252 
PROFESSOR FORBES ON THE EXTINCTION OF THE SOLAR RAYS 
der the proportion which the masses of air obliquely traversed,, and having a certain 
mean dampness, bear to the shorter ones with a different dose of moisture. The 
average dampness of the whole mass of air penetrated will be 
sum of (masses of air x dampness of each) 
sum of masses of air 
If c> be the ratio to saturation at any hour, cq the mean ratio required, and x the 
varying mass of air as before, we shall have 
ZxJ 
S X 
(5.) 
68. The following Table, resuming the results of Table E, contains these quantities. 
Table F, 25th Sept. 1832. 
Apparent 
time. 
1 
V 
log—. 
V 
X . 
log — 
V 
m= . 
X 
Dampness 
s. 
x S. 
h m 
mm. 
8 15 
1-075 
•0312 
450-8 
•0000692 
•512 
230-8 
9 
1-141 
•0573 
352-8 
•0001624 
•530 
187-0 
10 
1-214 
•0841 
286-8 
2932 
•489 
140-2 
11 
1-345 
•1287 
255-8 
5031 
•484 
123-8 
12 
1-219 
•0860 
245-8 
3499 
•452 
111-1 
1 
1-078 
•0327 
254-5 
1285 
•451 
114-8 
2 
1-207 
•0815 
284-6 
2864 
•526 
149-7 
3 
1-219 
•0860 
350-4 
2454 
•617 
216-2 
4 
1-206 
•0814 
500-6 
1626 
•620 
310-4 I 
4 30 
1-321 
•1209 
659-5 
1833 
•685 
451-7 
Sums 
•7898 
3641-6 
2035-7 
The solution is 
*7 898 
W = 3641-6 x 760 = "164831, 
M =1 ‘46161, 
\y] = -68418, 
Mean dampness of inter- 
cepted column cij 
69. Hence it appears that, on the hypothesis of uniform opacity, a standard atmo- 
sphere of 760 mm , or 29‘922 English inches of mercurial pressure, and having a mean 
dampness or ratio to saturation represented by -56 nearly, would transmit 68^ per cent., 
or stop 31^ per cent, of the incident heating rays, an estimate which agrees nearly 
with the mean of the results of Bouguer and Lambert, and mentioned above, and 
very closely with the separate and indirect results obtained by Professor Kamtz alone 
on this very occasion*. This confirmation is interesting, perhaps unexpected, as 
the present is I believe the very first direct measurement of the kind. 
j = -559013. 
* His estimate is -68, not deduced from this day’s observations alone, but from an extensive series. 
