and top of the tube, between an inner ceiling and floor and the exterior plates; the iron plates which form the cells being riveted and held in their places by angle irons.  The space between the sides of the tube is 14 feet; and the height of the whole, inclusive of the cells, is 22 feet 3-1/2 inches at the ends, and 25 feet 6 inches at the centre.  The total length of the tube is 412 feet.  One end of the tube is fixed to the masonry of the pier; but the other is so arranged as to allow for the expansion of the metal by changes of the temperature of the atmosphere, and it therefore, rests upon eleven rollers of iron, running upon a bed-plate; and, that the whole weight of the tube may not be carried by these rollers, six girders are carried over the tube, and riveted to the upper parts of its sides, which rest upon twelve balls of gun-metal running in grooves, which are fixed to iron beams let into the masonry.

The second of these vast railway bridges crosses the Menai Straits, which separate Caernarvon from the island of Anglesey.  It is constructed a good hundred feet above high-water level, to enable large vessels to sail beneath it; and in building it, neither scaffolding nor centering was used.

The abutments on either side of the Straits are huge piles of masonry.  That on the Anglesey side is 143 feet high, and 173 feet long.  The wing walls of both terminate in splendid pedestals, and on each are two colossal lions, of Egyptian design; each being 25 feet long, 12 feet high though crouched, 9 feet abaft the body, and each paw 2 feet 1 inches.  Each weighs 30 tons.  The towers for supporting the tube are of a like magnitude with the entire work.  The great Britannia Tower, in the centre of the Straits, is 62 feet by 52 feet at its base; its total height from the bottom, 230 feet; it contains 148,625 cubic feet of limestone, and 144,625 of sandstone; it weighs 20,000 tons; and there are 387 tons of cast iron built into it in the shape of beams and girders.  It sustains the four ends of the four long iron tubes which span the Straits from shore to shore.  The total quantity of stone contained in the bridge is 1,500,000 cubic feet.  The side towers stand at a clear distance of 460 feet from the great central tower; and, again, the abutments stand at a distance from the side towers of 230 feet, giving the entire bridge a total length of 1849 feet, corresponding with the date of the year of its construction.  The side or land towers are each 62 feet by 52 feet at the base, and 190 feet high; they contain 210 tons of cast iron.

[Illustration:  CONWAY CASTLE AND TUBULAR BRIDGE.]