DERRICK, a mechanical contrivance used for the same purposes as the crane, but recently so improved in size, strength, and mechanism, as to be able not only to raise a body of 1000 tons in weight, but to transport it from One place to another. The fel lowing description of the Great Floating Derrick, built in 1859, will convey an idea of the powers of this machine, and of the principles upon which it works. This derrick was built by the Thames iron ship-building company, at Blackwall. It consists of a flat-bottomed vessel, 270 ft. long, and 90 ft. across the beam, is divided throughout Into a number of water-tight compartments, which can be filled, so as to counterbalance any weight on an opposite side. From the deck of this floating steam-crane rises an .iron tripod 80 ft. high, on the top of which revolves a gigantic boom, 120 ft. long, and
above the boom the "king-post,' a continuation of the tripod, rises to the height of 50 feet. One arm of the boom is furnished with ten fourfold blocks; the chains attached tii.these blocks are passed across the king-post, brought over the other arm of the boom, aiid so descend to the other side of the vessel, where they are connected with two pow erful steam-engines, by means of which the weights are raised. This derrick is capable of being propelled by means of a series of bucket-paddle floats at the rate of 4 miles an bour. It is the invention of Mr. Bishop, an American. Derricks have been long in use in America, and have proved much more expeditious and economical titan any other species of lifting-power. They are chiefly used for lifting machinery or other great weights, and for raising wrecks.