When a bridge is put into position across the lock, a series of wicket girders which are attached to the upstream side of the floor of the bridge are let down into the water, the connection between the bridge and one end of each girder being made by an elbow joint. The other end goes down into the water, its motion being controlled by a cable attached some distance from the free end of the girder and paid out or drawn in over an electrically operated drum. This free end passes down until it engages a big iron casting embedded in the concrete of the lock floor. This makes a sort of inclined railway at an angle of about 30 degrees from the perpendicular, over which huge steel plates are let down into the water. There are six of these girders, and they are all made of the finest nickel steel. When they are all in position, a row of six plates are let down, and they make the stream going through the locks several feet shallower. Then another row of plates is let down on these, and the stream becomes that much shallower. Another row of plates is added, and then another, until there is a solid sheet of steel plates resting on the six girders, and they make a complete steel dam which effectively arrests the mad impulse of the water in Gatun Lake to rush down into the sea. The plates are moved up and down by electrical machinery, and are mounted on roller-bearing wheels, so that the tremendous friction caused by their being pressed against the girders by the great force of the water may be overcome. That the emergency dams will be effective is shown by the experience at the "Soo" locks, on the canal connecting Lakes Superior and Huron. There, a vessel operating under its own power, rammed a lock gate. Although the emergency dam had grown so rusty by disuse that it could be operated only by hand, it was swung across the lock and effectively fulfilled its mission of checking the maddened flow of the water. Another protective device for the locks is the big caisson gates that will be floated across the head and tail bays when it is desired to remove all the water from the locks for the purpose of permitting the lower guard gates to be examined, cleaned, painted, or repaired, and for allowing the sills of the emergency dams to be examined in the dry. The caisson gates are 112-1/2 feet long, 36 feet beam, and have a light draft of 32 feet and a heavy draft of 61 feet. When one is floated into position to close the lock, water will be admitted to make it sink to the proper depth. Then its large centrifugal pumps, driven by electric motors, will pump the water out of the lock. When the work on the lock is completed these pumps will pump the water out of the caisson itself until it becomes buoyant enough to resume its light draft, after which it will be floated away.