NATURAL HISTORY OF SELBORNE 167 



greatest quantity. Ivy leaves are smooth, and thick, and cold, 

 and therefore condense very fast ; and besides, evergreens im- 

 bibe 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 evap- 

 oration 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 wood-lands, 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 hogs- 

 heads 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 



