NATURAL HISTORY OF SELBORNE 103 



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, ever-greens 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. 



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. 1 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 



1 Vide Kalm's Travels to North- America. 



