464 ‘ Remarks on the 
requires; and that its winter abode is fixed 
by the article of food, which depends on the 
temperature of the place, and the appetite of 
the visitor. 
After making the foregoing imperfect re- 
marks on the southern retreats of the migrat- 
ing tribe, I come in course to the cause 
which invites these wanderers northward, to 
spend the summer in higher latitudes. No 
sooner has the sun touched the tropic of Capri- 
corn, than he begins to lessen his southern 
declination, and to shine more directly upon 
the -opposite hemisphere: every latitude of 
which experiences his animating influence in 
succession, commencing with the parts con- 
tiguous to the torrid zone, and proceeding 
gradually to the frozen regions within the 
arctic circle. The adyances of spring towards 
the north, keep pace with the diffusion of 
solar heat over the northern half of the globe: 
For the same plants flower much earlier im 
low than in high latitudes; and we may 
safely conclude that the same lethargic ani- 
mals, especially the same flies and other in- 
sects, will observe the like rule in quitting 
their winter quarters ; and will appear abroad 
in Italy much sooner than in Britain. The 
following comparative facts may serve to elu- 
cidate the slow progress of spring from the 
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