The sides, bottom, and top of these gigantic tubes are formed of oblong wrought iron plates, varying in length, width, and thickness, according to circumstances, but of amazing size and weight. They are so arranged as to obtain the greatest possible strength, the whole being riveted together in the strongest manner. In addition to the 1600 tons of wrought iron in each of the four large pieces, an additional 200 tons was used to form lifting frames, and cast iron beams for the purpose of attaching the tube to those huge chains by which they were elevated.