202 



THE NATURAL HISTORY 



[LETT. 



the middle, and not more than thirty feet in diameter, and con- 

 taining perhaps not more than two or three hundred hogsheads 

 of water, yet never is it 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-sized 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 con- 

 siderable 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 Vege- 

 table 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, &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. 



SELBORNE, Feb. 7, 1776. 



