Surron—Influence of Water- Vapour upon Nocturnal Radiation. 19 
to be the true interpretation of my results, it may not be amiss if 
some notice be taken of comparatively recent opinion :— 
W. M. Davis remarks that ‘“‘it has been supposed that the 
air was a better absorber of terrestrial radiation than of solar 
radiation; and thus the atmosphere has been compared to a trap 
which allowed sunshine.to enter easily to the Harth’s surface, but 
prevented the free exit of radiation from the Earth ; water-vapour 
in particular was thought to be very active in this selective process. 
The general temperature maintained by the atmosphere has been 
explained largely on these suppositions; but recent observations 
throw grave doubts upon both of them. Clear air allows the 
coarse-waved radiation from the Earth an easy outward passage. 
Water-vapour is, like clear air, a poor absorber of nearly all kinds 
of waves. It is true that the presence of excessively fine water- 
particles, sufficient only to make the air faintly hazy, greatly 
diminishes its power of transmission or diathermanence; but water- 
vapour—that is, water in the gaseous state—is found by experiment 
to be as poor an absorber as pure dry air.' The temperature of 
the air is, therefore, now explained as a result of its own absorp- 
tion and radiation, largely aided by suspended dust and by certain 
processes. . . . ”—such as conduction and convection.* Davis 
remarks further that “it is only water in the liquid state [7.e. in 
the form of haze or cloud] that exerts a strong control over radia- 
tion from the Earth. . . . If the temperature of the air is well 
above saturation, the range is relatively strong; if near saturation, 
the range is diminished even though no visible clouding of the 
sky occurs; if a thin, hazy cloud is formed, the range is greatly 
reduced.’”* 
1 Of. G. Magnus, “ On the Propagation of Heat in Gases’’: Phil. Mag., August, 
1861, pp. 1, 85:—* Although it might with certainty be predicted that the small 
quantity of aqueous vapour which air can take up at the ordinary temperature (not 2 
per cent. of its volume at 16° C.) could exercise no influence on the radiation, it 
appeared desirable to determine experimentally that this supposition was correct. . . - 
These experiments show that the water present in the atmosphere at 16° C. exercises 
no perceptible influence on the radiation. That such an influence should be felt as 
soon as part of the vapour separates as fog, appears very probable” (pp. 105-106). 
2 W. M. Davis, “‘ Elementary Meteorology,” 1894, p. 32. 
3 Ibid., p. 145. 
