soil into a drain without its place being supplied by air, unless there
is more water to supply it; so that drainage, in this way, manifestly
promotes the permeation of air through the soil.

But it is claimed that drains may be made to promote circulation of air
in another way, and in dry times, when no water is flowing through them,
by connecting them together by means of a header at the upper ends, and
leaving an opening so that the air may pass freely through the whole
system. Our friend, Prof. Mapes, is an advocate for this practice, and
certainly the theory seems well supported. It is said that in dry, hot
weather, when the air is most highly charged with moisture, currents
thus passing constantly through the earth, must, by contact with the
cooler subsoil, part with large quantities of moisture, and tend to
moisten the soil from the drains to the surface, giving off also with
the moisture whatever of fertilizing elements the air may bear with it.

This point has not escaped the notice of English drainers. Mr. J. H.
Charnock, an assistant commissioner under the Drainage act, in 1843,
read a paper in favor of this practice, but in 1849 he published a
second article in which he suggests doubts of the advantages of such
arrangements, and says he has discontinued their application. He says
they add to the cost of the work, and tend to the decay of the pipes,
and to promote the growth into the pipes, of any roots that may approach
them.

Mr. Parkes, in a published article in 1846, speaks of this idea, but
passes it by as of very little importance. Mr. Denton quotes the
authority of some of his correspondents strongly in favor of this
theory. After trying some experiments himself upon clay soil, he admits
the advantages of such an arrangement for such soil, in the following
not very enthusiastic terms:

"It will be readily understood that as clay will always contract rapidly
under the influence of a draught of air, in consequence of the rapid
evaporation of moisture from its surface, one of the benefits of
draining is thus very cheaply acquired; and for the denser clays it may
possibly be a desirable thing to do, but in the porous soils it would
appear that no advantage is gained by it."

Yet, notwithstanding this summary disposition of the question in
England, it is by no means clear, that in the tropical heat of American
summers, when the difference between the temperature of the air and the
subsoil is so much g