904 PEOrESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEinCAl EESEAECHES. 
or W— 12x60 
or substituting the values of a, 5, and c, 
W=1998-7+37058cos^-16559cos> 
By applying this formula for the places given in Table XIII., we obtain the folloTving 
numbers as representing the chemical action, expressed in degrees of light, which, at the 
time of the equinoxes, the whole heaven effects on the unit of area in each of these 
places, from sunrise to sunset:— 
Table XIII 
Melville Island 10590 
Reykjavik 15020 
St. Petersburg 16410 
Manchester 18220 
Heidelberg............. 19100 
Naples ... 20550 
Cairo 21670 
We need scarcely remark that these numbers, as well as the data from which they are 
obtained, only hold good for skies free from cloud and mist, and for situations not too 
far distant from the sea’s level. In places which lie nearer to the higher portions of the 
atmosphere, it is evident that the conditions will be totally different. The law accord- 
ing to which the atmospheric diffused light varies with the height above the sea, might 
in like manner be determined empirically, but such a series of experiments entail an 
expenditure of time and money which we have unfortunately been unable to afford. 
The chemical action of diffused sunlight becomes extremely irregular when either 
clouds or mists intercept the clear blue sky. The chemically active light, which imder 
these circumstances reaches the earth’s surface, varies as irregularly as do the other 
meteorological phenomena dependent on the thermic and hygrometric state of the 
atmosphere. Without at present entering into an explanation of a method by which, 
taking cognizance of these circumstances, photo-chemical determinations may be made, 
we will here give one example to show the variable nature of the diffuse light from a 
clouded sky. The following Table contains the results of a series of obseiwations made 
at Heidelberg on October 5, 1856. The columns marked I. give the time of obser- 
vation ; those marked II. the amount of light expressed in units of light radiated from 
a portion of sky at the zenith equal to the whole sky. 
Table XIV. 
I. 
II. 
I. 
II. 
I. 1 II. 
I. 
11. 
I. 
II. 
h m 
7 24 A.M. 
7 54 A.M. 
8 2 A.M. 
8 36 A.M. 
8 42 A.M. 
9 0 A.M. 
617 
670 
963 
1165 
1314 
1687 
h m 
9 12 A.M. 
9 15 A.M. 
9 30 A.M. 
9 50 A.M. 
9 56 A.M. 
10 2 A.M. 
1537 
1469 
1686 
1797 
1631 
1934 
h m 
10 47 A.M. 
10 53 a.m. 
11 0 A M. 
11 6 A.M. 
11 31 A.M. 
11 37 A.M. 
1881 
1731 
1881 
1921 
2274 
2231 
li m 
1 1 38 A.M. 
0 4 p.M. 
1 1 P.M. 
1 7 P.M. 
211 p.M. 
2 22 p.M. 
1650 
1660 
1494 
1475 
675 
836 
h m 
2 28 P.Af. 
3 20 p.M. 
3 27 P.M. 
3 33 P.M. 
4 16 p.M. 
4 41 p.M. 
765 
370 
450 
445 
304 
188 
