During the construction of the Forth Bridge, the young engineer Evelyn George Carey was given privileged access to the site in order to make a comprehensive photographic record of the bridge’s development. It was hoped that this visual documentation would restore public confidence in British engineering following the Tay Bridge disaster of 1879. In this photograph Carey uses volunteers, possibly the architects of the bridge Sir John Fowler and Sir Benjamin Baker, to demonstrate the cantilever principle. If you look closely you can see that the boy’s weight is sufficiently supported for his feet to rise off the ground – just as the cantilevers support the central girder of the bridge.