their shores, natural harbors are of rare
occurrence. Consequently, for the protection and convenience of
commerce, a system of artificial harbors has been adopted by the Federal
Government, and appropriations have been made from time to time by
Congress for this purpose; and officers of the United States Engineer
Corps have been appointed to carry on the work. It is to some extent a
new and peculiar kind of engineering, caused by the peculiar conditions
of the case.

[Footnote A: See _Atlantic Monthly_ for February.]

Most of the lake-towns are built upon rivers which empty into the lakes,
and these rivers are usually obstructed at their mouths by bars of sand
and clay. The formation of these bars is due to several causes. The
principal one is this:--The shores of the lakes being usually composed
of sand, this is carried along by the shore-currents of the lake and
deposited at the river-mouths. Another cause of these obstructions may
be found in the fact, that the currents of the rivers are constantly
bringing down with them an amount of soil, which is deposited at the
point where the current meets the still waters of the lake. A third
cause, as we are told by Col. Graham, in his Report for 1855, is the
following:--

"Although the great depth of Lake Michigan prevents the surface from
freezing, yet the ice accumulates in large bodies in the shallow water
near the shores, and is driven by the wind into the mouths of the
rivers. A barrier being thus formed to the force of the lake-waves, the
sudden check of velocity causes them to deposit a portion of the silt
they hold in suspension upon the upper surface of this stratum of ice.
By repeated accumulations in this way, the weight becomes sufficient to
sink the whole mass to the bottom. There it rests, together with other
strata, which are sunk in the same way, until the channel is obstructed
by the combined masses of ice and silt. In the spring, when the ice
melts, the silt is dropped to the bottom, which, combined with that
constantly deposited by the lakeshore currents, causes a greater
accumulation in winter than at any other season."

These bars at the natural river-mouths have frequently not more than two
or three feet of water; and some of them have entirely closed up the
entrance, although at a short distance inside there may be a depth of
from twelve to fifteen or even twenty feet of water.

The channels of these rivers have also a tendency to be deflected fr