146 NATURAL HISTORY OF SELBORNE. 



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 advan- 

 tageous 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 vallies 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 beside. This pond, it is true, is 

 overhung with two moderate beeches, that, doubtless, at times afford it 

 much supply : but then we have others as small, that, without the aid 

 of trees, and in spite of evaporation from sun and wind, and perpetual 

 consumption by cattle, yet constantly maintain a moderate share of 

 water, without overflowing in the wettest seasons, as they would do if 

 supplied by springs. By my journal of May, 1775, it appears that 

 " the small and even considerable ponds in the vales are now dried up, 

 while the small ponds on the very tops of hills are but little affected." 

 Can this difference be accounted for from evaporation alone, which 

 certainly is more prevalent in bottoms'? or rather have not those 

 elevated pools some unnoticed recruits, which in the night time counter- 

 balance the waste of the day ; without which the cattle alone must soon 

 exhaust them ? And here it will be necessary to enter more minutely 

 into the cause. Dr. Hales, in his Vegetable Statics, advances, from 



* Vide Kalm's Travels to North America. 



