454 Remarks on the 
‘so generally, as the regular appearance of 
those birds which visit the northern climates 
in spring, and disappear as regularly at the 
approach of winter. » But though many facts 
have been collected, relating to the manners 
of these singular birds, by the industry of 
naturalists, their history still remains involved 
- in much obscurity and perplexed with difficul- 
ties; many of which in wy opinion arise from 
a negligent or an injudicious arrangement of 
the facts already ascertained. Philosophers 
have been induced by this oversight, to take 
partial views of the subject ; and to entertain 
very discordant notions respecting the winter 
retreat of the birds in question. All parties, 
however, are unavimous in concluding, that 
the regularity of their visits in spring is inti- 
mately connected with the apparent motion 
of the sun betwixt the tropics, whose northern 
declination is increasing at the time of their 
appearance, and consequently the temperature 
of the northern hemisphere is also advancing 
towards the heat of summer in every latitude. 
The Philosophers, who have undertaken to 
discuss this curious question in natural history, 
agree then, in ascribing the alternate appear- 
ance and disappearance of the swallow tribe, 
the cuckoo, the wryneck, and a majority of 
the British warblers, to the vicissitudes of 
