NATURAL HISTORY OF SELBORNE. 209 



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 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 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 ; and that the air, when loaded 

 with fogs and vapours, and even with copious dews, can alone 

 advance a considerable and never-failing resource. Persons that 

 are much abroad, and travel early and late, such as shepherds, 

 fishermen, etc., can tell what prodigious fogs prevail in the night 



