ON THE MIGRATION OF BIRDS. 9 
Still the temperature, at the time of the disappear- 
ance of the birds under consideration, is found greatly 
in excess. We will now examine how nearly the 
mean of October corresponds with that of April. 
A comparative view of the mean temperature of April and 
October from 1814 to 1828 inclusive. 
1814, | 1815.| 1816.| 1817.] 1818, | 
Migiiilis coteaee acon 49-4| 45-2| 45:5] 468] 45-1 
October s.ssssvecsesccsecee, 463| 491| 506| 463] 565 
Difference of means) —3'1| +39} +51] — 5/+11-4 
1819. | 1820.) 1821.| 1822.] 1823. 
Pipe tienes etal 50°5| 50-7| 485| 465] 440 
QetobOr jecri-crssiartssieccas 501] 47°7| 505] 51-0] 47-2 
Difference of means| — 4} —3:0} +2°0] +45] +32 
1824, | 1825.| 1826.| 1827.| 1828. 
48-6| 47:8| 47-2) 47-4| 46-4 
48-4| 51:5} 524] 528| 50-2 
April 
October .. 
Difference of means|) +28} +8°7| +5°2| +54] +3:8 
General mean for April 47°-1, for October 50°. Difference of 
means +2°-9. 
A near approximation is observable in the tempe- 
rature of these months, allowing for the unusual 
warmth of the latter in 1818; yet the Stonechat, Pied 
Wagtail, Swallow, House-Martin, and Land-Rail are 
almost the only summer birds seen in October, and 
they generally retire before the termination of the 
third week; though Bats, Dormice, Hedgehogs, and 
