190 NATURAL HISTORY OF S15LBORNE. 
nrnch 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 ovei'flowing 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 seem*s to fall. 
I am, &c. 
LETOER 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 stmcture 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. 
