130 NATURAL HISTORY OF SELBORNE. 
autopsia! Though there is endless room for observation in the 
field of nature, which is boundless, yet investigation (where a man 
endeavours 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 Dif- 
ference between the Present Temperature of the Air in Italy,” &c., 
have fallen in my way; and gave me great satisfaction : 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. 
P.S. Swallows appear amidst snows and frost. 
