ants and vines with mats or
boards, or even with thin cloth, and thus protect them from frost. If
the covering touch the plants, they are often frozen, the heat being
conducted off, by contact, to the covering, and thence radiated. Dew
then is an effect, but not a cause, of cold. It imparts warmth, because
it can be deposited only on objects cooler than itself.

It has been supposed by many that the light of the moon promotes
putrefaction. Pliny and Plutarch both affirm this to be true. Dew, by
supplying moisture in the warm season, aids this process of decay. We
have seen that dew is most abundant in clear nights; and although all
clear nights are not moonlight nights, yet all moonlight nights are
clear nights; and this, perhaps, furnishes sufficient grounds for this
belief, as to the influence of the moon.

The quantity of dew deposited is not easily measured. It has, however,
been estimated by Dr. Dalton, to amount, in England, to five inches of
water in a year, or 500 tons to the acre, equal to about one quarter of
our rain-fall during the six summer months!

Deep and well-pulverized soils attract much more moisture, in every
form, from the atmosphere, than shallow and compact soils. They, in
fact, expose a much larger surface to the air. This is the reason why
stirring the ground, even in the Summer drought, refreshes our fields of
Indian corn.

CHAPTER XVII.

INJURY OF LAND BY DRAINAGE.

Most Land cannot be Over-drained.--Nature a Deep
drainer.--Over-draining of Peaty Soils.--Lincolnshire Fens; Visit
to them in 1857.--56 Bushels of Wheat to the Acre.--Wet Meadows
subside by Drainage.--Conclusions.

Is there no danger of draining land too much? May not land be
over-drained? These are questions often and very naturally asked, and
which deserve careful consideration. The general answer would be that
there is no danger to be apprehended from over-draining; that no water
will run out of land that would be of advantage to our cultivated crops
by being retained. In other words, soils _generally_ hold, by capillary
attraction, all the moisture that is of any advantage to the crops
cultivated on them; and the water of drainage would, if retained for
want of outlets, be stagnant, and produce more evil than good.

We say this is generally true; but there are said to be exceptional
cases, which it is proposed to consider. If we bear in mind the
condition of most soils in Summer, we shall