osphere.

The question was very fully studied by Melloni and others, but little
more was added to the explanation given by Wells until 1885, when John
Aitken of Falkirk called attention to the question whether the water of
dewdrops on plants or stones came from the air or the earth, and
described a number of experiments to show that under the conditions of
observation in Scotland, it was the earth from which the moisture was
probably obtained, either by the operation of the vascular system of
plants in the formation of exuded dewdrops, or by evaporation and
subsequent condensation in the lowest layer of the atmosphere. Some
controversy was excited by the publication of Aitken's views, and it is
interesting to revert to it because it illustrates a proposition which
is of general application in meteorological questions, namely, that the
physical processes operative in the evolution of meteorological
phenomena are generally complex. It is not radiation alone that is
necessary to produce dew, nor even radiation from a body which does not
conduct heat. The body must be surrounded by an atmosphere so fully
supplied with moisture that the dew-point can be passed by the cooling
due to radiation. Thus the conditions favourable for the formation of
dew are (1) a good radiating surface, (2) a still atmosphere, (3) a
clear sky, (4) thermal insulation of the radiating surface, (5) warm
moist ground or some other provision to produce a supply of moisture in
the surface layers of air.

Aitken's contribution to the theory of dew shows that in considering the
supply of moisture we must take into consideration the ground as well as
the air and concern ourselves with the temperature of both. Of the five
conditions mentioned, the first four may be considered necessary, but
the fifth is very important for securing a copious deposit. It can
hardly be maintained that no dew could form unless there were a supply
of water by evaporation from warm ground, but, when such a supply is
forthcoming, it is evident that in place of the limited process of
condensation which deprives the air of its moisture and is therefore
soon terminable, we have the process of distillation which goes on as
long as conditions are maintained. This distinction is of some practical
importance for it indicates the protecting power of wet soil in favour
of young plants as against night frost. If distillation between the
ground and the leaves is set up, the temperature of t