CANALS AND CANALIZED RIVERS.) The new entrance lock at Ymuiden, begun in 1921, is the largest in the world (see TABLE I). It is a remarkable structure of re inforced concrete resting on piles of the same material. The lock has a floor of reinforced concrete under and near the caisson sills but in the main chamber the natural bed is left uncovered between the side walls the foundations of which are, however, sheet piled (fig. 19). The lock was built on fine waterlogged sand, the excava tions being kept dry by means of the ring well system of pump ing. In a few other instances, including one of the large locks at Bremerhaven, an artificial floor has been dispensed with except at the ends of the lock chamber.