Sometimes a bridge is made movable by swinging it round on a pivot instead of drawing it up on a hinge or axis; and sometimes, as in the case of a bridge over the Arun for the Brighton and South Coast Railway, it is made to slide on wheels backwards and forwards from the abutment. Floating or pontoon bridges are made by placing planks on pontoons, or boats anchored by cables. The longest in the world is probably at Calcutta, across the Hooghly. It is 1530 feet in length, there being twenty-eight pontoons in pairs. These are of iron, 160 feet long, and with ends shaped like wedges; they support a road-way of 3-inch timbers, forty-eight feet wide, and raised on tressel work. An opening can be made for ships by removing four pontoons and floating them clear of the passage way. Great bridges present some of the most remarkable triumphs of the engineer. They rank beside the express locomotive and the ocean liner as among the great constructive achievements of mankind. Daring in design, and bold in execution and in sweep of span, they have been developed along several principles; and so solidly have they been built, so sound are the laws of their being, that it seems as though they will live as long as the everlasting hills. [Illustration] [Illustration] REMARKABLE TUNNELS AND THEIR CONSTRUCTION. CHAPTER I. HOW BRUNEL MADE A BORING-SHIELD. “I watched the worm at work and took my idea from that tiny creature!” “A worm! Was it an ordinary worm?” “Oh no, it was the naval wood-worm—_Teredo Navalis_; it can bore its way through the hardest timber. I was in a dockyard and I saw the movements of this animal as it cut its way through the wood, and the idea struck me that I could produce some machine of the kind for successful tunnelling.” “Well, it has been brilliantly successful.” “I looked at the animal closely, and found that it was covered with a couple of valvular shells in front; these shells seem to act as a shield, and after many attempts I elaborated the boring-shield which was used in hollowing out the Thames Tunnel.” This statement, which we can imagine to have been made by Sir Marc Isambard Brunel to a friend, is no doubt in substance quite true. A writer in the “Edinburgh Encyclopædia” says, that Sir M. I. Brunel informed him, “that the idea upon which his new plan of tunnelling is founded, was suggested to him by the operations of the _Teredo_, a testaceous worm, covered with a cylindrical shell, which eats its way through the hardest wood.” Two or three attempts had already been made to drive a tunnel under the Thames, but they had ended in failure. In 1823, Brunel came forward with another proposal, and he ultimately succeeded. This illustrious engineer must not be confounded with his son—who was also a celebrated engineer—Isambard Kingdom Brunel. There were two Brunels, father and son, even as there were two Stephensons, George and Robert.