THE CUCKOO. 217 



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 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 evaporarion 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 experi- 

 ment, that " the moister the earth is, the saore 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 coobiess, 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 

 teU what prodigious fogs prevail in the night on elevated 

 downs, even ia 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. 



LETTEE LXXII. 



TO THE SAME. 



Selborne, April 3, 1770. 

 Deab Sir, — Monsieur Herissant, a French anatomist, seems 

 persualed that he has discovered the reason why cuckoos * 

 do not hatch theii* own eggs ; the impediment, he supposes, 



• The cuckoo is the largest of insectivorous birds, and luust requne a great 



