Engineering Wonders of the World
Volume I

178 ENGINEERING WONDERS OF THE WORLD. a Dreadnought may be comfortably dry-docked in them. In the case of a mercantile dock, however, breadth is of more importance than length, to give easy passage through the open lock at high water. CROSS SECTION OF A DRY DOCK. The white circles at the ends of the invert are the culverts through which, water enters or leaves the dock. A lock entrance enables vessels to enter or leave a dock so long before or after high water as there is adequate depth, in the out- , side channel or tidal basin. The i .ocks depth of water on what is called the sill of a dock entrance—in other words, the step on the floor of the entrance against which the dock gates close—deter- mines the draught at which vessels may enter the dock. There may be greater depth of water in the dock itself, and also in the lock, but the draught that counts is that over the sill. As a rule, the depth is the same over the sills of both pairs of gates to a lock chamber. When a third pair of gates is interposed, as is sometimes done, to facilitate the locking of smaller vessels, all three sills have the same depth. But in some cases, where the tidal flow gives plenty of depth outside before abso- lute high water, the sill of the outer gates is almost as deep as the bed of the approach channel. Obviously by an arrangement of this character the fullest possible use is made of every period of high water. Structurally, a lock chamber consists of two quay walls and a paved bottom. The floor or bottom resembles an arch inverted—“ in- vert ” is, in fact, the technical description of it — and is designed to resist the upward pressure of water under the chamber when the water in _ How . Constructed. the lock is low. By sup- porting the quay walls at their toes, it also prevents their being pushed forward by the variations of water pressure to which they are exposed by continual locking and unlocking. The entrances or gateways are flanked by double side walls, and between each pair of side walls is a sluice for the filling and empty- ing of the lock. The ground on which the roller paths are laid is called the gate floor. Beyond this, towards the entrance channel, as far as the extremities of the outer side walls, the construction is continued in the form of an apron, while another apron finishes off what may be called the entrance works on the inner side of the sill. At high water, with both gates open, vessels are at liberty to come or go as they please. For some time before or after high water vessels may, in certain circum- stances , be locked inward or outward. But as a rule dock gates are closed before the ebb begins, and are kept closed until the time of the next flood. The opening and shutting is generally effected by means of chains attached to the gates and wound by hydraulic power on to drums. A more modern method is repre- sented by a hinged hydraulic ram operated from a recess on either side. This acts on the inner side of the gate, pushing it forward or pulling it back, as the need may be. Although, in naval dockyards such as Ports- mouth and Devonport, lock entrances and locks between basins are occasionally used as dry docks, they differ somewhat in construction from dry docks. Dry“dockinS P a Ship. The sides oi a dry dock are stepped to facilitate the docking of ships. The gates are opened at high water, and the vessel to be docked is floated in. Along the centre line of the dock floor blocks are securely laid, and on these the ship settles down as