of Selborne 177 



droughts of summer. On chalk-liills T say, because in 

 many rocky and gravelly soils springs usually break out 

 pretty high on the sides of elevated grounds and moun- 

 tains ; 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 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, with- 

 out overflowing in the wettest seasons, as they would do if 

 supplied by springs. By my journal of May 1775, it ap- 

 pears 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 counterbalance 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 of moist earth." Hence we see 

 that water, by its coolness, is enabled to assimilate to 

 itself a large quantity of moisture nightly by condensation ; 



M 



