in water, or the land on which the towers are to stand is unstable, then
the problem is much more difficult.

For bridges like those that connect New York and Brooklyn, the towers of
which rest on bed-rock below the river's bottom, caissons are sunk and
the massive masonry is built upon them. If you take a glass and sink it
in water, bottom up, carefully, so that the air will not escape, it will
be noticed that the water enters the glass but a little way: the air
prevents the water from filling the glass. The caisson works on the same
principle, except that the air in the great boxlike chamber is highly
compressed by powerful pumps and keeps the water and river ooze out
altogether.

The caissons of the third bridge across the East River were as big as a
good-sized house--about one hundred feet long and eighty feet wide. It
took five large tugs more than two days to get one of them in its proper
place. Anchored in its exact position, it was slowly sunk by building
the masonry of the tower upon it, and when the lower edges of the great
box rested on the bottom of the river men were sent down through an
air-lock which worked a good deal like the lock of a canal. The men, two
or three at a time, entered a small round chamber built of steel which
was fitted with two air-tight doors at the top and bottom; when they
were inside the air-lock, the upper door was closed and clamped tight,
just as the gates leading from the lower level of a canal are closed
after the boat is in the lock; then very gradually the air in the
compartment is compressed by an air-compressor until the pressure in the
air-lock is the same as that in the caisson chamber, when the lower door
opened and allowed the men to enter the great dim room. Imagine a room
eighty by one hundred feet, low and criss-crossed by massive timber
braces, resting on the black, slimy mud of the river bottom; electric