Smeaton next appears in the character of a bridge-builder. The handsome bridges at Perth, Coldstream, and Banff were erected by him. With reference to the first of these, it should be explained that the Tay being subject to frequent inundations, it was requisite that great care should be taken with the foundations, which were laid down by means of coffer-dams. That is, a row of piles was driven into the river-bed, and round about and between them was thrown a quantity of gravel and earth mixed together, so as to render the enclosed space impervious to water. Pumping power was then applied, and the bed of the river within the coffer-dam was laid completely dry; after which the soil was excavated to a proper depth, and a firm foundation obtained for the piles. Piles were driven into the earth underneath the intended foundation-frame, and then the building was carried upwards in the usual way. [Sidenote: The Perth bridge.] The Perth bridge is a handsome structure, consisting of seven principal arches, and measures about nine hundred feet in length. It was completed and opened for traffic in 1772. His success in this notable undertaking secured him a very considerable amount of engineering business in the North. At Edinburgh he found employment in improving the water-supply for that city; at Glasgow, in strengthening and securing the old bridge. Far more important were the works he executed in designing and constructing the Forth and Clyde Canal, which links together the east and west coasts of Scotland, the North Sea and the Irish Sea. [Sidenote: The Forth and Clyde Canal.] After a careful examination of the various lines which had been proposed for the canal, Smeaton strongly recommended the adoption of the most direct route possible, and suggested that the depth of the canal should be sufficient to accommodate vessels of large burden. Lord Dundas, the principal promoter of the scheme, adopted Smeaton's ideas, and took the necessary steps for obtaining an Act to authorize the construction of the Forth and Clyde Canal, which was accordingly commenced in 1768. This canal runs nearly parallel with the famous wall of Antoninus, erected by the Romans to protect the southern Lowlands from the predatory attacks of the wild tribes of Caledonia. It begins at a point near Grangemouth, on the Forth, and ends at Bowling, on the Clyde, a few miles below Glasgow. Its length is about 38 miles, and it includes 39 locks, with an elevation of 156 feet from the sea to the summit-level.