PONDS ON CHALK HILLS. 231 



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 on the vales are now 

 dried up, while the small ponds on the very tops 

 of hills are but little affected." Can this differ- 

 ence be accounted for from evaporation alone, 

 which certainly is more prevalent in bottoms ? or 

 rather, have not those elevated pools some unno- 

 ticed recruits, which in the night-time counterba- 

 lance 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, ad- 

 vances, 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-fail- 

 ing 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. 



