558 Prof. Roscoe — Registration of Action of Daylight, [Dec. 22, 



the curves given represent graphically the results of these douhle observa- 

 tions. From the close agreement of these curves, it is seen that the method 

 is available for practical measurement. 



In order to show that the method can be applied to the purposes 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 graphically by a curve. 



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

 ever, when the object was either to control the accuracy of the measure- 

 ment or to record the great changes which suddenly occur when the sun 

 is obscured or appears from behind a cloud, the determinations 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 chemi- 

 cal 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 a];)parent change in 

 the amount of light could be noticed by the eye. The remarkable ab- 

 sorptive 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 hourly intensities, giving the daily mean chemical intensities of the 

 place, a rough method of integration is resorted to : this consists in deter- 

 mining the weights of the areas of paper inscribed between the base-line 

 and the curve of daily intensity, that chemical action being taken as 1000 

 which the unit of intensity would produce if acting continuously for 

 twenty-four hours. The remarkable differences observed in the chemical 

 intensity on two neighbouring days is noticed on the curves for the 20th 

 and 22nd of June 1864: the integrals for these days are 50"9 and 119, 

 or the chemical actions on these days are in the ratio of 1 to 2*34. 



The chemical action of light at Manchester was determined at the winter 

 and summer solstices, and the vernal and autumnal equinoxes : the results 

 of these measurements are seen by reference to the accompanying curves. 

 The integral for the winter solstice is 4* 7, that of the vernal equinox 36*8 ; 

 that of the summer solstice 119, and that of the autumnal equinox 29' 1. 

 Hence, if the chemical action on the shortest day be taken as the unit, 

 that upon the equinox will be represented by 7, and that upon the longest 

 day by 25. 



