314 Royal Society : — 



Altitude. Chemical Intensity. 



; Found. Calculated. 



7 15 . 0-050 0-050 



24 43 0-200 0-19(5 



84 34 0-306 0276 



53 37 0437 0435 



62 30 ,...0-518 ,.0-506 



A similar relation is found to hold good for the Para observa- 

 tions. Assuming the same relation to exist at Kew as at Heidel- 

 berg 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 integrals of chemical intensity for each 

 month, from April 1865 to March 1867 inclusive, have been ob- 

 tained. Curves exhibiting the daily rise and fall for each of the 

 twenty-four months, as well as a curve showing the biennial varia- 

 tion of chemical intensity for the same period, accompany the paper. 

 The curve of yearly chemical intensity is found to be un symme- 

 trical about the vernal and autumnal equinoxes ; thus in spring 

 and autumn the results are as follows : — 



1865 & 1867. Mean Ch. Lit, 



March 1867 30'5 



April 1865 97-8 



September 1865 107-8 



August 1865 88-9 



1866. Mean Ch. Int. 



March 34-5 



April 52-4 



September 70*1 



August 94*5 



Or for 100 chemically active rays falling during the months of 

 March and April 1865, 1866, and 1867 at Kew there fell in the 

 corresponding autumnal months 167 rays, the sun's mean altitude 

 being the same. 



The author discusses the probable causes of this autumnal maxi- 

 mum ; he finds that it is not due to variation in the amount of 

 cloud, and believes that it is to be explained by a les3 amount of 

 atmospheric opalescence in the autumn than in the spring. 



The yearly integral for the twelve months January to March 

 1867 and April to December 1865, is 55'7, whereas that for the 

 twelve months of the year 1866 is 54-7. 



II. 'Para Observations. 



All the knowledge we possess concerning the distribution and 

 intensity of the chemically active rays in the tropics is derived 

 from the vague statements of photographers. According to their 

 observations it appears that the difficulty of obtaining a good pho- 

 tograph increases as we approach the equator ; and more time is 

 said to be needed to produce the same effect upon a sensitive 

 plate under the full blaze of a tropical sun than is required in 

 the gloomier atmosphere of London. Thus in Mexico, where 

 the light is very intense, from twenty minutes to half an hour is 

 stated to be required to produce photographic effects which in 

 England occupy but a minute. Hence the existence of a pecu- 



