GROUND BUILDERS. 29 
exhaled, because it is sooner reached by the heat 
emanating from the interior of the earth. The ex- 
istence of this interior heat is still more obviously 
proved by the water of springs, which, on its first 
issuing through the sand, will not freeze, even in 
severe frost, till it is cooled down by exposure to 
the cold atmosphere, when it freezes as readily as 
the water of the next pond. 
These facts will be found, as we proceed, to be 
closely connected with the subject of birds’ nests 
built on the ground; and on that account we may 
also mention that the interior temperature of the 
earth is more uniform than the surface, which is 
exposed to the alternate influence of the sun and 
the cold air of the night; a circumstance of no little 
importance in the hatching of eggs. That birds 
have a very correct notion of the requisite temper- 
ature for hatching, is strikingly proved by the os- 
trich (Struthio camelus),and several sea birds, which. 
only sit on their eggs during the night or in moist or 
gloomy weather, and at other times leave them al- 
together to the influence of the sunshine. 
In by far the greater number of instances, dryness 
seems to be a no less indispensable condition than 
warmth. But some birds are known to make use 
of moist rather than dry materials for their nests. 
This is well exemplified in a bird peculiar to Amer- 
ica, called by Wilson the willet (Totanus semipalma- 
tus, Laru.), from its reiterated shrill cry of Pill-will- 
willet, Pill-will-willet, which is loud enough to be 
heard at the distance of half a mile. This bird is 
very similar to the snipes, but may be readily dis- 
tinguished from them by short webs at the joinings 
of the toes. The willets generally begin to lay 
about the twentieth of May, which is said, from 
some unknown cause, to be two weeks later than it 
was twenty years ago. “Their nests,” says Wil- 
son, “are built on the ground, among the grass of 
the salt marshes, pretty well towards the land or 
C2 
