the lock-gate, an accident which has occurred on the big locks that
connect Lake Superior with Lake Huron. So catastrophic would be such a
mishap, releasing as it might this immense accumulation of water, that
it seemed desirable at whatever expense to provide additional
safeguards against it. There are in the first place cross-chains,
tightening under pressure, which may be drawn across the bows of a ship
that threatens to become unmanageable. Secondly, the lock-gates are
doubled at the entrance to all the locks, and at the lower end of the
upper lock in each flight. And, thirdly, each flight of locks can be
cut off from the lake by an "emergency dam" of peculiar construction.
It is essentially a skeleton gate, which ordinarily lies uplifted along
the top of the lock-wall, but can be swung across, lowered, and
gradually closed against the water by letting down panels. In its
ordinary position it lies high above the masonry--conspicuous from some
distance out at sea as a large cantilever bridge, swung in air.

Peculiar difficulties have been encountered in establishing the
foundations of the locks. The lowest of each flight are planted in deep
morasses, and could only be settled by removing vast masses of estuary
slime to a depth of 80 feet below sea-level. The sea was cut off and a
dredger introduced, which gradually cleared its way down to the bottom
rock. But the troubles which the American engineers will remember are
those which have presented themselves in the Culebra cutting. The
channel is nine miles long. Its average depth is between 100
and 200 feet, but at one point it reaches 490 feet. The formation
of the ground varies extraordinarily. At some points it is
rock; at others rock gives place to contorted layers of brilliantly
colored earth which is almost as restless as quicksand. Unfortunately,
it is at places where the cutting is deepest that its banks are most
unstable. The sides of the lowest 40 feet of the excavation--the actual
water channel--are cut vertically and not to a slope; in a firm
formation this reduces the amount of excavation, but in loose material
it must apparently have increased the risk of slides. But, however this
may be, slips on a gigantic scale were inevitable. The cutting is an
endeavor to form precipitous slopes of crumbling material under a
tropical rain-fall: it may be likened to molding in brown sugar under
the rose of a watering-pot. The banks have been in a state of constant
m