The Natural History of Selborne 271 



Now we have many such little round ponds in this district ; and 

 one in particular on our sheep-down, three hundred feet above my 

 house ; which, though never above three feet deep in the middle, 

 and not more than thirty feet in diameter, and containing perhaps 

 not more than two or three hundred hogsheads of water, yet never 

 is 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 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 con- 

 stantly 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 " 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, &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. I am, &c. 



