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. The construction of the tubes is thus described in the London Illustrated News, from which this account is derived:

"In order to carry out this vast work (the construction of the tubes), eighty houses have been erected for the accommodation of the workmen, which, being whitewashed, have a peculiarly neat and picturesque appearance; among them are seen butcher's, grocer's, and tobacconist's shops, supplying the wants of a numerous population. A day school, Sunday school, and meeting-house also conspicuously figure. Workshops, steam-engines, store-houses, offices, and other buildings meet the eye at every turn; one is led to conclude that a considerable time has elapsed since the works were commenced, yet it is little more than two years ago. A stranger, on coming to the ground, is struck with wonder when for the first time he obtains a near view of the vast piles of masonry towering majestically above all the surrounding objects—strong as the pillars of Hercules, and apparently as endurable—his eyes wander instinctively to the ponderous tubes, those masterpieces of engineering constructiveness and mathematical adjustment; he shrinks into himself as he gazes, and is astonished when he thinks that the whole is the developed idea of one man, and carried out, too, in the face of difficulties which few would have dared to encounter."