IN PASSING THROUGH THE ATMOSPHERE. 
241 
column contains the zenith distance ; the second, its secant ; the third, the refraction 
g g 
by Ivory’s Table ; the fourth, the thickness by the formula 58 /i. j6 sin ^ ; the fifth, the 
corresponding - mass of air in millimetres of mercury; the sixth, the thickness by 
Bouguer’s Table. 
Table of Atmospheric Thicknesses corresponding- to different Elevations. 
f “ 0 Refraction ) 
= 58"-36 x sinZ. D.J 
Z. D. 
Sec. Z. D. 
Refraction. 
B. 
B x 760 mm . 
B'>uguer’s 9 
formula. 
O / 
0 
1-0000 
"o-oo 
1-0000 
760-0 
1-0000 
10 
1-0154 
10-30 
1-0164 
772-4 
1-0153 
20 
1-0642 
21-26 
1-0651 
809-5 
1-0642 
30 
1-1547 
33-72 
1-1556 
878-2 
1-1547 
40 
1-3054 
48-99 
1-3060 
992-5 
1-3050 
50 
1-5557 
69-52 
1-5550 
1181-7 
1-5561 
60 
2-0000 
100-85 
1-9954 
1516-4 
1-9903 
70 
2-9238 
159-16 
2-9023 
2205-8 
2-8998 
75 
3-8637 
214-70 
3-8087 
2894-7 
3-8046 
77 30 
4-6202 
257-74 
4-5237 
3438-0 
80 
5-7588 
320-19 
5-5711 
4234-0 
5-5600 
82 30 
7-6613 
418-59 
7-2343 
5498-1 
85 
11-4737 
593-96 
10-2165 
7762-7 
10-2002 
86 
14-3356 
707-43 
12-1512 
9234-9 
12-1401 
87 
19-1073 
866-76 
14-8723 
11303-0 
14-8765 
88 
28-6537 
1101-35 
18-8825 
14350-7 
19-0307 
89 
57-2987 
1466-8 
25-1374 
19104-4 
25-8067 
90 
infinite 
2072- 
35-5034 
26982-4 
35-4955 
Section IV. — Account of the following Observations. 
46. In 1832 Sir John Herschel was good enough to direct my attention to the im- 
portant use which might be made of his actinometers, to find the loss of solar radiation 
by simultaneous observations at the top and bottom of a mountain ; and furnished 
with two instruments marked G. 7 and B. 2, which had formerly been used by Captain 
Foster in the arctic regions, I made very numerous observations in Switzerland and 
elsewhere that summer*. 
47- We have already seen that, upon certain postulates (such as the uniform opacity 
of the air), the diminution of the effect of solar radiation in passing through the atmo- 
sphere may be ascertained by observations at any station, at varying altitudes. But 
it is certainly very interesting to test and confirm this indirect result by simultaneous 
observations at different heights in the atmosphere. By this means too, the influence 
of the meteorological conditions of a column of air directly under experimentmay be 
immediately ascertained. Balloon observations would theoretically be the most satis- 
factory, but they appear to offer practical difficulties, in this case nearly insuperable. 
Simultaneous observations at the top and bottom of a high insulated mountain were 
* See Note A. at the end of this paper on the Scale of the Actinometers. 
MDCCCXLII. 2 I 
