THE NATURAL HISTORY OF SELBORNE. 179 
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 chalky 
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 counter- 
balance 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 
