200 NATURAL HISTORY OF SELBORNE. 
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 chalky districts will 
allow that they ever saw springs in such a soil but in valley 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 con- 
stantfy 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 smalland even con- 
siderable ponds in the vales are now dried up, while the smali 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 ex- 
periment, 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 
* Vide Kalm’s Travels to North America. 
