274 



METEOROLOGY AND ALLIED SUBJECTS. 



At noon and during the afternoon notable diminutions of the chemical 

 intensities occur j so for equal solar altitudes the intensity before noon 

 is greater than in the afternoon. 



From all these observations it follows that the diurnal change in 

 chemical intensity is as complicated a function of the solar altitude as 

 is the change in atmospheric moisture, transparency, &c. 



It is almost certain that the intensib^ is directly and principally de- 

 pendent upon the variation of the hygrometric condition. 



EJiowing the normal daily curve of chemical intensity we easily com- 

 pute the annual curve, and the variation with latitude. In this respect 

 Pernter compares the observations of Bunsen and Roscoe, Marchand, 

 who observed daily for four years, and those at Kew and Greenwich, 

 and Southern Europe, and finally Par4, in Brazil. The following table 

 gives a series of relative numbers as observed by Marchand : 



Months. 



January--^ 

 Febmary . 



March 



April . 



May 



June 



July 



August 



September , 

 October ... 

 November. 

 December. 



Annual mean. 



We now come to the question of the ratio of the intensities of the diffuse 

 daylight and the direct sunlight. The results of observation by Bunsen 

 and Eoscoe on the separate values of these intensities are expressed in 

 the equations for H and S above given, and according to which the 

 following table is computed : 



From this table we see in a striking manner that up to an altitude of 

 1(P the chemical intensity of direct sunlight is inappreciable, a result 

 that is abundantly confirmed by more recent observations, and yet 

 direct photometric measures of the ratio between the intensities H and 



