Surron—ZInfluence of Water- Vapour upon Nocturnal Radiation. 21 
are attributable to the partial cessation of the evaporation from 
‘skin and lungs; but the thermometrie effects, such as the 
diminution of the daily range of temperature under a clear 
‘sky, which becomes very noticeable when the relative humidity 
is high, can be due only to strong absorption of the long- 
waved terrestrial radiations; and it is interesting to note that 
the difference between the absorption of liquid and vaporous 
water lies chiefly in the greater absorption of longer waves by 
the former.’ 
Again, “the direct effect of the Sun’s rays upon a normal sur- 
face is less in the tropics than in temperate regions, and less at sea- 
level than upon a mountain-top, owing to the difference in the 
aqueous component of the air; and the ability of the solar radiation 
to maintain a high temperature in the torrid zone or at sea-level is 
due to the accumulation of the thermal energy imparted to the 
Harth’s surface by reason of the retention of the escaping radiation 
from that surface by a moist and highly absorbent atmosphere 
rather than to the direct power of the sunbeam. . . . The penetrative 
power of the incoming is greater than that of the outgoing rays; 
and this relative difference, which increases with the amount of 
moisture in the air, produces an accumulation of thermal energy 
_at the Harth’s surface which would generate a very high tempera- 
ture were it not that the sign of the function is reversed after 
sundown. .. . The heat entrapped through the differential trans- 
mission of solar and terrestrial radiation by aqueous vapour and 
carbon dioxide is mainly stored in the lower layers of the atmo- 
sphere, and because the absorption of air heavily loaded with 
moisture is nearly complete for its own radiation.’” 
J. H. Poynting and J. J. Thomson take practically the same 
view as Very :—“ One result obtained by Tyndall is that water- 
vapour has a very considerable absorption for dark radiations—a 
result which was contradicted by other experimenters. The 
various methods, however, by which Tyndall obtained evidence of 
the absorption leave little doubt that his experiments warranted 
1F. W. Very, ‘‘ Atmospheric Radiation,’’ p. 100. 
* Ibid., pp. 125, 126. I find a good deal of difficulty in following Very’s argument 
sometimes, particularly in discerning the dividing line between experimental and 
speculative results. 
