course a
considerable--probably by far the larger,--part of the heat taken up in the
process of evaporation is furnished by the air; but the amount abstracted
from the soil is great, and is in direct proportion to the amount of water
removed by this process; hence, the more we remove by draining, the more
heat we retain in the ground.

The season of growth is lengthened by draining, because, by avoiding the
cooling effects of evaporation, germination is more rapid, and the young
plant grows steadily from the start, instead of struggling against the
retarding influence of a cold soil.

*Temperature.*--The temperature of the soil has great effect on the
germination of seeds, the growth of plants, and the ripening of the crops.

Gisborne says: "The evaporation of 1 lb. of water lowers the temperature
of 100 lbs. of soil 10 deg.,--that is to say, that, if to 100 lbs. of soil,
holding all the water it can by attraction, but containing no water of
drainage, is added 1 lb. of water which it has no means of discharging,
except by evaporation, it will, by the time that it has so discharged it,
be 60 deg. colder than it would have been, if it had the power of discharging
this 1 lb. by filtration; or, more practically, that, if rain, entering in
the proportion of 1 lb. to 100 lbs. into a retentive soil, which is
saturated with water of attraction, is discharged by evaporation, it
lowers the temperature of that soil 10 deg.. If the soil has the means of
discharging that 1 lb. of water by filtration, no effect is produced
beyond what is due to the relative temperatures of the rain and of the
soil."

It has been established by experiment that four times as much heat is
required to evaporate a certain quantity of water, as to raise the same
quantity from the freezing to the boiling point.

It is, probably, in consequence of this cooling effect of evaporation,
that wet lands are warmest when shaded, because, under this condition,
evaporation is less active. Such lands, in cloudy weather, form an
unnatural growth, such as results in the "lodging" of grain crops, from
the deficient strength of the straw which this growth produces.

In hot weather, the temperature of the lower soil is, of course, much
lower than that of the air, and lower than that of the water of warm
rains. If the soil is saturated with water, the water will, of course, be
of an even temperature with the soil in which it lies, but if this be
drained off, warm air