898 PEOFESSOE BTOSEN AXD DE. H. E. EOSCOE’S PHOTO-CHEMICAE EESEAECHES. 
different nights may, by care in the selection of nights, be reduced to within yery narrow 
limits.” 
Hence we think that we are justified in setting aside the alterations in the extinction 
of the cloudless sky, and in considering the chemical illumination of the earth’s surface 
as being merely a function of the sun’s zenith-distance. !\Ve are at the same time 
perfectly well aware that the constants contained in our formulae, derired as they are 
from a comparatively small number of observations, cannot lay claim to any very great 
degree of accuracy. Still we believe, and we hope in the sequel to show, that the amount 
of reliability which our experiments present is sufficient to permit us to deduce there- 
from the empirical laws according to which the chemical energy proceeding from the 
sun is diffused over the earth’s surface when the sky is unclouded. 
The following formula gives the value of the zenith distance (<p) for the observed times 
of experiment : 
cos (p— cos § . cos t cosj:>+ sin ^ . sin^j (5.) 
where S signifies the sun’s declination on the day of the experiment (-{-22° 39' 30"); 
j? gives the latitude of the place (49° 24' 25"), and t represents the sun’s hour-angle. 
In the following Table the observations of June 6, 1858, are arranged with the cor- 
responding zenith-distance of the sun. The first column contains the times of obseiTa- 
tion in true solar time ; the second gives the sun’s hour-angle corresponding to the time 
of observation; the third contains the corresponding zenith-distances of the sun; and 
the fourth shows the amount of light which the unit of area receives from the whole 
sky, compared with that which the same area receives at the same time ffiom a portion 
of sky at the zenith equal in area to the of the whole heavens. 
Table VII. 
Xiunber of 
experiment. 
Time oi 
I. 
observation. 
II. 
t honr-angle of 
the sun. 
III. 
0 zenith distance 
of the sun. 
rv. 
I. 
1 
h 
5 
m 
38 
A.M. 
O 
95 
30 
76 
25 
762 
2 
6 
8 
A.M. 
88 
0 
71 
44 
738 
3 
7 
25 
A.M. 
68 
45 
59 
19 
637 
4 
8 
52 
A.M. 
47 
0 
45 
24 
481 
5 
9 
32 
A.M. 
37 
0 
39 
28 
432 
6 
10 
9 
A.M. 
27 
45 
34 
31 
405 
7 
10 
43 
A.M. 
19 
15 
30 
45 
360 
8 
11 
26 
A.M. 
8 
30 
27 
34 
324 
9 
0 
1 
P.M. 
0 
15 
26 
47 
307 
10 
0 
32 
P.M. 
8 
0 
27 
29 
330 
11 
0 
57 
P.M. 
14 
15 
29 
1 
360 
12 
1 
24 
P.M. 
21 
0 
31 
27 
407 
13 
2 
4 
P.M. 
31 
0 
36 
11 
429 
14 
2 
38 
P.M. 
39 
30 
40 
54 
455 
15 
3 
21 
P.M. 
50 
15 
47 
26 
522 
16 
3 
57 
P.M. 
69 
15 
53 
10 
590 
17 
4 
42 
P.M. 
70 
30 
60 
28 
652 
18 
5 
20 
P.M. 
80 
0 
66 
38 
683 
19 
6 
2 
P.M. 
90 
1 
30 
73 
18 
755 
