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Royal Society : — 



of actual registration, the author gives the results of determinations 

 of the varying intensity of the chemical action of total daylight at 

 Manchester on more than forty days, made at the most widely 

 differing seasons of the year. These measurements reveal some of 

 the interesting results to which a wide series of such measurements 

 must lead. They extend from August 1863 to September 1864 ; and 

 Tables are given in which the details of observations are found, 

 whilst the varying chemical intensity for each day is expressed graphi- 

 cally by a curve. 



Figure showing curves of daily chemical intensity at Manchester, 

 in spring, summer, autumn, and winter. 



As a rule, one observation was made every half-hour ; frequently, 

 however, when the object was either to control the accuracy of the 

 measurement or to record the great changes which suddenly occur 

 when the sun is obscured or appears from behind a cloud, the deter- 

 minations were made at intervals of a few minutes or even seconds. 



Consecutive observations were carried on for each day for nearly a 

 month, from June 16th to July 9th, 1864 ; the labour of carrying 

 out these was not found to be very great, and the results obtained are 

 of great interest. By reference to the Tables, it is seen that the 

 amount of chemical action generally corresponds to the amount of 

 cloud or sunshine as noted in the observation ; sometimes, however, a 

 considerable and sudden alteration in the chemical intensity occurred 

 when no apparent change in the amount of light could be noticed 

 by the eye. The remarkable absorptive action exerted upon the 

 chemically active rays by small quantities of suspended particles of 

 water in the shape of mist, or haze, is also clearly shown. For the 

 purpose of expressing the relation of the sums of all the various 



