LXXl.] 
OF SELBORNE. 
201 
moisture ; and so render tliose districts habitable by condensa- 
tion alone. 
Trees in leaf have such a vast proportion more of surface than 
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 dii]) 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 intelb'gent with hints concerning 
what sorts of trees they should plant round small ponds that 
they w^ould wish to be perennial ; and show them how advan- 
tageous some trees are in preference to others. 
Trees perspire profusely, condense largely, and check eva- 
poration 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 w^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 wdth chalky 
districts will allow tliat they ever saw springs in such a soil, but 
only 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 w^e 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 
^ Vide Kalni's Travels in North America. 
