he leaves cannot
fall much below the original dew-point because the supply of water for
condensation is kept up; but if the compensation for loss of heat by
radiation is dependent simply on the condensation of water from the
atmosphere, without renewal of the supply, the dew-point will gradually
get lower as the moisture is deposited and the process of cooling will
go on.

In these questions we have to deal with comparatively large changes
taking place within a small range of level. It is with the layer a few
inches thick on either side of the surface that we are principally
concerned, and for an adequate comprehension of the conditions close
consideration is required. To illustrate this point reference may be
made to figs. 1 and 2, which represent the condition of affairs at 10.40
P.M. on about the 20th of October 1885, according to observations by
Aitken. Vertical distances represent heights in feet, while the
temperatures of the air and the dew-point are represented by horizontal
distances and their variations with height by the curved lines of the
diagram. The line marked 0 is the ground level itself, a rather
indefinite quantity when the surface is grass. The whole vertical
distance represented is from 4 ft. above ground to 1 ft. below ground,
and the special phenomena which we are considering take place in the
layer which represents the rapid transition between the temperature of
the ground 3 in. below the surface and that of the air a few inches
above ground.

[Illustration: FIG. 1.]

[Illustration: FIG. 2.]

The point of interest is to determine where the dew-point curve and
dry-bulb curve will cut. If they cut above the surface, mist will
result; if they cut at the surface, dew will be formed. Below the
surface, it may be assumed that the air is saturated with moisture and
any difference in temperature of the dew-point is accompanied by
distillation. It may be remarked, by the way, that such distillation
between soil layers of different temperatures must be productive of the
transference of large quantities of water between different levels in
the soil either upward or downward according to the time of year.

These diagrams illustrate the importance of the warmth and moisture of
the ground in the phenomena which have been considered. From the surface
there is a continual loss of heat going on by radiation and a continual
supply of warmth and moisture from below. But while the heat can escape,
the moistur