BIRDS OF PASSAGE. 
199 
I 
be ascribed to its very advanced state of consolidation, and to 
its having but a small share of phosphoric or nebulous matter 
in its construction. 
That of the year 1S07 was more affected, and although 
considerably condensed, showed clearly that it conveyed a 
great quantity of nebulosity to the perihelion passage. 
The comet of last year contained with little solidity a most 
abundant portion of nebulous matter, on which, in its approach 
to the perihelion, the action of the sun produced those beau- 
tiful phenomena, which have so favourably afforded an oppor- 
tunity for critical observations. 
a planet in a 
perihelion pas. 
T iat of 1807 
was more ne- 
bulous. 
And tl.e pre« 
ceding comet 
was still more 
so. 
X. 
Remarks on the Summer Birds of Passage , and on Migration 
in general. By Mr. John Gough. 
P ERHAPS no phenomenon in the history of animated na- The migration 
ture has engaged the attention of men of observation* ot birds is not 
11 . . ,, , . yet well un- 
in' all ages and countries, so generally as the regular appearance derstood. 
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 his- 
tory still remains involved in much obscurity, and perplexed 
with difficulties : many of which, in my 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 5 and to entertain very 
discordant notions respecting the winter retreat of the birds 
in question. All parties, however, are unanimous in conclud- depends on 
ing, that the regularity of their visits in spring is intimately the seasons, 
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 under- 
taken to discuss this curious question in natural history, agree, 
then, in ascribing the alternate appearance and disappearance 
of the swallow tribe, the cuckoo, the wryneck, and a ma- 
jority 
