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 "Encyclopaedia 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. Although Eads took care to protect his special innovations by patent, he was most willing to explain them with care to other engineers and to have others profit by his improvements; and several of the mechanical novelties of his bridge are now in the commonest use, and have been taken advantage of even in such famous structures as the Brooklyn Bridge.