Cornish bridge, certainly had not." The form of the tubes is an oval, 17 feet in its longest diameter, and 12 feet in its shortest. They are bent into an elliptical curve, with a rise in the middle of twenty-eight feet. With the roadway and suspension chains attached, each tube weighs 1,100 tons. The total weight of wrought iron in the bridge, when completed, will be 2,650 tons; of cast iron, 1,200; of masonry and brickwork there will be about 17,000 cubic yards; and of timber, about 14,000 cubic feet. The second tube, which is in every respect like the first, was completed on the 30th of June last, and on the 10th of July was successfully placed in position between the central pier and the Devonshire side of the river. The operation of elevating it began on the 9th of August, and it has now reached nearly the level of the first one, the tube being raised six feet in a week. The engraving on the other side is a view of this wonderful structure in its completed form. Its appearance is far more light and elegant than that of the Britannia Bridge, but it remains to be seen whether it will be equally steady under a gale of wind, and whether any vibration of the suspended roadway will interfere with the rapid motion of the trains. As the South Devon Railway has only one line of rails for the greater portion of its length, but a single roadway is provided on the Royal Albert Bridge. The progress of railway locomotion has not only given rise to the construction of new kinds of bridges, but it has directed mechanical science to devise better means of applying the strength of materials. On the South Devon and Cornwall Railways are to be seen wooden viaducts, carrying the line over valleys at great heights, constructed with such slender timbers, that, to an inexperienced eye, they seem frightfully frail for the support of heavy railway trains. [Illustration: ROYAL ALBERT BRIDGE, OVER THE TAMAR, AT SALTASH.] We must not omit to notice, among the remarkable bridge erections connected with railways, the viaduct across the valley of the Boyne, which passes over the river close to the town of Drogheda, at a height of 95 feet. The central portion of the viaduct is supported on four piers, 90 feet above high water mark, with a span in the centre of 250 feet, and on each side of 125 feet. This elevated portion of the work is approached on the southern side by twelve arches, of 60 feet span each, and on the north by three similar arches. The viaduct is constructed of limestone and iron lattice-work, and is calculated to bear 7,200 tons. During the erection of this viaduct the railway trains were carried over the valley on a wooden platform, without side railings, supported by scaffold-poles; and the crackling of the timbers, as the carriages passed over it, and the dizzy height at which they were carried through the air, produced a sensation of terror in nervous passengers, that was fully justified by the apparent danger. SELF-ACTING ENGINES.