352 PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
Tension of vapour. 
•888. 
•849. 
•805. 
•749. 
•708. 
•659. 
•605. 
•554. 
•435. 
Fall of temperature from 1 
6 h 40 m p.m. to 5 11 40 m a.m. J 
0 
6-0 
O 
7-1 
00 
O 
8-5 
10°*3 
12° 6 
O 
121 
O 
13 T 
16° 5 
These results, if correct, and I am not aware that they have ever been questioned, 
show in the most impressive manner the influence of the aqueous vapour of our atmos¬ 
phere on our planet’s radiation. As the vapour diminishes, the door opens, which 
permits the escape of the earth’s heat. The halving of the vapour tension nearly 
trebles the refrigeration of the thermometer. 
Equally clear is the evidence given by General Strachey as to the action of aqueous 
vapour upon the radiation of the sun. Here are the results :—- 
Tension of vapour. 
■824. 
•737. 
•670. 
•576. 
•511. 
•394. 
Rise of temperature from 5 h 40™ A.M. 1 
to l h 40 m p.m. .. .. .. J 
12° 4 
15-1 
19° 3 
22° 2 
24° 3 
27°0 
This table is the exact complement of the last. There the fall of temperature was 
powerfully promoted by the withdrawal of the vapour. Here the rise of temperature 
is powerfully promoted by the same cause. * 
But the most impressive illustration of the action of aqueous vapour is now to be 
referred to. In 1865 I subjected to examination the radiation from the electric light 
produced by a battery of 50 of Grove’s cells, and found, by prismatic analysis, the 
invisible calorific radiation to be 7*7 times the visible. The determination was after¬ 
wards made by the method of filtration, whereby the one class of rays was detached 
with great sharpness from the other, and both of them rendered measurable. By this 
method the invisible radiation was found to be 8 times the visible. A close agree¬ 
ment was therefore established between the results of the two methods. Computed 
from the diagram of Muller the invisible radiation of the sun is twice the visible. 
This smaller ratio might, of course, be referred to the original quality of the solar 
* Mr. Hill, Meteorological Reporter for the North-West Provinces of India, in a paper recently presented 
to the Royal Society describes an attempt to determine the “ Constituent of the Atmosphere which absorbs 
Radiant Heat.” He uses for this purpose the careful observations made by Messrs. J. B. N. Hennessey 
and W. H. Cole, at Mussoree and Dehra respectively. From the absence of symmetry in the quantities 
of heat received by the actinometer on both sides of noon at Mussoree, and from the existence of this 
symmetry at Dehra, he infers the periodic lifting and lowering of the absorbing constituent above and 
below the higher station. He finds the variation of the absorption coefficient to follow the variations of 
vapour tension. From this and from a similar result obtained by a second method of calculation, he draw T s 
the conclusion “ that there can be very little error in agreeing with Dr. Tyndall that the absorptive 
power of dry air is sensibly nothing, and that the total absorptive power of the atmosphere is due to the 
water vapour it contains.” A most interesting discourse on Solar Heat, by M. Violle, of Grenoble, will 
be found in the Revue Scientifique for 1878, p. 944. I guard myself against saying that the diather¬ 
mancy of dry air is perfect. 
