ecomes of the waters of Lake Superior. This
vast lake has but one visible outlet, namely, the River of St. Mary;
while it receives the waters of a large number of rivers, some of which
are of greater dimensions than the St. Mary. What, then, becomes of the
surplus water? 2d. The difficulty of explaining whence come the waters
of Huron and Michigan. Very few rivers flow into these lakes, and
their volume of water is such as to fortify the belief that it must be
supplied through the subterranean river entering the Straits.

A large number of facts have been collected by Messrs. Foster and
Whitney on the subject of these oscillations of the Lake level; and,
in fact, these phenomena have been for a long time familiar to the
residents on the Lake shores. They are generally attributed by
scientific men to atmospheric disturbances, which, by increasing or
diminishing the atmospheric pressure, produce a corresponding rise
or fall in the water-level. These are the sudden and irregular
fluctuations.

The gradual fluctuations are probably caused by the variable amount of
rain which falls in the vast area of country drained by the Lakes. Thus,
at Fort Brady, where the mean of five years' observations is 29.68
inches, the extremes are 36.92 and 22.44.

An idea has been long prevalent among the old residents, derived from
the Indians, that there is a variation of the Lake surface which extends
over a period of fourteen years,--that is, the Lakes rise for seven
years, and fall for seven years. The records kept by accurate observers
at various points on the Lakes for the last ten years do not seem to
confirm this theory; but it has been well established by the recent
observations of Colonel Graham, at both ends of Lake Michigan, that
there is a semi-diurnal lunar tide on that lake of at least one third of
a foot.

The evaporation from this great water-surface must be immense. It has
been estimated at 11,800,000,000,000 cubic feet per annum; and in this
way alone can we account for the difference between the volume of water
which enters the Lakes and that which leaves them at the Falls of
Niagara. Immense as is the quantity of water which pours over the Falls,
it is small in comparison with the floods which combine to make up the
Upper Lakes.

In the year 1832, about the close of the Black Hawk War, the tonnage of
the Lakes was only 7,000 tons. In 1845 it had increased to 132,000 tons,
and in 1858 it was 404,301 tons. Or, if we ta