Forth, a river and firth of Scotland. The river is formed by two head-streams, Duchray Water and the Avondhu, which, rising on and not far from Ben Lomond, at altitudes of 3000 and 1900 feet, run 14 and 9 miles to a confluence near Aberfoyle, the Avondhu traversing Lochs Chon and Ard. From their confluence, 80 feet above sea-level, the Forth itself winds 39 miles to Stirling, then 12 1/2 (the ' Links of Forth') to Alloa, the distances in a straight line being only 18 1/2 and 5| miles. It receives the Teith, Allan Water, and Devon, and traverses or divides Stirling, Perth, and Clackmannan shires. The Firth of Forth extends 51 miles eastward from Alloa to the German Ocean, between Clackmannanshire and Fife on the north, and Stirlingshire and the Lothians on the south. It has a width of 1/2 mile at Kincardine, 3 miles above Bo'ness, 1 1/4 at Queensferry, 5 between Granton and Burntisland, 17 at Prestonpans, and 8 1/4 at Elie. Its waters, 3 to 37 fathoms deep, encircle the islands of Inchkeith (fortified 1878-81), Inch-colm (with a ruined abbey), Cramond, etc., whilst at the entrance are the Bass Rock (q.v.) and the Isle of May, on which last and on Inchkeith are lighthouses. Rivers falling into it are the Carron, Avon, Almond, Water of Leith, Esk, and Leven. White fish are plentiful. In 1882-90 a great cantilever railway bridge was erected across the firth at Queensferry. It consists of two main spans of 1700 feet each, and two of 675, its total length, inclusive of piers, being 8296 feet, or a little over 1 1/2 mile. The clear headway under the centre of the bridge is 152 feet at high-water, and the highest part of the bridge is 361 feet. Designed by Fowler and Baker, the bridge with approaches cost £3,368,000. Above the bridge is the roadstead of St Margaret's Hope, and Rosyth, the new naval base (1904).