PONDS ON HILLS. 233 



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 jour- 

 nal of May, 1775, it appears that " the small and 

 even considerable ponds on the vales are now dried 

 up, while the small ponds on the very tops of hills 

 are but little affected." Can this difference be ac- 

 counted 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 ex- 

 haust 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 an equal surface of moist earth." Hence we 

 see that water, by its coolness, is enabled to assimi- 

 late to itself a large quantity of moisture nightly by 

 condensation ; arid that the air, when loaded with 

 fogs and vapours, and even with copious dews, can 

 alone advance a considerable and never-failing re- 

 source. 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 ele- 

 vated 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. 



