PERENNIAL PONDS. 
189 
those that are naked, that, in theory, their condensations should 
greatly exceed those that 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 con- 
dense very fast ; and besides ever-greens 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 advantageous 
some trees are in preference to others. 
Trees perspire profusely, condense largely, and check evapora- 
tion so much, that woods are always 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 v^ater 
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 wiU allow that they ever saw springs in such a soil but 
in vaUeys 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 con- 
taining 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 over- hung 
with two moderate beeches, that, doubtless, at times afFord it 
* Vide Kalin's Travels to North-America. 
