238 NATURAL HISTORY 
though never above three feet deep in the middle, 
and not more than thirty feet in diameter, and con- 
taining, perhaps, not more than two or three hun. 
dred 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 besides. ‘This pond, it is true, is over- 
hung by 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 over- 
flowing 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 on 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 nighttime 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 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 
