198 PONDS ON THE SUMMITS OF CHALK HILLS. 



containing pernaps not more than two or three hundred hogs- 

 heads 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 besides. 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 maintain a moderate share of water, without over- 

 flowing 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 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 w r aste 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 an equal surface of moist earth." 

 Hence we see that water, by its coolness, is enabled to assimi- 

 late 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 

 earl}'' 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. 



* Fogs are much more frequent in cold seasons, and cold countries, than 

 in such as are warm ; because, in the former, the aqueous particles, being 

 condensed almost as soon as they proceed from the surface of the earth, are 

 incapable of rising into the higher portions of the atmosphere. If the 

 oold be augmented, the fog freezes, attaching itself in small icicles to the 

 branches of trees, and to the hair and clothes of persons exposed to it, to 

 the bJades of grass, and other substances, ED. 



