104 NATURAL HISTORY OF SELBORNE 



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 pre- 

 valent 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 it's 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, &c., can tell what prodigious 

 fogs prevail in the night on elevated downs, even in the 

 hottest parts of summer ; and how much the surfaces of 

 things are drenched by those swimming vapours, though, 

 to the senses, all the while, little moisture seems to fall. 



I am, &c. 



