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, LATH.), from its reiterated shrill cry of Pill-will- 

 willet, Pill-will- ivillet, 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 



