244 PROFESSOR FORBES ON THE EXTINCTION OF THE SOLAR RAYS 
screen between the sun and the acti nometer, and observe the effect of all other in- 
fluences besides solar radiation for an equal space of time ; and so on alternately sub- 
tracting algebraically the shade-effect from the sun-effect, so as to get the total in- 
fluence of the solar rays. An example will make this clearer. 
Paris. — M. Arago’s Magnetic Cabinet, June 10, 1833. 
Actinometer. 
Hour. 
Sun or shade. 
Reading. 
Diff. 
Mean shade 
effect. 
Sun-effect 
minus shade- 
effect. 
j 
h m 
S 
B. 2. 
12 26 
0 
shade. 
18-3 
} 
27 
0 
10-9 
27 
28 
0 
0 
o 
10-9 
40*2 
j 29*3 
>--6-7 
36-0 
28 
29 
0 
0 
shade. 
O* G* 
O 4* 
Tf CO 
J - 6-0 
* 
29 
30 
0 
0 
O 
34-2 
64-3 
| 30-1 
>-6-3 
36*4 
30 
31 
0 
0 
shade. 
64-3 
57-7 
j - 6-6 
J 
G. 7. 
32 
0 
shade. 
11-6 
j - 7-2 
33 
0 
4-4 
—> 
33 
34 
0 
0 
o 
4-4 
28-8 
j 24-4 
>-6-85 
31-25 
34 
35 
0 
0 
shade. 
28-8 
22'3 
j - 6-5 
35 
36 
0 
0 
o 
22-3 
46-6 
| 24-3 
>-6-45 
30-75 
36 
37 
0 
0 
shade. 
46-6 
40-2 
| - 6-4 
B. 2. 
38 
39 
0 
0 
shade. 
33-7 
23*5 
| -10-2 
39 
40 
0 
0 
o 
23-5 
50-7 
j 27*2 
>-9-3 
36-5 
40 
41 
0 
0 
shade. 
50-7 
42-3 
| — 8-4 
- 
41 
42 
0 
0 
G 
42-3 
70-7 
j 28-4 
>-8-55 
36-95 
42 
43 
0 
15 
shade. 
70-7 
59-8 
| - 8-7* 
53. The following are the results of various series of observations similarly con- 
ducted. 
* Reduced to l m interval. 
