LOCK, on a river or canal, is an arrangement of two parallel floodgates, by which communication is secured between two reaches of different levels. When locks were first introduced, is not known within a hundred years, nor is it dlear whether Holland or Italy can claim the distinction of having first employed them. This much, however, can be affirmed with certainty, that at the beginning of the 17th c. locks existed in both countries, and it is probable that they were arrived at gradually by successive improve ments in the mode of rendering shallow rivers navigable. Obviously, the first step would have been to dam the stream across at intervals, leaving gates in the dams for the passage of vessels. This measure would have divided the river into reaches or steps, each, as the source was approached, being higher above the sea than the one last passed. But the passage up or down—and especially up—such a stream must be extremely slow, as at each dam a vessel must wait until the gate has been opened, and the level equalized in the reach it is in, and that on which it is proposed to enter. Where the reaches were far apart, a large body of water would require to be raised or lowered, and the process could not but be tedious. The medimval engineers next tried to place the dams as near together as possible, but expense limited this. The course then was to build two dams, with floodgates, just far enough apart to allow a vessel to float within. Under this arrangement, only the section between the dams had to be raised or lowered. The post of thus doubly damming a wide river, however, was very great, and it was an easy transition of idea to remove the passage from the main stream alto gether and construct a lock with double gates which should open at one end above and at the other below the dam or weir. The economy of money in building, and of time and water in working, was obvious; and on this principle all locks are now made, wherever there is traffic of any importance. The arrangement consists of two pairs of
gates, opening up the stream, and offering-, when shut, a salient angle to the stream or upper pressure. The effect is that the weight above only tends to close the gates still tighter. When a vessel is to be brought from one level to the other, it is floated into the " pound," as the space between the upper and lower gates is called. The 'gates are then shut, and a sluice in the lower part of the upper gate raises the surface of the pound, or the sluice in the lower gate depresses it in a few minutes to the level of the upper or lower reach, as the case may be. These sluices are worked by racks in the gates, and the ponderous gates themselves are moved with the aid of long and heavy levers. Of course, one pair of gates must always be shut, or the two reaches would speedily assimilate their levels.
On canals where water is scarce, a reservoir, equal in size to the lock, is formed at its side. When the pound is to be emptied, the water is run into the reservoir until it and the lock are at the same level, which will be half height. The reservoir is then closed, and the remaining water in the lock run off through the lower sluices in the usual way. On refilling the lock, before opening the npper sluices, one-quarter the quantity required can be obtained from the reservoir, thus effecting a saving of many tons of water at each filling.
On rivers advantage is taken of islands for the formation of weirs (q.v.) and locks. On the Thames the locks are from .2 to 3 m. apart, and the river is locked by upwards of 50 locks from Teddington to Lechlade. Oti canals, to economize superintendence, ; the locks are usually constructed in " ladders" of several close tog-ether, like a flight of steps. As the pressure on lock-gates is very great, and varies with the height of water 1 above, the rise in one lock is rarely more than 8 or 9 ft., although in some instances 12 ft. have been accomplished, and in a very few cases even more.