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 take Chicago, the chief city of the Lakes, we find that her imports and exports were,—­

Imports.  Exports.  In 1836, $ 325,203 $ 1,000 " 1851, 24,410,400 5,395,471 " 1859, estimated 60,000,000 24,280,890

In the year 1858, there were on the Lakes,—­

  American vessels, 1,194.  Tonnage, 399,443
  Canadian " 321. " 59,580

  Value of American tonnage on the
  Lakes, $16,000,000

  Value of Lake commerce, import
  and exports, $600,000,000

  Number of seamen employed, 13,000

Taking the island of Mackinac as the geographical centre of this navigation, we find the distances as follows:—­

Miles.  From Mackinac to head of Lake Superior 550 " " " Chicago 350 " " " East end of Georgian Bay 300 " " " Buffalo 700 " " " Gulf of St. Lawrence 1,600

Or ninety thousand miles of lakes and rivers, extending half across the continent.

The following table shows the amount of tonnage belonging to different cities in 1857:—­