THE OTJCKOO. 217 



spite of evaporation from sun and wind, and perpetual con- 

 sumption by cattle, yet constantly maintain a moderate 

 share of water, without overiiowing in the wettest seasons, 

 as they would do if supplied hy 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 waste of the day, without 

 which thfe 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 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 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 thiugs are drenched by those swimming 

 vapours, though to the senses all the while Uttle moisture 

 seems to fall. 



LETTER LXXII. 



TO THE SAME. 



Selborne, April 3, 1776. 

 Deae Sib, — Monsieiu: Herissant, a French anatomist, seems 

 persuaded that he has discovered the reason why cuckoos * 

 do not hatch their own eggs ; the impediment, he supposes, 



* The cuckoo is tlie largest of insectivorous birds, and must require a great 



