lights shine dimly, showing the half-naked workmen toiling with
tremendous energy by reason of the extra quantity of oxygen in the
compressed air. The workmen dug the earth and mud from under the
iron-shod edges of the caisson, and the weight of the masonry being
continually added to above sunk the great box lower and lower. From time
to time the earth was mixed with water and sucked to the surface by a
great pump. With hundreds of tons of masonry above, and the watery mud
of the river on all sides far below the keels of the vessels that passed
to and fro all about, the men worked under a pressure that was two or
three times as great as the fifteen pounds to the square inch that every
one is accustomed to above ground. If the pressure relaxed for a moment
the lives of the men would be snuffed out instantly--drowned by the
inrushing waters; if the excavation was not even all around, the balance
of the top-heavy structure would be lost, the men killed, and the work
destroyed entirely. But so carefully is this sort of work done that such
an accident rarely occurs, and the caissons are sunk till they rest on
bed-rock or permanent, solid ground, far below the scouring effect of
currents and tides. Then the air-chamber is filled with concrete and
left to support the great towers that pierce the sky above the waters.

[Illustration: THE SPIDER-WEB-LIKE VIADUCT ACROSS CANON DIABLO
The slender steel structure supporting a loaded train that stretches
along its entire length.]

The pneumatic tube, which is practically a steel caisson on a small
scale operated in the same way, is often used for small towers, and many
of the steel sky-scrapers of the cities are built on foundations of this
sort when the ground is unstable.

Foundations of wooden and iron piles, driven deep in the ground below