of trees, from which the constant evaporation of water
goes on, is very great. The consequence is that trees may be regarded as
pumping-engines. It is from this cause that foresters have noticed that
clay lands are apt to become wetter after the trees growing upon them
have been cut down.[42]

_Capacity for Heat in Soils._

A property which depends largely on those we have just been considering
is the capacity soils possess of absorbing and retaining heat.[43] The
temperature of a soil, of course, largely depends on the temperature of
the air; but this, we must not forget, depends also on the soil itself.
The heat given forth by the sun's rays strikes the soil, with the result
that, while so much of its heat is absorbed, a certain portion--and this
will vary according to the nature of the soil--of its heat is radiated
into the air.

The changes in the temperature of the soil naturally take place more
slowly than the changes in the temperature of the air; the depth of soil
thus affected by those changes varies also in different climes. It has
been calculated that in temperate climes the changes of temperature
occurring from day to night are not felt much below three feet down.

_The Explanation of Dew._

We have, it may be stated, generally two processes going on. During the
day the soil is engaged in absorbing its heat from the sun's rays; when
night comes, and the sun goes below the horizon, the air is chilled
below the temperature of the soil, which radiates out its stored-up heat
into the air. The result is that the temperature of the soil is soon
reduced below the temperature of the air, and the moisture, present in
the air in the form of vapour, coming in contact with the cold surface
of the earth, is condensed into dew, which is deposited, and is seen
best early in the morning before the sun has had time to evaporate it
again. Dew is most abundant in summer-time, for the reason that the
difference in temperature of the day and night is then greatest. In
winter-time it is seen as hoar-frost.

_Heat of Soils._

The temperature of a soil, however, is due to other sources than the
sun's rays. Whenever vegetable matter decays, there is always a certain
amount of heat generated. Soils, therefore, in which there is a large
amount of decaying vegetable matter, are certain to receive more heat
from this source than soils of more purely mineral nature.

_Heat in Farmyard Manure._

A good example of the a