The longest spans on the Tay Bridge were 245 feet; it was over two miles long, and had ninety spans. It was an iron girder bridge, and was opened on the 31st of May, 1878. Not to be beaten, however, after the panic had subsided, another and more stable bridge was constructed, also a girder, but not so high in elevation, and sixty feet further up the river. It was opened in 1887, and is 10,779 feet long, with 85 piers, the navigable channel being under four of the spans, the centre spans being 245 feet wide. It will be seen at once that the cantilevers at the Forth Bridge cover very much wider spans; and the channel being so deep, the impossibility of building piers will also be obvious. The best place for the bridge was marked by the projection of the Inverkeithing peninsula on the north shore, and also the Inchgarvie rock in the channel itself. The peninsula brought the two shores together, reducing the space to be bridged, and the rock gave firm support for a pier. Still there were the two immense spans of 1700 feet to be crossed, and the engineers decided on the cantilever principle. Thus, though the Tay Bridge was the longest in the world, the Forth presented by far the greatest spans—viz., the two main spans of 1700 feet each, in addition to which there are two of 675 feet each, and fifteen of 168 feet each. The total length of this magnificent bridge, which Sir Benjamin Baker rightly claimed was the most wonderful in the world, is somewhat over 1½ miles in length, or 8296 feet, including the piers, while almost a mile is bridged by the huge and superb cantilevers. This is, perhaps, the great marvel. The clear space under the centre is no less than 152 feet at high-water, while the highest portion is 361 feet above the same mark. And now, how was this great bridge constructed? Workshops were erected at South Queensferry, and the mammoth cantilevers were put up there piece by piece. They were fitted together and then taken plate by plate to the bridge itself. The shops were lit by electricity, and furnished with appliances for bending, cutting, moulding, holing, and planing plates. The workshops were surrounded by quite a maze of railways. But what of the piers, without which all these preparations would be unavailing? Now the foundations of piers are usually laid by means of cofferdams; that is, piles of timber are driven down through the water into the bed of the river close together, and the interstices filled with clay; or a casing of iron may be used instead. The water in the enclosure thus formed can be pumped out and excavation proceeded with, and the foundations laid. Cofferdams are sometimes made of iron boxes or caissons with interstices fitted with felt, and caissons of this kind about 12½ feet long and 7 feet wide were used in constructing the Victoria Embankment on the Thames.