.Flood-goles.—The tail-end of the lock, which connects a navigable canal with the stream, must obviously be so arranged as to protect the lock-chamber from the entrance of the highest flood-water. This portion of the lock is therefore provided with a supplementary set of gates, called "flood-gates," that open out toward the stream. In the case of tide-locks at the entrance of ship-canals, it is cnstomary to provide the upper gates with additional sections, one above the other, so that the height of the upper gates, when necessity requires, may be raised sufficiently to protect the lock against an unusual high tide.
Construction of Lock-chomber.—The lock-chamber comprises two parts —the walls and the bottom. They are built either of stone or of timber; more rarely, of cast iron. Timber walls for this purpose are subject to leakage, and require frequent repairs. Masonry walls are best made of liewn stone laid in hydraulic cement. The bottom is formed generally of a stout timber platform extending out beyond the walls. This foundation is usually a grillage (pl. 50, fig. 6)—that is, several courses of stout timbers laid crosswise and securely bolted or tree-nailed to the tops of a set of wooden piles sawed off to a uniform height. Upon the upper course of this grillage a close platform of timber is bolted, and the joints are calked as additional security against leakage. It is customary to excavate the soil around the piles to the depth of several feet, and to fill this space up to the level of the under surface of the platform with well-rammed gravel, stone spawls, etc. The bottom is sometimes constructed of masonry, and in situations where an upward pressure is to be feared it is built with reverse arches—that is, arches having their concave faces directed upward.
Construction of Head-boy IT'alls.—The walls and bottom of the heads of the lock are formed either of timber or of stone. As these parts are re quired to resist the pressure of the upper-level water, special care must bc taken to secure solidity and to prevent leakage through the sides or the bottom, by which the surrounding earth would be washed away, thus en dangering the structure. To accomplish this, the walls are defended by sheet-piling (16/. 41, fig. 17), by which they are enclosed, and which is car ried below the bottom; the greatest care is bestowed upon the construc tion. Locks have also been built entirely of concrete (for example, the lock at the embouchure of the Franz-Joseph Canal into the Danube). Some of the locks on the Ellesmere Canal near Beeston Castle, England, have their walls and floors of cast-iron plates.
Construction of Lock-gotes.—The construction of timber lock-gates is shown on Plate 52 (fig. 3). For the attachment of the leaves of the gates, the lower end of the turn-post is furnished with a pin that is let into and turns upon a socket provided for the purpose, while the upper part of the post is held by an iron collar firmly anchored into the masonry. It is
found preferable to form the socket in the lower end of the turn-post and to attach the pin to the bottom of the chamber, for with this arrangement sand is not likely to work its way into the socket. The gates, that they may not sag, are strongly braced, as shown in the Fig-ure. To lessen their weight, larg-e gates are furnished on their lower face with rollers or are made hollow. Of late, cast- aud wrought-iron gates have been introduced with much success.
To prevent the entrance of water to the lock-chamber while the gates are being repaired, grooves are provided in the so-called " head-bays" and " tail-bays" for the insertion of watertight bulkheads of beams before and behind the gates respectively. In the case of small and comparatively narrow locks, the gates may be made of a single piece.
Operation of Locks.—The operation of locking- a vessel is substantially as follows: If a vessel is going up stream, the lower gates are opened, the vessel passes into the lock, the lower gates are then closed, and the water from the upper level is admitted to the lock through the sluices iu the upper gates or through the side-channels above described until the water level in the lock is the same as that of the upper level. The upper gates are then opened, and the vessel passes out. When the vessel is passing down stream, these operations are reversed. The depth of the lock will vary from 6 to 25 feet, according to the nature of the canal and the kind of traffic for which it is designed.
The Suez Canal is thus far the most important ever completed. The canalization of the Isthmus of Suez had been attempted as early as the fourteenth century B. c., though it appears that the plan then was to form a connecting waterway between the Red Sea and the Nile. This project was afterward successfully carried out, and the canal in question is known to have been in existence until about the eighth century of the Christian era. The first Napoleon projected the connecting of the Mediterranean and the Red Sea, but, having- been misled by defective'preliminary sur veys, he abandoned the enterprise. To De Lesseps is due the honor of having revived the project, and of having- successfully completed the most important engineering work of modern times—not, perhaps, so inuch in respect of the difficulties to be surmounted, which were not of a very for midable character, as in respect of the results that have followed the open ing of the new highway.