raised by pumping it out, used to close a sluiceway or entrance to a dock.
It works in grooves in the dock walls, and acts as a lock-gate.
See Plate XIX. page 884.
（Military Engineering.) A temporary military bridge supported on flat-bottomed boats or floats, termed pontons.
The use of boats or floats for supporting temporary bridges is of great antiquity.
Darius Hytaspes and his army crossed the Bosphorus on a bridge of this kind in order to invade Greece 493 B. C., and his successor Xerxes constructed one across the Hellespont, 480 B. C., for the same purpose, of which we have a description in Herodotus.
Its length was 500 paces.
Ships were used as pontons; suspension-cords of flax and biblos united them; transverse beams were laid on the ropes, planks on the beams, soil on the planks, and the armies crossed thereon.
Cyrus, according to Xenophon, threw over the Meander a bridge supported on seven boats.
Pompey crossed the Euphrates by a boat-b