182 Observations on Periodicai Birds: 
swallow, and martin withdraw, than» it is 
when they appear; and with regard to the 
cuckoo and swift, this is uniformly the case 
in a very remarkable degree: but, as the 
motions of the periodical birds may be sup- 
posed to be influenced by the weekly or 
monthly, rather than by the daily mean tem- 
perature; let us compare the mean tempera- 
ture 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. | 
A comparative view of the mean temperature of 
April and September, from 1817 lo 1821 
inclusive. 
Gen. 
means, 
Years.| 1817. | 1818. | 1819. | 1820, | 1821. 
April..| 46.8 | 45.1 | 50.5 | 50.7 | 48.5 | 48,3 
Sept. ..| 58.9 | 58.3 |57.1'| 56.1 [58.4 | 57.7 
Dif. of SHE Cowal eae ene 
ie ot | 4.12. 11-413.9|46.6 }45. 4 |49,9 +9-4 
Still the temperature at the time of the dis- 
appearance of the birds under consideration 
is found greatly in excess. We will now 
examine how nearly the mean of October 
corresponds with that of Aipril. 
