wide, and from thirty to forty feet long. They are strengthened by
cross-ties of timber, uniting together the outward walls of the crib.
Piles are usually driven down into the clay, inside of these cribs, and
they are covered with a deck or flooring of plank. As the action of the
currents is constantly tending to remove the bed on which the cribs
rest, and thus cause them to tilt over, their bottoms are constructed
in a sort of open lattice-work, with openings large enough to allow the
stones with which they are loaded to drop through and supply the place
of the earth which is washed away.

The effect of these piers is to concentrate and deepen the
river-channel, and to retard the formation of bars, though they do not
wholly prevent it. In the spring it is often necessary to employ the
services of a steam-dredge-boat to cut through the bar, before vessels
can pass out.

The portion of these cribs above water is found not to last more than
ten or fifteen years; so that it is now recommended to replace them with
piers of stone masonry, wherever the material is easy of access.

As to the cause of the shore-currents which produce this mischief, Col.
Graham says, in one of his Reports,--

"The great power which operates to produce the littoral or shore
currents of the lake is the prevailing winds; just as the great ocean
current called the Gulf Stream is produced by the trade-winds. The
first-mentioned phenomenon is but a miniature demonstration of the same
principle which is more boldly shown in the other. The wind, acting
in its most prevalent lakeward direction, combined with this littoral
current, produces the great power which is constantly forming sand-bars