This is composed of two long metal tubes let down through the decks somewhat apart from each other, but joined at their lower ends, which are pierced with holes for the admission of water. Above the upper part of the tubes is a sprocket-wheel worked by crank handles; over this wheel, and passing through both tubes, is an endless chain, furnished at certain distances with bucket valves or pistons, turning round a friction-roller. The whole, when set in motion by means of the crank handles, passing down one tube and up the other, raises the water very rapidly.