of Total Dayliyhl at Kew and Para in 1865-67. 313 



"On the Chemical Intensity of Total Daylight at Kew and Para 

 in 1865-67." By Henry E. Roscoc, F.R.S. 



This communication contains the results of a regular series of 

 measurements of the chemical action of daylight, carried out at 

 the Kew Observatory, through the kindness of Dr. Balfour Stewart, 

 according to the method described by the author in the Philoso- 

 phical Transactions for 1864, p. 605. Tne observations extend 

 over a period of two years, from April 1, 1865 to March 31, 1867. 

 The second part of the communication gives the results of ob- 

 servations upon the Intensity of the Chemical action of Sunlight 

 under the Equator, made at Para in latitude 1° 28' S. during 

 the month of April 1866. 



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, show- 

 ing 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-mentioned paper, are given. The first result which these ob- 

 servations yield is that the mean chemical intensity for hours equi- 

 distant from noon is constant ; that is, the mean chemical inten- 

 sities are equal for equal altitudes of the sun ; thus the mean of 

 all the observations made about 9 h 30 m A.M. corresponds with 

 the mean intensity at 2 h 30 m r.M. 



Mean of Times Mean Chern. 

 of* Observation. Intensity. 



Mean of 552 Morning Observations") Qll - 1m n , 1nr 



• i n r* " rthr r ... J 'i-L U J.UO 



m 186o-67. J 



Mean of 529 Afternoon Observations 1 2 ii 07m 0-107 



in 1865-67. J '" 



Hence the author concludes that, when the disturbing causes of 

 variation in amount of cloud &c. are fully eliminated by a suffi- 

 cient number of observations, the daily maximum of chemical in- 

 tensity corresponds to the maximum of sun's altitude. The author 

 then shows, from measurements made at varying altitudes of the 

 sun at Heidelberg and Para, that the relation between sun's alti- 

 tude and chemical intensity may be represented by the equation 



CI a = CI -f const. a, 



where CI a represents the chemical intensity at a given altitude 

 (o) in circular measure, CI the chemical intensity at the alti- 

 tude 0, and const, (a) a number to be calculated from the obser- 

 vations. 



The agreement of the chemical intensities as found at Heidel- 

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



Phil. Mag. S. 4. Vol. 31. No. 230. Oct. 1867. Y 



