12 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 60 



of all three days with this instrument indicated that the scattered 

 solar radiation received from the sky was in excess of the long-wave 

 radiation lost to the sky. 



The bolometric observations at different parts of the sky were 

 reduced by graphical methods so as to give the mean brightness of 

 the various zones, and these were summed up with regard to their 

 relative areas, so as to give the total effect of the sky on a horizontal 

 surface. The values given are for noon observations, and are stated 

 in calories per square centimeter per minute. In the daylight obser- 

 vations with the nocturnal radiation apparatus the sun was screened 

 away by a broom held at about 2 meters distance. The results 

 represent therefore the whole sky except a part very close to the 

 sun. The results of both kinds of observations are collected below : 



Table 3. Radiation of the Sky. Bassour, 1912. 



a. BOLOMETRY OF SKY RADIATION. 



Direct solar beam in calories. 

 Fraction added by the sky . . . 

 Sky radiation in calories 



1.25 

 0.174 



0.217 



MEASUREMENTS OF SKY RADIATION BY NOCTURNAL APPARATUS. 



Before sun-rise 



Noon 



After sun-set 



Total sky radiation in calories 



calories. 

 —O.194 

 + O.067 

 —0.2l8 

 O.273 



Although the results show a large divergence in percentage, it is 

 not great in calories ; and the mean result of all experiments, namely 

 0.245 calories per square centimeter per minute, probably represents 

 the total sky radiation to within 0.05 calories. 



We will now give, as a mean for the three days, noon measure- 

 ments of the three quantities of radiation: (1) That which reaches 

 the earth in the direct beam of the sun. (2) That which reaches a 

 horizontal surface by scattering from the whole sky. (3) That 

 which is absorbed from the direct beam of the sun and from the sky 

 radiation by the vapors of the atmosphere. 



These quantities are as follows : 



