134 Observations on Periodical Birds. 
Now, if the periodical summer birds, when 
they withdraw, do not migrate into more 
genial climates, they must retire to suitable 
retreats, in which they pass the winter months 
in a state of torpidity. But where are these 
suitable retreats to be found? The notion 
of the submersion of these birds in lakes, 
ponds, and rivers, is too absurd to merit a 
moment’s consideration, as they are not only 
specifically lighter than water, but quite 
unfitted for existence in it by their organ- 
ization. 
Mr. Gough in his remarks on migration, 
published in the Memoirs of this Society, 
Vol. Il. New Series, from a consideration 
of the laws that regulate the temperature of 
the earth at all moderate depths beneath its 
surface,* clearly establishes the fact, that 
deep caverns cannot be the winter retreats of 
the periodical summer birds, as their tem- 
perature is not far from the maximum when 
these birds retire; and is near the minimum 
about the time that they begin to appear. 
He then proceeds to observe, (p. 461—2,) 
that “‘very few arguments will be now re- 
quired to demonstrate the impossibility of the | 
analogy which is supposed to connect the 
* Those who wish for information on this subject, may consult 
Saussure’s Voyages dans les Alpes. Tome III, chapitre XVIII. 
