OF SELBORNE 177 



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 wiU 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 litde 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 over-hung 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 hiUs 

 are but little affected." Can this difference be accounted 

 for from evaporation alone, which certainly is more preva- 

 lent 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 experiment, 

 that " the moister the earth is the more dew falls on it in 

 a night : and more than a double quantity of dew falls 

 on a surface of water than there does on an equal surface 



M 



