268 



METEOEOLOGY AND ALLIED SUBJECTS. 



the earth ; indeed still more important, since the latter is genernllj' con- 

 ditioned ui)on the former. My present object will be attained if it gives 

 occasion to the more careful study of the changes in the weather and 

 the anomalies in our zone as compared with the temperature conditions 

 of the lower latitudes. {Z. 0. G. M., XIV., 1879, p. 40.) 



The total intensity of daylight has been investigated by Stelling, who 

 has applied Roscoe's photochemical method of observation, depending 

 on the law that the darkening upon chloride of silver paper is in pro- 

 portion to the product of the intensity of the light and the duration of the 

 exposure. We take the following abstract from a review of his work 

 by Pernter. Stelling's method of determining a scale for measuring 

 the amount of the discoloration consisted in simplj^ exposing various 

 pieces of prepared paper to the influence of the action of daylight during 

 periods of time whose duration was very accurately determined. 



The comparison of liis own results with those of Bunsen and Eoscoe 

 was accomplished by means of a sheet of normal black that Stelling 

 received from Eoscoe himself. The coincidence of the normal black with 

 the scale adopted by Stelling was at the point 158, and this })oint was 

 determined weekly during the entire series of observations in order to 

 allow for any change that might take i)lace in the position of the nor- 

 mal point. In this work freshly prepared slips of paper were always 

 employed, in view of the fact that Eoscoe had shown that in nearly all 

 cases an irregular bleaching of the prepared papers took place during 

 six or eight weeks, but that after this time the black tint remained un- 

 changed for many months. Stelling finds that the dryness and the age 

 have little influence, but the method of silvering is imi)ortaut, and that 

 the sih^ering must be done immediately after the filtration. Stelling's 

 practical application of his results to meteorology relate esi)ecially to the 

 question, '• What influence has the cloudiness ui)on the total intensity 

 of daylight?" To this end he first determines the intensity upon clear 

 days, or those on which the cloudiness does not exceed 5 per cent. The 

 observed normal intensity is indicated by the following table of obser- 

 vations at St. Petersburg : 



1874. Intensity. 



November 3 0. 041 



November 18 0. 038 



December 31 0.027 



1875. 



January 24 0. 038 



February 13 0. 082 



March 5 0.113 



1875. Intensity. 



March 16., 0.100 



April 4 0.201 



May 2 0.204 



May 19 0.359 



June 1 0. 440 



June 21 0. 352 



July IG 0.300 



Julv29 0.257 



The condition of the sky is divided by him into three portions, namely : 

 first, as partially cloudy; second, sufficiently cloudy to hide the sun; 

 third, comx^letely covered, uniform gray sky. 



