10 ON THE MUGRATION OF BIRDS. 
various coleopterous and dipterous insects, &c. are 
visible till the close of November, and even through 
the winter when the weather is open. In the year 
1820 Long-eared Bats were observed nearly to the 
middle of November; and in 1821 they were first 
noticed on the 23rd of April, the mean temperature 
for the day being 50°:5, and were seen through No- 
vember to the Sth of December, when the mean 
temperature was 46°, the mean for November being 
47°, which is within 1°-5 of that of April for the same 
year. 
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 such 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 considera- 
tion, as they are not only specifically lighter than 
water, but quite unfitted for existence in it by their 
organization. 
Mr. Gough, in his remarks on migration, published 
in the ‘Memoirs of the Literary and Philosophical 
Society of Manchester,’ vol. ii. new series, from a con- 
sideration of the laws which regulate the temperature 
of the earth at all moderate depths beneath its sur- 
face *, clearly establishes the fact that deep caverns 
* Those who wish for information on this subject may consult 
Saussure’s ‘ Vovages dans les Alpes, tome iii. chap. xviii. 
