The improvements in the processes of putting in the foundations of bridges have been as great as those above water. All have shortened greatly the time necessary, and have made the results more certain. The American system may briefly be described as an abandonment of the old engineering device of coffer-dams, by which the bed of the river is enclosed by a water-tight fence and the water pumped out. For this we substitute driving piles and sawing them off under water; or sinking cribs down to a hard bottom through the water. In both cases we sink the masonry, built in a great water-tight box (called a caisson) with a thick bottom of solid timber, until it finally rests on the heads of the piles sawn to a level, or on the top of a crib which is filled with stone, dumped out of a barge. Sometimes it is filled with concrete lowered through the water by special apparatus.[6] Another process, developed within the last twenty years, is to sink cribs through soft or unreliable material to a harder stratum by compressed air. This is an improvement on the old diving-bell. The air, forced into the bell-shaped cavity, expels the water and allows the men to work and remove the material, which is taken up by a device called an air-lock. The crib slowly sinks, carrying the masonry on its top. By this means the foundations of the Brooklyn bridge and of the St. Louis bridge were sunk a little over 100 feet below water. A recent invention is that of a German engineer, Herr Poetsch, who freezes the sand by inserting tubes filled with a freezing mixture, and then excavates it as if it were solid rock. The process of sinking open cribs through the water by weighting them and dredging out the material was followed at the new bridge recently built over the Hudson at Poughkeepsie, where the cribs were sunk 130 feet below water, and at the bridge building over the Hawkesbury River, in Australia. The Hawkesbury piers are sunk to a depth of 175 feet below water, and are the deepest foundations yet put in. The writer (who derives his knowledge from being one of the designing and executive engineers of both these bridges) sees no difficulty in putting down foundations by this process of open dredging to even much greater depths. The compressed-air process is limited to about 110 feet in depth. IV.