METEOROLOGY AND ALLIED SUBJECTS. 



2G9 



In the first condition, for a, cloudiness of from .1 to .7, Stelling con- 

 cludes that such partial cloudiness exercises no perceptible influence 

 upon the intensity. For the second condition, the sky .9 cloudy or 

 less, he finds that the obscuration of the sun always brings a lowering 

 of the intensity, which on the average amounts to 30 per cent. 



For the third condition, the heavens completely covered with the 

 uniform gray tint, he finds that this condition lowers the intensity of 

 the sunlight, on the average, to less than one-half. In general, all these 

 measures of the effect of cloudiness depend on an unknown factor, 

 namely, the thickness and density of the clouds themselves. In gen- 

 eral, the minima occur when fog or rain is associated with the horizontal 

 stratus clouds. 



The following table shows the monthly maxima and minima: 



This table of maxima shows that only in two months did the maxi- 

 mum occur on days with perfectly clear sky ; in all other cases the sky 

 was more or less cloudy and in December completely covered. In the 

 cloudy season Stelling frequently observed similar cases in which high 

 intensities occurred during cloudy weather, a i)henomenon which still 

 awaits future explanation. The following table shows the mean 

 intensity of the light for each month at St. Petersburg, so far as the 

 observations extend : 



1874. Monthly mean. 



November 0. 035 



December 0. 017 



1875. 



January 0. 027 



February 0. 0G4 



1875. Monthly mean. 



March 0. 120 



April 0. 103 



May 0.277 



June 0. 292 



July 0. 227 



It is evident that the photochemical and the photometric methods 

 must be combined with the thermometric in order to attain a complete 

 determination of the absorption in the earth's atmosphere. {Z. 0. G. 

 il/., 1879, XIV, p. 48.) 



The memoir of Wiener on the distribution of the solar radiation over 

 the earth's surface, which was published by him in 187G in Carlsruhe, 



