieflj^ absorbed on a hollow blackened 

 metallic cone at the rear of a test-tube-shaped blackened metallic 

 chamber. A measured current of water continually removes the 

 heat produced on the cone and on the inner walls of the chamber. 



Figure 3. — Diagram of the wator-flow pyiheliometpr 



A A, ray absorber ; B B, vestibule ; C, measured aperture ; Di D2, electrical thermometer ; 

 E F, entering and emerging ^\ater flow; G H, electrical test coils; K K, Dewar 

 vacuum flask 



An electrical thermometer measures the rise of temperature thus 

 produced in the water current. For test purposes, known quantities 

 of heat may be introduced at the cone by measured electrical cur- 

 rents. The accuracy of the instrument, which is very satisfactory, 

 is measured by the equivalence of heat introduced and heat found. 

 The instrument is represented in Figure 3. 



THE FUNDAMENTAXf SOLAR-CONSTANT METHOD 



The fundamental solar-constant method as worked out by Langley 

 involves determining the intensity of all parts of the solar spectrum 

 repeatedly on a day of unchanging clearness, so as to disclose the 

 increase of intensity of each spectral ray which occurs as the sun 

 mounts higher and higher. For a ray of homogeneous wave length, 

 the intensity is connected with the length of path in the atmosphere 

 by the exponential formula of Lambert and Bouguer : 



6=6^11^; or log e=m log a-{-log c„ 



Here e is the observed intensity; e^ that which would be observed oiitside 

 the atmosphere ; a is the fraction transmitted with vertical sun ; and m is the 



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