p.3

[4]

59 CHICAGO ACADEMY OF SCIENCES.

general phenomena intended to be represented will remain
substantially the same.

The well known property of water in large bodies, by
which it imparts heat more slowly to the air above in winter,
and absorbs it more slowly in summer, than the adjacent land,
may be stated as the chief cause of this remarkable deflection
of the isothermal lines as they approach Lake Michigan, a
body of water about three hundred miles in length, and fifty
miles in average breadth. The water varies less above or
below the mean annual temperature, than the land.

But it would be wrong to attribute the whole of this re-
markable effect upon the climate of the lake region to the
slow-conducting power of water; there is another cause, here-
tofore overlooked, which materially aids in producing the
result under consideration.

When the horticulturalist wishes to protect his tender plants
from the severity of winter cold, he directs a wall to be built
along the northern border; and the enterprising farmers on
the prairies of Illinois plant trees on their northern boundary
lines for a similar purpose. It is upon the same principle that
the elevated mountain range, extending from a little west of
Lake Superior, through the British Possessions to the coast of
Labrador, protects the whole lake region from the northern
blasts of winter which would otherwise sweep over it. The
elevation of this range north and north-west of Lake Superior,
is from 1,035 to 1,900 feet above the sea level; in Labrador
it is 2,240 feet.3 Of the intermediate portion little is accu-
rately known; but as it is said, at some points, to reach the
limits of perpetual snow, we may infer that this great garden
wall protects the lake region in no inconsiderable degree from
the excessive cold of winter. It is the absence of this moun-
tain system west of Lake Superior, that allows the cold winds
from the arctic regions in the water to spread over the
country west of the Mississippi river, depressing the tempera
ture in a very remarkable manner on and about the 95th de-
gree of west longitude.4

3 H. Y. Hind's Report, &c.-Toronto, 1860.

4 Prof. Joseph Henry, Patent Office Agricultural Report, 1856, p. 485, and map, pl. iv.