168 WHITE 



the wettest seasons, as they would do if supplied by springs. 

 By my journal of May 1/75, 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 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 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 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 vapors, 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, etc., 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 vapors, though, to the 

 senses, all the while, little moisture seems to fall. 



I am, etc. 



LETTER XXX 



SELBORNE, April yd, 1776. 



DEAR SIR, Monsieur Herissant, a French anatomist, seems 

 persuaded that he has discovered the reason why cuckoos do 

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

 from the internal structure of their parts, which incapacitates 

 them for incubation. According to this gentleman, the crop, 

 or craw, of a cuckoo does not lie before the sternum at the 

 bottom of the neck, as in the gaUma, cohimba, etc., but 

 immediately behind it, on and over the bowels, so as to make 

 a large protuberance in the belly. 



