350 NATURAL HISTORY 



but then we have others as small, that, 

 without the aid of trees, and in spite of 

 evaporation from sun and wind, and perpe- 

 tual 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 in the 

 " vales are now dried up, while the small 

 ponds on the very tops of hills are but 

 little aifected." 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; with- 

 out which the cattle alone must soon ex- 

 haust them ? And here it will be necessary 

 to enter more mirmtely 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 



