146 
NATURAL HISTORY OF SELBORNE. 
exceed those tliat are stripped of their leaves ; but, as the former imbibe 
also a great quantity of moisture, it is difficult to say which drip most ; 
but this I know, that deciduous trees that are entwined with much ivy 
seem to distil the greatest quantity. Ivy-leaves are smooth, and thick, 
and cold, and therefore condense very fast ; and besides, evergreens 
imbibe very little. These facts may furnish the intelligent with hints 
concerning what sorts of trees they should plant round small ponds 
that they would wish to be perennial ; and show them how advan- 
tageous some trees are in preference to others. 
Trees perspire profusely, condense largely, and check evaporation so 
much, that woods are alwaj^s moist ; no wonder, therefore, that they 
contribute much to pools and streams. 
That trees are great promoters of lakes and rivers appears from a 
well-known fact in North America ; for, since the woods and 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 vallies 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 alfords 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 Vegetable Statics, advances, from 
* Vide Kalm's Travels to North America. 
