NATURAL HISTORY OF SELBORNE. 197 



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 

 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 mountains ; 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. 



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 affords drink for three hundred or four 

 hundred sheep, and for at least twenty head of large cattle 



