28o The Natural History of Selborne 



of a mountain, keep their heads constantly enveloped with 

 fogs and clouds, from which they dispense their kindly never- 

 ceasing moisture ; and so render those districts habitable by 

 condensation 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 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 advantageous some trees 

 are in preference to others. 1 



Trees perspire profusely, condense largely, and check 

 evaporation 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 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 



* Vide Kalm's " Travels to North America." 



1 It is now well known that dry districts have had their rainfall largely 

 increased by being planted with trees, while districts once moist have been 

 rendered arid and desert by clearing off the timber. ED. 



