NATURAL HISTORY OF SELBORNE. 147 



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. 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 things are drenched by those swimming vapours, though, to 

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



I am, &c. 



LETTEE XXX. 



TO THE SAME. 



SELBORNE, April 3rd, 1776. 



DEAR SIR, Monsieur Herissant, a French anatomist, seems per- 

 suaded 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 gallince, columbce, 

 &c., but immediately behind it, on and over the bowels, so as to make 

 a large protuberance in the belly.* 



Induced by this assertion, we procured a cuckoo ; and, cutting open 

 the breast-bone, and exposing the intestines to sight, found the crop 

 lying as mentioned above. This stomach was large and round, and 

 stuffed hard, like a pincushion, with food, which, upon nice examination, 

 we found to consist of various insects ; such as small scarabs, spiders, 

 and dragon-flies ; the last of which we have seen cuckoos catching on 

 the wing as they were just emerging out of the aurelia state. Among 

 this farrago also were to be seen maggots, and many seeds, which 

 belonged either to gooseberries, currants, cranberries, or some such 

 fruit ; so that these birds apparently subsist on insects and fruits ; nor 

 was there the least appearance of bones, feathers, or fur, to support the 

 idle notion of their being birds of prey. 



The sternum in this bird seemed to us to be remarkably short, 

 between which and the anus lay the crop, or craw, and immediately 

 behind that the bowels against the back-bone. 



It must be allowed, as this anatomist observes, that the crop placed 

 just upon the bowels must, especially when full, be in a very uneasy 

 situation during the business of incubation; yet the test will be to 

 examine whether birds that are actually known to sit for certain are 

 not formed in a similar manner. This inquiry I proposed to myself to 

 make with a fern-owl, or goat-sucker, as soon as opportunity offered : 



* Histoire de I'Acad&nie Eoyale, 1752. 



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