3. =Locks.=--An ordinary lock is shown in fig. 29A. The space above the head gates is called the “head bay,” and that below the tail gates the “tail bay.” The floor of the lock is often an inverted arch. Sometimes the floor is of cast-iron. The “lift wall” is generally a horizontal arch. The gates when closed press at their lower ends against the “mitre sills”; and the vertical “mitre posts” at the edges of the gates meet and are pressed together. The gate, in opening and closing, revolves above the cylindrical “heel post”--which stands in the “hollow quoin” of the lock wall--and when fully open is contained in the “gate recess.” A lock is always strongly built, of masonry or concrete. The walls have to withstand the earth pressure when the lock is laid dry for repairs. The floor has to withstand the scouring action from the sluices. Regarding the upward pressure of the water when the lock is empty, see CHAP. X., _Art. 3_. The lift or difference in the water-levels of the two reaches of a barge canal is generally from 4 to 9 feet, but occasionally it is much more. [Illustration: FIG. 29A.] The gates of small locks are generally of wood and are counterbalanced. Those of large locks are of wood or steel, and the weight is generally taken by rollers. Ordinary wood should not be used if the _Teredo navalis_ exists in the waters. An iron gate, if enclosed on all sides by plating, is buoyant, and the rollers and anchor straps which hold the upper ends of the heel posts are thus relieved of much weight. The gates of the Panama Canal locks are 110 feet long and 7 feet thick, and the height ranges from 48 feet to 82 feet. The sluices for filling and emptying a lock are placed in the gates or in the walls. The gates and sluices are generally worked by hydraulic power or by electricity. Locks are frequently arranged in flights. There are, in a few instances, 20 to 30 locks in a flight, the total lift being 150 to 200 feet. By this means the number of gates is reduced, the tail gates of one lock being the head gates of the rest, and there is a saving in labour in working the locks. Let L be the volume of water contained in a lock between the levels of the upper and lower reaches, and let B be the submerged volume of a boat. The “lockage” or volume of water withdrawn from the upper reach of the canal is shown in the following statement:--