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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



where </> is the zenith angle. For the radiation E to the whole sky, 

 we consequently have : 



£ = 360! J d<f> = 360 



f'2 



R cos <f> sin <j>d(f> 



(2) 



•90 60 30 30 60 9C 



Zenith distance. 



Fig. i ib. — Radiation to different parts of the sky. Curves I, II: Mt. Whit- 

 ney, 1913. Water-vapor pressure; 3.6 and 1.5 mm. Hg. Curve dotted, 

 Bassour, 1912. Water-vapor pressure; 5 mm. Hg. Temperature of instru- 

 ment higher at Bassour. Compare table IX. 



This integration can conveniently be effected in a mechanical way 

 by measuring the areas given by ( i ) . The curves that represent the 

 radiation from a horizontal surface to different parts of the sky are 

 shown in figure 12. The whole areas included between the curves 

 and the ;r-axis must be proportional to the total radiation. In 

 measuring the areas we must take into consideration the fact that the 



