REGISTRATION OE THE CHEMICAL ACTION OE TOTAL DAYLIGHT. 621 
1864, when the whole sky was apparently unclouded throughout the day; at 9 h 25' a.m. 
the chemical intensity was found to be (M3; at 10 h , without any visible change in the 
light, the chemical action sank to 007, and continued at this point for more than half 
an hour, rising again to (Ml at 11 o’clock. That this diminution of the chemical 
activity arises from the presence of mist, or of suspended particles of water imper- 
ceptible to the eye, is rendered probable by the very powerful absorptive action which 
a light haze or mist exerts upon the chemical rays. Thus on March 18, 1864, the 
action at 8 l1 a.m., when a light mist obscured the sun, amounted to 00026, whereas the 
normal action for that day and hour, with an unclouded sky, is twenty-five times as 
large. It is scarcely necessary to remark that on this occasion the ratio of decrease of 
visible luminosity was not nearly so great. The same absorptive action of mist is well 
seen in the following measurements on September 27 and 28, 1864. 
September 
27, clear sun. 
September 28, 
, sun obscured by haze. 
Time. 
Intensity. 
Weather. 
Time. 
Intensity. 
Weather. 
h m 
10 0 A.M. 
0-13 
Clear sky and direct sun. 
h m 
10 0 A.M. 
0-016 
Hazy. 
10 30 
0-17 
10 30 
0-039 
„ 
11 0 
0-18 
11 0 
0-053 
„ 
11 30 
0-13 
11 30 
0-075 
„ [pearing. 
12 40 p.m. 
0-16 
„ 
12 0 
0-042 
Sunshine, haze gradually disap- 
1 10 
0-13 
12 45 p.m. 
0-056 
1 40 
0-17 
: i o 
0-053 
„ 
2 10 
0-14 
„ 
1 30 
0-10 
Haze gone. 
2 15 
0-12 
For the purpose of expressing the relation of the sums of all these various hourly 
intensities, giving the daily mean chemical intensity of the place, a rough, but sufficiently 
accurate method of integration may be resorted to. This consists simply in cutting 
the curves out in strong homogeneous paper or cardboard, and in determining in each 
case the weights of the paper enclosed between the base-line and the curve. A por- 
tion of the paper of given size is cut out between every four or five curves, and the 
small variations in weight caused by irregularity in the thickness of the paper thus 
allowed for. 
In the following Table the numbers are compared with the action, taken as 1000, 
which would be produced by light of the intensity 1 acting uniformly throughout the 
twenty-four hours. 
4 Q 
MDCCCLXV. 
