270 The Natural History of Selborne 



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 soils springs usually 

 break out pretty high on the sides of elevated grounds and moun- 

 tains : but no person acquainted with chalky districts will allow that 

 they ever saw springs in such a soil but 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. 



* 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. 



