Suction dredges are particularly adapted for excavating sandy bottoms. One type used for dredging channels consists of a large steam vessel with large bins into which the dredged material is pumped. At each side of the boat there is a long pipe which may be let down into the water. Each pipe terminates in a drag or footpiece with grated opening which is designed to be dragged along the bottom as the vessel slowly steams ahead. Powerful pumps suck a stream of water up the pipes which carries with it a quantity of sand. The sand and water flow into the bins, the solid matter settling to the bottom while the liquid flows out over the top. When the settlings have filled the tanks, the drags are pulled up and the vessel steams out to sea. Here doors in the bottom of the bins are opened and the material drops through. The idea of opening up the bottom of a boat to empty it seems rather startling until we consider that the bins are sealed off from the rest of the boat and do not contribute to its buoyancy. The sand that is dumped out of them is much heavier than the water that takes its place when the bin doors are opened. CANAL DIGGING UNDER WATER The Ambrose channel in lower New York Bay was dredged by means of suction dredges. The channel is forty feet deep at low water and the bottom had to be excavated from ten to twenty-five feet to attain this depth. About seventy million cubic yards of material had to be excavated or nearly a third as much as was excavated in the Panama Canal. The Ambrose channel is seven miles long while the Panama Canal is forty-five miles in length. Two of the larger dredges each had a capacity of forty-five hundred cubic yards in their bins or enough to load a train about a mile long, composed of 175 cars. It took less than three hours to fill the bins. The openings in the gratings of the drags measured about eight by nine inches and any stones or solid matter small enough to pass through them was easily sucked up into the bins. When a pile of stones of larger diameter was encountered a deep hole was dredged around it and then by means of a water jet the stones were forced into the hole. The material sucked up by a dredge is sometimes dumped into a scow alongside. This makes the structure of the dredge less expensive, but where work has to be conducted in bodies of water exposed to storms it is more expedient to let the dredge collect the material within its own hull. LAND BUILDING WITH DREDGES