.12 



Prof. Eoscoe on the Chemical Intensity 



[June 2Q, 



I. Kew Observations. 



The Kew measurements do not profess to exhibit the changes in 

 chemical intensity which occur from hour to hour, but they give, with 

 accuracy, the mean monthly chemical intensity, showing the rise and 

 fall with the changing seasons of the year, and they enable us to deduce 

 the mean monthly and yearly chemical intensities at Kew for 1865-67. 



Tables showing the daily mean chemical intensity obtained from the 

 daily observations, according to the method described in the above-men- 

 tioned paper, are given. The first result which these observations yield 

 is that the mean chemical intensity for hours equidistant from noon is 

 constant ; that is> the mean chemical intensities are equal for equal alti- 

 tudes of the sun; thus the mean of all the observations made about 

 9^ BO"" A.M. corresponds with the mean intensity at 2^' 30°" P.M. 



Mean of Times Mean Cheui. 

 of Observation. Intensity. 



Mean of 529 Afternoon Observations" 



in 1865-67. 

 Mean of 552 Morning Observations 



in 1865-67. 



Hence the author concludes that when the disturbing causes of varia- 

 tion in amount of cloud, &c. are fully eliminated by a sufficient number 

 of observations, the daily maximum of chemical intensity corresponds to 

 the maximum of sun's altitude. The author then shows from measure- 

 ments made at varying altitudes of the sun at Heidelberg and Para, 

 that the relation between sun's altitude and chemical intensity may be 

 represented by the equation 



OI«==CIo-r const. < 



where CI„ represents the chemical intensity at a given altitude {a) in 

 circular measure, CIq the chemical intensity at the altitude 0, and 

 const, (a) a number to be calculated from the observations. 



■The agreement of the chemical intensities as found at Heidelberg 

 with the calculated results is seen in the following Table : — 



Altitude. 



Chemical Intensity. 





round. 



Calculated. 



7 15 



0-050.. 



0-050 



24 43 



0-200.. 



0-196 



34 34 



0-306.. 



0-276 



53 37 



437.. 



0-435 



62 30 



0-518.. 



0-506 



A similar relation is found to hold good for the Para observations. 

 -Assuming the same relation to eiist at Kew as at Heidelberg and Para, 

 the values of the mean monthly intensity at noon have been calculated 

 .from the observations at 2.30 and 4.30 p.m., and the mean monthly in- 

 tegrals of chemical intensity fol* each monthj from April 1865 to March 



.O'^ 41^^^ 0-105 

 2^27™ 0107, 



