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 their dense atmosphere a candle when blown out would at once light again. This was before the days of electric lighting: otherwise we may be sure that that would have been used, as so many other modern inventions were. For the first time in any such work, the last pier sunk had telegraphic communications with the offices on shore; which must have been comforting to workmen starting out to their labor in the dead of winter with two weeks' provisions. The dense air of the chambers caused not only discomfort to the ears, but also in the case of some of the workmen a partial paralysis. There was no previous experience to go by, but every precaution seen to be necessary was taken; the hours of work were made very short, the elevator was provided, medical attendance and hospital care were given free. After the first disasters no man was allowed to work in the air-chambers without a doctor's permit. And it is known that in helping the sufferers with his private means, Eads was as charitable as ever. Out of 352 men employed in the various air-chambers, 12 died. Eads, with his wonted generosity of praise, printed in his yearly report the names of all the men who worked in the deepest pier from its beginning till it touched bed-rock. It is interesting to note in passing that of all the workmen in the blacksmith's yard only the head smith himself could lift a greater weight than the designer of the bridge.
The superstructure consisted mainly of three steel arches, by far the longest that had ever been constructed; the first to dispense with spandrel bracing; and the first to be built of cast-steel. The "Encyclopædia Britannica" called them "the finest example of a metal arch yet erected." They were built out from the piers from both ends to meet in the middle; and were put into place entirely without staging from below,--once again, the first instance of such a proceeding. All the necessary working platforms and machinery were suspended from temporary towers built on the piers; and thus while the arches were being put up, navigation below was not interfered with. This throwing across of the 500-foot arches without the use of false works has been ranked with the sinking of the piers "through a hundred feet of shifting quicksands," as producing "some of the most difficult problems ever attempted by an engineer." One problem, caused by the fault of the contractors, presented itself when they came to insert the central tubes to close the arches. The tubes were found to be two and a half inches too long to go in, although they would be only the required length when they were in. It was left for Eads to insert them. Shortening them would of course have lowered the arch. Eads, who was just starting for London on financial business of the bridge, cut the tubes in half, joining them by a plug with a right and left screw. Then he cut off their ends, for the plug would make them any required length by inserting or withdrawing the screws a little. Then he went away. As it would have been much cheaper not to use this device, his assistants tried for hours to shrink the tubing by ice applications, and thus to get the arches closed; and there is a popular tradition in Saint Louis that they succeeded; but it was excessively hot weather, and they did not succeed. The screw-plug tubes, of course, were easily put in. Any part of this steel work can be at any time safely removed and replaced,--another structural feature original in this bridge.