of Edinburgh^ Session 1863-64. 
203 
2. On the Protection of Vegetation from Destructive Cold 
every night. By Professor W. Thomson. 
The effect of dew in protecting vegetation every clear still night 
of summer was long ago pointed out by Dr Wells ; the correctness 
and acuteness of whose views on this subject have been generally 
recognised. The hypothesis recently put forth by Dr Tyndall, that 
absorption of radiant heat by aqueous vapour in the atmosphere is 
an effective defence against destructive degrees of cold, and the 
ready acceptance yielded to it by some of our highest authorities 
in the popular promulgation of the truths of science, seem to 
render it necessary to recall attention to Dr Wells’ admirable work. 
In the first place, when Dr Tyndall announces, as a result of his 
experiments on radiant heat, that, “ It is perfectly certain that 
more than ten per cent, of the terrestrial radiation from the soil of 
England is stopped within 10 feet of the surface of the soil,” by 
the absorption it suffers from aqueous vapour; it must be remarked 
that this absorption cannot go on at the same rate through any 
great thickness of air. For at the same rate half the radiant heat 
would be absorbed in 70 feet; J in 140 feet; in 210 feet, and so 
on, which is inconsistent with known facts ; as, for instance, the 
influence of clouds on terrestrial radiation. Hence the quality of 
rays which passes through the lowest 10 feet of air suffers less than 
ten per cent, of absorption in the next 10 feet; and it is quite 
certain that after passing through several times 10 feet of air, the 
radiant heat must, by having been deprived of the part of it liable 
to absorption by aqueous vapour, be in a condition in which not 
one per cent, is absorbed from it in its passage through 10 feet of 
clear air. Whatever influence true vapour of water really does 
exercise in checking, by its absorption, the loss of heat by radiation 
from the earth’s surface, it is, even in the most humid conditions of 
optically clear atmosphere, insufficient to prevent heavy dews ; far 
less than the latent heat of which, taken from the blades of grass, 
or other finer parts of plants, would leave them destroyed by frost. 
In point of fact, therefore, heat actually is radiated away into 
very high terrestrial atmosphere and distant interstellar air or 
