622 
PROFESSOR ROSCOE ON A METHOD OE METEOROLOGICAL 
Daily Mean Chemical Intensities at Manchester, 1863-64. 
(Intensity 1*0 acting for 24 hours = 1000.) 
Date. 
Intensity. 
Date. 
Intensity. 
Date. 
Intensity. 
1863. 
1864. 
1864. 
August, 26 
40-5 
March 19 
36-8 
June 28 .. 
26-6 
27 
29-8 
April 19 
78-6 
29 
26-7 
Sppt. 4 
41-8 
20 
85-3 
30 
64-4 
16 . 
30-8 
June 16 
100-7 
July 1 
61-5 
23 
12*4 
17 
47-2 
19-1 
24 
18-7 
18 
118-7 
4 
51-2 
25 
18-1 
20 
50 '9 
5 
76-2 
28 
29'1 
21 
99-0 
6 
78-9 
Dee. 21 
3-3 
22 
119-0 
7 
39-1 
4*7 
23 
81-4 
8 1 
72-2 
25 
83-0 
9 
83-6 ! 
27 
83-0 
Sept. 26 
48-8 
The remarkable differences observed in the chemical intensity on two neighbouring 
days is shown on fig. 17, in which the curves for the 20th and 22nd June 1864 are 
represented. The integrals for these days are 50 ‘9 and 119’; or the total chemical 
action on the 20th and 22nd June is in the ratio of 1 to 2*34. 
The chemical action of daylight at Manchester at the winter and summer solstice, 
and the vernal and autumnal equinoxes, is clearly seen by reference to the curves on 
fig. 18, in which the actions on September 28, 1863, December 22, 1863, March 19, 1864, 
and June 22, 1864, are represented graphically. These days were chosen out from 
amongst the observations made near the required periods, as being days upon which the 
sun shone most brightly, and as therefore giving the nearest approach to the maximum 
actions for the several periods in question. The integral for the winter solstice is 4*T, 
that of the vernal equinox 36'8, that of the summer solstice is 119, and that of the 
autumnal equinox 29 T. Hence if the total 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. From these numbers, as well as from the curves (fig. 18), it is seen 
that the increase of chemical action from December to March is not nearly so great as 
that from March to June. With the small amount of experimental data which we as 
yet possess upon this subject, it is useless to attempt to give an explanation of the 
probable cause of this difference ; suffice it to say that it does not appear to be mainly 
produced by the absorptive action exerted by the direct sunlight in passing through the 
different lengths of the columns of air which the rays have to traverse on the days in 
question. 
In carrying out a regular series of meteorological observations upon the variation of 
mean daily chemical intensity at any spot, a fair average result may be obtained by a 
much smaller number of observations than is necessary when the object is to indicate 
the rapid changes occurring in the intensity. Thus, for instance, if determinations had 
been made on the following days once every two hours, viz. at 8 b , 10 h a.m., 12 h , 2 h , 4 h , 
