p.2

902

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(Wis Authors)

TRANSACTIONS OF THE 58

ART. II. On the Climate of the Country bordering upon the
great North American Lakes. By I. A. LAPHAM, LL. D., of
Milwaukee, Wis.

Among the more important results deduced from the great
mass of meteorological observations accumulated within the
last thirty or forty years, in the United States, none are more
interesting than those that show the effect of local causes
upon the climate of particular districts. Thus, in the valley
of the Hudson river, in the State of New York, the prevailing
winds are from the north or south; while in the valley of its
tributary, the Mohawk, they are from the east or west-these
being the directions of the valleys.1 We often see indications
of a strong current of air passing over deep valleys, while it
is quite calm below. On the borders of our great lakes,
atmospheric currents frequently flow in opposite directions at
the top of steeples, and on the ground.2 It is to represent
one of the most striking of these local phenomena that the
accompanying map (Plate X) of the State of Wisconsin, has
been constructed, on which the points of equal mean tempera-
ture for the month of January are connected with black lines,
902 and those for July with red. It will be seen that the former
TE approach Lake Michigan with a north-easterly direction, while
the latter have a direction towards the south-east, thus cutting
each other at a considerable angle, and showing the remark-
able effect of that lake in elevating the mean temperature of
winter, and depressing that of summer. To show these modi-
fications of temperature in their maximum, the coldest and
warmest months have been selected; but if we contrast in
like manner the mean temperature of the three winter months
with that of the summer months, the same phenomena would
be seen, though less in degree. The map is based upon obser-
vations made at various points, and continued during several
years. Longer-continued observations may require some modi-
fication of the exact position of these several lines, but the

1 HOUGH, Results of Meteor. Obs., &c., p. xi, Albany, 1855.

2 KIRTLAND, Am. Jour. of Science, 2d Ser. XIII (1852), p. 217.