198 NATURAL HISTORY OF SELBORNE. 



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 main- 

 tain 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 prevalent in 

 bottoms 1 or rather, have not those elevated pools some un- 

 noticed 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 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, etc., 

 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. 



