300 NATURAL HISTORY 



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 sea- 

 sons, 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 preva- 

 lent 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 ex- 

 periment, 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. 



