190 NATURAL HISTORY OF SELBORNE. 



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 overflowing in the wettest 

 seasons, as they would do if supplied by springs. By my jour- 

 nal of May 1775, it appears that "the small and even considera- 

 ble ponds in the vales are now dried up, while the small ponds 

 on the very tops of hills are but little affected." Can this dif- 

 ference 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 counter- 

 balance 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 vapours, and even with copious dews, can 

 alone advance a considerable and never-failing resource. Per- 

 sons 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 things are drenched by 

 those swimming vapours, though, to the senses, all the while, 

 little moisture seems to fall. 



I am, &c. 



LETTER XXX. To THE HON. DAINES BARRINGTON. 



DEAR SIR, Selborne, April 3, 1776. 



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 gallin<e, columba, &c. but immediately behind it, 



