470 Remarks on the t 
of May. The latter town lies south of east 
thirty miles from the former, in a moun- 
tainous district not far from the source of the 
Ribble. 
The preceding instances, with other facts 
of a similar nature, shew how absolutely the 
motions of the birds under consideration, are 
regulated inthe vernal months by local causes 
affecting local temperature; and the principal 
object of the present Essay may be called an 
attempt to demonstrate, that the same leading 
eause, naturally connected with the article of 
food, compels them to traverse the temperate 
zone, Wholly or in part, twice in the course 
of the year. When the phanomena of mi- 
gration are considered in this way, winter 
and summer birds of passage become rela- 
tive terms belonging to the place of obser-) 
vation. For instance, the twite inhabits the 
southern parts of Britain during the cold 
months, but returns to the hills of Yorkshire 
in spring; and if we may judge from the op- 
posite climates of the torrid and frigid zones, 
the former will have no visitors but in winter, 
and the latter none excepting in summer, 
The intermediate space on the surface of the 
globe is the chief scene of their operations. 
It is here that the temperature of the atmo- 
sphere undergoes great variations, but never 
