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.

I am, etc.

LETTER XXX.

SELBORNE, _April_ 3_rd_, 1776.

Dear Sir,--Monsieur Herissant, a French anatomist, seems persuaded that
he has discovered the reason why cuckoos do not hatch their own eggs; the
impediment, he supposes, arises from the internal structure of their
parts, which incapacitates them for incubation. According to this
gentleman, the crop, or craw, of a cuckoo does not lie before the sternum
at the bottom of the neck, as in the _gallinae_, _columbae_, etc., but
immediately behind it, on and over the bowels, so as to make a large
protuberance in the belly.

Induced by this assertion, we procured a cuckoo; and, cutting open the
breast-bone, and exposing the intestines to sight, found th