5D THE NATURAL HISTORY OF SELBORNE. 
undertaking for a man unsupported and alone to begin a 
natural history from his own autopsia!* Though there is 
endless room for observation in the field of nature, which is 
boundless, yet investigation (where a man endeavors to be 
sure of his facts) can make but slow progress, and all that 
one could collect in many years would go into a very narrow 
compass. 
Some extracts from your ingenious “Investigations of the 
Difference between the Present Temperature of the Air in 
Italy,” etc., have fallen in my way, and gave me great satis- 
faction: they have removed the objections that always arose 
in my mind whenever I came to the passages which you quote. 
Surely the judicious Virgil, when writing a didactic poem for 
the region of Italy, could never think of describing freezing 
rivers, unless such severity of weather pretty frequently 
occurred ! 
Two swallows have appeared amidst snows and frost. 
LETTER VI. 
SELBORNE, May 2zst, 7770. 
The severity and turbulence of last month so interrupted 
the regular process of summer migration, that some of the 
birds do but just begin to show themselves, and others are 
apparently thinner than usual; as the white-throat, the black- 
cap, the redstart, the fly-catcher. JI well remember that after 
the very severe spring in the year 1739-40, summer birds of 
passage were very scarce. ‘They come hither probably with a 
southeast wind, or when it blows between those points; but 
in that unfavorable year the winds blowed the whole spring 
and summer through from the opposite quarters. And yet 
1 Autopsia — ocular observation. 
