PEorEssoE BimsEN ajstd de. h. e. eoscoe’s photo-chemical EESEAECHES. 903 
Table XIII. 
MelTille 
Island. 
EeykiaTik. 
Petersburg. 
Manchester. 
Heidelberg. 
Naples. 
Cairo. 
h m 
6 A.M. or 6 p.M. 
7 A.M. or 5 P.M. 
8 A.M. or 4 P.M. 
9 A.M. or 3 P.M. 
1 0 A.M. or 2 P.M. 
1 1 A.M. or 1 P.M. 
12 A.M. 
2-77 
8*06 
12-61 
16-20 
18-78 
20*32 
20-83 
2-77 
11-32 
18-22 
23-34 
26-76 
28-67 
29*30 
2-77 
12*49 
20*13 
25-64 
29*20 
31*14 
31*74 
2*77 
14*19 
22*81 
28*72 
32*30 
34*10 
34-67 
2*77 
15-09 
24*21 
30*24 
33*74 
35*43 
3.5*91 
2*77 
16-84 
26*77 
32*87 
35-80 
37*20 
37*49 
2*77 
18*59 
29*15 
35*03 
37*58 
38*23 
38-30 
The curves (fig. 11, Plate XLVI.) dra’wn from these numbers show graphically 
the hourly variation in the chemical difiused light on the equinox at the several 
localities. 
We may now proceed to the consideration of the chemical actions produced by 
difiuse light upon the horizontal unit area of earth’s surface during a longer period of 
time. 
Let Wx signify, as before, the action efiected by the light in one minute, when the 
sun’s hour-angle is t ; and let W be the action during a period in which the hour-angle 
increases from to then we have 
AV 
w^dt, 
where the unit in which the hour-angle is expressed is the angle whose arc is equal to 
the radius. We have, however. 
io^r=.a-\-h cos <p-\-c cos^ <p, 
and cos<p=sin^sin^-|-cos^cos^cos^; 
hence when sin S sin p = a, and cos h cos = (3, 
+^c/3^(sin sin 
( 10 .) 
This expression is somewhat simplified, if the action for the whole day, from sunrise to 
sunset, be calculated; for then — and 
and hence 
W= 
24x60 
COS — tan ^ tan^, or = — ^ 
If the calculation be made for the time of the equinoxes, the equation becomes much 
more simple ; for in this case 
^=0, a=0, (5 — COSp, = 
6 c 2 
