'The Natural History of Selborne 281 



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 

 the wettest seasons, as they would do if supplied by springs. 

 By my journal of May 1775, i* 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 evapora- 

 tion 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 

 experimept, 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 



