Migrations of North American Birds. 345 
The clue to these peculiarities attending the interchange of 
Species of the two continents will be found in the study of the 
Jaws of the winds of the northern hemisphere, as developed by 
Prof. Henry and Prof. Coffin. These gentlemen have shown 
(see Prof. Henry’s articles on Meteorology, “ Report of Commis- 
sioner of Patents for 1856,” page 489) that ‘the resultant mo- 
tion of the surface atmosphere, between latitudes 82° and 58° 
in North America, is from the west, the belt being twenty de- 
grees wide, and its greatest intensity in the latitude of 45° 
This, however, must oscillate north and south at different sea- 
Sons of the year with the varying declination of the sun. South 
of this belt, in Georgia, Louisiana, etc., the country is influ- 
enced, at certain seasons of the year, by the northeast trade- 
Winds; and north of the same belt by the polar winds, which 
On account of the rotation of the earth, tend to take a direction 
toward the west. It must be recollected that the westerly direc- 
tion of the belt here spoken of is principally the resultant o 
the southwesterly and northwesterly winds alternately predomi- 
nating during the year.” easy 
From these considerations and facts, therefore, we are entitled 
they can make is that of England, whence the fact that most of 
the species have occurred in the British Islands as well as 
Heligoland, equally well fitted to attract stragglers and furnish 
Iceland being in the latitude of the reverse current, from east 
i si caught up by the winds and 
Carried off would soon reach Greenland, only a few hun 
miles distant. This may be the principal agency of supply from 
urope to Greenland, as most European land birds are only met 
With there at rare intervals, although as Greenland lies north of 
Iceland, there may be a regular migration to some extent. 
__ As remarked, the prevailing direction of the winds, whether 
Violent or moderate throughout the year, as well as during the 
Period in which our birds are on either their spring or autumnal 
Migration, is from America toward Europe. itven should their 
Irection be reversed and that rare pheno 
. Hortheaster,” occur, it would merely have 
ing the birds back upon our own coast, or i 
Am, Jour. Sc1.—SEconp Sextes, Vou. XLI, No. 123.—Mar, 1 
44 
menon, &@ summer 
the effect of bring- 
nto the interior, the 
