. These were
adapted in design from some he had seen in use in France, and had
examined during a trip his doctors ordered him to make in 1868. Eads
himself compared them to inverted pans. They were open at the bottom,
but perfectly air-tight everywhere else. They had several important
features which were entirely original. Such caissons, sunk to the
bottom, have the masonry of the pier built on top of them even while
they are sinking; and workmen inside them keep removing the sand from
underneath, and throwing it under the mouths of pipes which suck it up
to the surface of the river. Evidently the caissons must be filled with
compressed air to equalize the external pressure, which is constantly
increasing as ever deeper water is reached; they must also have an
opening connecting with the surface; and to admit of passing from the
ordinary atmosphere to the denser one, there must be an air-lock.
Before this bridge was built, the air-lock had always been placed at
the top of the entrance shaft, where, as the caisson sank and the shaft
was lengthened, it had to be constantly moved up. Eads placed it in the
air-chamber of the caisson itself, where it never had to be moved; and
thus, as the shaft was not filled with compressed air, less was needed,
and there was less danger of leaks. Another of his useful innovations
was to build his shaft of wood, and another was to put a spiral
stairway into it. Indeed, in the last pier he put an elevator into the
shaft. Moreover, he was the first person to run his pipes for
discharging the sand, not through the shaft, but through the masonry
itself; and he invented a very simple and effectual new sand-pump,
which was worked by natural forces without machinery. All these
improvements and various others seem to have been thought of so easily,
that we are inclined to wonder why clumsier methods had ever been in
use. He described them all in his reports and his letters about the
bridge in a style which is not only clear but actually fascinating even
to a person who has scant scientific knowledge or taste.

One of the piers was sunk 110 feet below the surface of the river,
through ninety feet of gravel and sand. Eads's theories were justified
by finding the bed-rock so smooth and water-worn as to show that at
times it had been uncovered. This was the deepest submarine work that
had ever been done, and Eads tells us in his reports many interesting
experiments he made in the air-chambers. In thei