SELBORNE, _Feb._ 7_th_, 1776.
Dear Sir,--In heavy fogs, on elevated situations especially, trees are
perfect alembics; and no one that has not attended to such matters can
imagine how much water one tree will distil in a night's time, by
condensing the vapour, which trickles down the twigs and boughs, so as to
make the ground below quite in a float. In Newton Lane, in October,
1775, on a misty day, a particular oak in leaf dropped so fast that the
cart-way stood in puddles and the ruts ran with water, though the ground
in general was dusty.
In some of our smaller islands in the West Indies, if I mistake not,
there are no springs or rivers; but the people are supplied with that
necessary element, water, merely by the dripping of some large teak
trees, which, standing in the bosom of a mountain, keep their heads
constantly enveloped with fogs and clouds, from which they dispense their
kindly never-ceasing moisture; and so render those districts habitable by
condensation alone.
Trees in leaf have such a vast proportion more of surface than those that
are naked, that, in theory, their condensations should greatly exceed
those that are stripped of their leaves; but, as the former imbibe also a
great quantity of moisture, it is difficult to say which drip most: but
this I know, that deciduous trees that are entwined with much ivy seem to
distil the greatest quantity. Ivy-leaves are smooth, and thick, and
cold, and therefore condense very fast; and besides, evergreens imbibe
very little. These facts may furnish the intelligent with hints
concerning what sorts of trees they should plant round small ponds that
they would wish to be perennial; and show them how advantageous some
trees are in preference to others.
Trees perspire profusely, condense largely, and check evaporation so
much, that woods are always moist; no wonder, therefore, that they
contribute much to pools and streams.
That trees are great promoters of lakes and rivers appears from a well-
known fact in North America; for, since the woods and forests have been
grubbed and cleared, all bodies of water are much diminished; so that
some streams that were very considerable a century ago, will not now
drive a common mill. Besides, most woodlands, forests, and chases, with
us abound with pools and morasses; no doubt for the reason given above.
To a thinking mind few phenomena are more strange than the state of
little ponds on the summits of chalk hills, many of which are never dry
in the most trying droughts of summer. On chalk-hills I say, because in
many rocky and gravelly soils springs usually break out pretty high on
the sides of elevated grounds and mountains: but no person acquainted
with chalk districts will allow that they ever saw springs in such a soil
but in valleys and bottoms, since the waters of so pervious a stratum as
chalk all lie on one dead level, as well-diggers have assured me again
and again.
Now we have many such little round ponds in this district; and one in
particular on our sheep-down, three hundred feet above my house; which,
though never above three feet deep in the middle, and not more than
thirty feet in diameter, and containing perhaps not more than two or
three hundred hogsheads of water, yet never is known to fail, though it
affords drink for three hundred or four hundred sheep, and for at least
twenty head of large cattle beside. This pond, it is true, is overhung
with two moderate beeches, that, doubtless, at times afford it much
supply: but then we have others as small, that, without the aid of trees,
and in spite of evaporation from sun and wind, and perpetual consumption
by cattle, yet constantly maintain a moderate share of water, without
overflowing in the wettest seasons, as they would do if supplied by
springs. By my journal of May, 1775, it appears that "the small and even
considerable ponds in the vales are now dried up, while the small ponds
on the very tops of hills are but little affected." Can this difference
be accounted for from evaporation alone, which certainly is more
prevalent in bottoms? or rather have not those elevated pools some
unnoticed recruits, which in the night time counterbalance the waste of
the day; without which the cattle alone must soon exhaust them? And here
it will be necessary to enter more minutely into the cause. Dr. Hales,
in his Vegetable Statics, advances, from experiment, that "the moister
the earth is the more dew falls on it in a night: and more than a double
quantity of dew falls on a surface of water than there does on an equal
surface of moist earth." Hence we see that water, by its coolness, is
enabled to assimilate to itself a large quantity of moisture nightly by
condensation; and that the air, when loaded with fogs and vapours, and
even with copious dews, can alone advance a considerable and
never-failing resource. Persons that are much abroad, and travel early
and late, such as shepherds, fishermen, etc., can tell what prodigious
fogs prevail in the night on elevated downs, even in the hottest parts of
summer; and how much the surfaces of things are drenched by those
swimming vapours, though, to the senses, all the while, little moisture
seems to fall.