Another engineering device, very necessary in a land where foundations are so frequently built under water, is the enclosed caisson with compressed air, as shown in detail in this exhibit. It was originally invented by M. Triger to keep the water expelled from the sheet-iron cylinders which he sunk through quick-sands in reaching the coal-measures in the vicinity of the river Loire in France. The seams of coal in this district lie under a stratum of quicksand from fifty-eight to sixty-six feet in thickness, and they had been inaccessible by all the ordinary modes of mining previously practised. The system has been much amplified and improved since, especially in sinking the foundations of the St. Louis and the New York East River bridges, and does not require specific description. An improved air-lock, by which access is given from the exterior to the working chamber at the part where the men work in an atmosphere sufficiently condensed to exclude water from the lower open end--like a tumbler inverted in water--is the principal addition which America has made to the device. We need not go abroad to find long bridges, but the great bridge, with three immense iron trusses and eight smaller ones, over the Wahal near Bommell would be respectable anywhere. Our Louisville bridge is a parallel example for length, but the truss is different. The dikes and jetties of the new embouchure of the Meuse embrace the same features of extending a river's banks into deep water, and by confining the stream making it scour out its own bed, as now so successfully practised by Captain Eads in one of the passes of the Mississippi River. Limbs and saplings made into gabions and staked together form mattresses, and by loading with stone are sunk in position. They soon become silted up, and are practically solid. Others are made and laid upon them _ad libitum_, and at last raise the crest above the level of the sea, the last course being laid with the advantage of high-water spring tides. This foundation supports courses of pitched masonry on its side, and these protect the stone or gravel embankment, which forms a roadbed. The river's water, instead of, as formerly, depositing its silt at the embouchure as its motion is arrested on reaching the open sea, carries its silt along and deposits it farther out: if a favorable shore-current occurs, it is swept away laterally, and so disposed of. The maritime canal of Amsterdam is another late success of this remarkable people, which leads the world in dikes and drainage of low lands, as the Italian does in the art and area of irrigation. The present canal may satisfy the great and still rising commerce of Amsterdam, the previous ship-canal, fifty-one miles in length, built in 1819-25 at a cost of $4,250,000, and deep and wide enough to float two passing frigates, having proved insufficient.