words, by
evaporation. Water is evaporated by the heat of the sun merely, and even
without this heat, in the open air. It is evaporated at very low
temperatures, when fully exposed to the air. Even ice evaporates in the
open air. We often observe in Winter, that a thin covering of ice or
snow disappears from our roads, although there has been no thawing
weather.

In another chapter, we have considered the subject of "Evaporation and
Filtration," and endeavored to give some general idea of the proportion
of the rain which escapes by evaporation. We have seen, that evaporation
proceeds much more rapidly from a surface of water, as a pond or river,
than from a land surface, unless it be fully saturated, and that
evaporation from the water exceeds the whole amount of rain, about as
much as evaporation from the land falls short of the amount of rain.
Thus, by this simple agency of evaporation, the vast quantities of water
that are constantly flowing, in all the rivers of the earth, into the
sea, are brought back again to the land, and so the great system of
circulation is maintained throughout the ages.

As evaporation is greatest from a water-surface, so it is greater, other
things being equal, according to the wetness of the surface of any given
field. If the field be covered with water, it becomes a water-surface
for the time, and the evaporation is like that from a pond. If, as is
often the case, the water stands on it in spots, over half its surface,
and the rest is saturated, the evaporation is scarcely less, and has
been said to be even more; while, if the surface be comparatively dry,
the evaporation is very little.

But what harm does evaporation do? and what has all this scientific talk
to do with drainage? These, my friend, are very practical questions, and
just the ones which it is proposed to answer; but we must bear in mind
that, as Nature conducts her grand affairs by systematic laws, the small
portion of her domain which for a brief space of time we occupy, is not
exempted from their operation. Some of these laws we may comprehend, and
turn our knowledge of them to practical account. Of others, we may note
the results, without apprehending the reasons of them; for it is true--

"There are more things in Heaven and earth, Horatio,
Than are dreamt of in your philosophy."

Discussions of this kind may seem dry, though the subject itself be
moisture. They belong, certainly, to the topic under conside