202 
THE NATURAL HISTORY 
[LETT. 
the middle, and not more than thirty feet in diameter, and con- 
taining perhaps not more than two or three hundred hogsheads 
of water, yet never is it 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 ])ond, it is true, is overhung 
with two moderate-sized 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 con- 
siderable ponds in the vales are now dried up, while the small 
ponds on the very tops of liills 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 Vege- 
table 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 
w^ater, 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, &c., 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. 
Selborne, Feh. 7, 1776. 
