d and the air. If we place a wet cloth on the head, and hasten
the evaporation of the water by fanning, we cool the head; if we wrap a
wet napkin around a pitcher of water, and place it in a current of air,
the water in the pitcher is made cooler, by giving up its heat to the
evaporating water of the napkin; when we sprinkle water on the floor of a
room, its evaporation cools the air of the room.

So great is the effect of evaporation, on the temperature of the soil,
that Dr. Madden found that the soil of a drained field, in which most of
the water was removed from below, was 6-1/2 deg. Far. warmer than a similar
soil undrained, from which the water had to be removed by evaporation.
This difference of 6-1/2 deg. is equal to a difference of elevation of 1,950
feet.

It has been found, by experiments made in England, that the average
evaporation of water from wet soils is equal to a depth of _two inches per
month_, from May to August, inclusive; in America it must be very much
greater than this in the summer months, but this is surely enough for the
purposes of illustration, as two inches of water, over an acre of land,
would weigh about _two hundred tons_. The amount of heat required to
evaporate this is immense, and a very large part of it is taken from the
soil, which, thereby, becomes cooler, and less favorable for a rapid
growth. It is usual to speak of heavy, wet lands as being "cold," and it
is now seen why they are so.

If none of the water which falls on a field is removed by drainage,
(natural or artificial,) and if none runs off from the surface, the whole
rain-fall of a year must be removed by evaporation, and the cooling of the
soil will be proportionately great. The more completely we withdraw this
water from the surface, and carry it off in underground drains, the more
do we reduce the amount to be removed by evaporation. In land which is
well drained, the amount evaporated, even in summer, will not be
sufficient to so lower the temperature of the soil as to retard the growth
of plants; the small amount dried out of the particles of the soil, (water
of absorption,) will only keep it from being raised to too great a heat by
the mid-summer sun.

An idea of the amount of heat lost to the soil, in the evaporation of
water, may be formed from the fact that to evaporate, by artificial heat,
the amount of water contained in a rain-fall of two inches on an acre,
(200 tons,) would require over 20 tons of coal. Of