8 ON THE MIGRATION OF BIRDS. 
is when they appear ; and with regard to the Cuckoo 
and Swift, this is uniformly the case in a remarkable 
degree: but, as the motions of the Periodical Birds 
may be supposed to be influenced by the weekly or 
monthly rather than by the daily mean temperature, 
I shall compare the mean temperature of April, the 
month in which most of the Summer Birds are first 
seen, with that of September, the month in which 
they chiefly retire, prefixing the sign + to the differ- 
ence of the means when the temperature at the time 
of their departure is in excess, and the sign — when 
it is in defect. 
A comparative view of the mean temperature of April and 
September from 1814 to 1828 inclusive. 
1814, | 1815.| 1816.| 1817.} 1818. 
PreMed tale Cat 49-4] 452] 455] 468! 451 
September...........00..666 55°5| 544] 560} 58:9) 58:3 
Difference of means} +671} +9:2/+10°5 jt121 +13-2 
1819.} 1820.) 1821.) 1822. 1823. 
pesado eee 50°5| 507| 485 485] ato 
September ....csccvascnsese 571) 561) 584] 5441 53:8 
Difference of means} +66/ +54] +99; +79; +98 
1824. | 1825.| 1886.| 1827." 1828. 
Dri oa te adaindcorliee 456| 473| 492! a4! 484 
September.......ssssess. 570| 596| 567) 562) 578 
Difference of means!-+11-4 ie a8) 43:8 | 
General mean for April 47°-1, for September 56°6. Difference of 
means +995, 
