peoi:essoe bunsen and de. h. e. eoscoe’s photo-chemical EESEAECHES. 909 
degrees of light, which the sun shining freely upon the insolation-vessel would have 
produced if no disturbing influences were acting, is obtained from the direct observations 
of our instrument {Wq) by means of the following formula : — 
njNA 
10000 
( 11 .) 
The subjoined Table (XVI.) gives a series of observations made, as just described, on 
the 3rd of August, 1857, and on the 14th and 15th of September, 1858, the sky being 
perfectly cloudless and the gas having reached the maximum degree of sensibility. In 
the series of observations made on August 3, 1857, in which two mirrors were employed, 
namely, the heliostat mirror of speculum metal and another mirror of steel, the data 
for the calculation of A and S were as follows 
X=17"-34; X,=1894"-6; ^;=72° 50'; MEi=45° 7'; ^=+17° 29'; _p=49° 24'. 
In the experiments made on September 14 and 15, 1858, ui which only one mirror 
was used, the data were 
^ — 21"’22; Xj — 1912"; MRi=73° 44'; S=-|-3° 26' for the 14th, and 
^=+3° 4' for the 15th; ^™49° 24'. 
Table XVI. 
! August 3, 1857. Barometer=0™'7560. 
I. 
II. 
III. 
IV. 
V. 
True time. 
Sun’s zenith- 
Observed 
Wo. 
8. 
distance. 
action, vj^. 
h m 
7 59 
57 35 
8*70 
63-13 
0-384 
8 42 
50 51 
12-11 
89-21 
0-378 
9 14 
46 8 
12-56 
92-96 
0-376 
September 14, 1858. Barometer=0‘"*7550. 
8 1 
t 
68 34 
8-84 
26-23 
0-63 f 
i 
September 15, 
1858. Barometer=0“-7562. 
I 7 9 
76 30 
1-52 
5-54 
0-637 
7 26 
73 49 
4-22 
15-50 
0-633 
7 40 
71 37 
6-09 
22-43 
0-631 
8 0 
68 34 
7-56 
27-85 
0-631 
8 7 
67 30 
8-38 
38-87 
0-631 
8 26 
64 42 
12-48 
45-85 
0-633 
8 54 
60 48 
17-09 
62-59 
0-634 
9 14 
58 11 
18-51 
67-61 
0-636 
The increase in the chemical action on diminution of the sun’s zenith distance, as 
seen in the numbers of the second and fourth columns, arises from the extinction of the 
chemical lays effected by passage through the atmosphere. In order to obtain from 
these obsei’vations the law of this extinction, we may, without sensible error, disregard 
MDCCCLIX. 6 D 
