Engineering Wonders of the World
Volume I

186 ENGINEERING WONDERS OF THE WORLD. TYPICAL DOCK ENTRANCE WITH GATES. Dock-closing caissons are of two main types —rolling caissons, which replace gates for locks vertically. In shape, gates vary considerably irrespectively of the material of which, they are made, although they may be grouped into three main classes—namely, straight gates, curved gates, and segmental gates. The last- named type is held by eminent engineers to be theoretically the best; but the second type— closing against a straight sill by means of a projecting sill-piece at the bottom—finds more general favour. CROSS SECTION OF “ SHIP ” TYPE CAISSON FOR CLOSING DOCK. and entrances at Portsmouth and Chatham ; and floating caissons, or ship caissons, used both, in this country and abroad for closing Caisson Types. SECTION OF ROLLING CAISSON FOR CLOSING A DOCK ENTRANCE. the entrances to graving docks. The rolling caisson is box-shaped, with vertical sides of iron or steel plates. It has air öhambers, and is prevented from floating, when fully immersed, by ballast. When the dock is open it is withdrawn into in the side of the dock en- trance ; to close the en- trance it is drawn out on rollers across the gateway. This type is held to be a chamber constructed for it HALF ELEVATION OF “ SHIP ” CAISSON. The dotted line round the edge indicates the projection which engages with a groove in the quay. superior to gates in sev- eral respects,