72 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



stronger than the outgoing effective temperature radiation. The 

 same was indicated by the nocturnal instrument, which, on two 

 different occasions, showed, in one case no appreciable radiation in 

 any direction, and in the other case a faint positive radiation from 

 the sky. If we correct for the reflection of the bright strip the two 

 instruments seem to be in general agreement with each other, show- 

 ing the radiation from the sky to be positive in the middle of the day, 

 under the conditions of the place. Lo Surdo found the same to be 

 the case at Naples, where he observed during some summer days. 

 On the other hand, Homen's observations at Lojosee in Finland, 

 show that there the radiation during the daytime had the direction 

 from earth to sky, and that consequently the effective temperature 

 radiation was stronger (and very much stronger) than the incoming 

 diffused light. The observations of the two observers are naturally 

 in no way contradictory. The total radiation during the daytime is 

 a function of many variables, which may differ largely from place to 

 place. It is dependent on the effective temperature radiation to the 

 sky. This radiation is probably about the same in different lati- 

 tudes, a circumstance which will be discussed below ; the effect of 

 the higher temperature in low latitudes being counterbalanced by 

 a high humidity. Thus we must seek the explanation in the behavior 

 of the other important term, the scattered skylight. The strength 

 of this light is dependent upon the diffusing power of the atmosphere : 

 the molecular scattering and the scattering by dust, smoke, and other 

 suspended particles in the air. For a not too low transmission of the 

 air, the intensity of the skylight must increase with a decrease in the 

 transmission power, so that the skylight is intense when the solar 

 radiation is feeble, and vice versa. 



There is nothing to indicate that the scattering power of the atmos- 

 phere is larger as a rule in low latitudes than at high ones, and I am 

 therefore inclined to think that we ought not to ascribe the high 

 intensity of the skylight in low latitudes to that cause. But the in- 

 tensity of skylight is affected by another important factor — the 

 height of the sun above the horizon. The nearer the sun approaches 

 the zenith, the more intense must be the light reaching us from the 

 diffusing atmosphere. The theory of scattered skylight, with due 

 consideration of the so-called " self-illumination " of the sky, has 

 been treated in a very interesting and remarkable paper by L. V. 

 King. 1 In his paper King gives curves and equations representing 



x Phil. Trans. Roy. Soc. London, Ser. A, Vol. 212, pp. 375~433- 



