22G NATURAL HISTORY 



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 con- 

 sumption 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 arc but little affected." Can this 

 difference be accounted for from evaporation alone, which 

 certainly is more prevalent in bottoms ? or rather, have 

 not thoso 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 watT than there docs 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 cv 7 en 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 va- 

 pours, though, to the senses, all the while, little moisture 

 seems to fall. 



