MECHANICS AND USEFUL ARTS. 27 



the deck. In the centre of the deck, lengthwise, and near one side of the 

 vessel, is erected a pier, at the top of which is a boom extending over the 

 water on the side where the weight is to be lifted, and over the deck on the 

 other side. The pier and boom resemble a cross, and constitute the derrick 

 proper. A wrought-iron arched truss, of 0-5 tons weight, passes from stem to 

 stern ; two trusses of 50 tons each cross the deck diagonally, and two more im- 

 mense trusses pass under the legs of the derrick. The object of them all is to 

 distribute the weight of the demck and its load equally over the whole deck, 

 and to enable the vessel to resist the leverage of the derrick. The pier is 

 composed of three wrought-iron legs, forming a tripod, each 80 feet high, 

 and weighing, together, 65 tons. They are surmounted by a horizontal plate, 

 or disk, near the edge of which is a trough filled with cannon balls. A sim- 

 ilar plate rests on the balls. On the top plate lies the boom. The balls be- 

 tween the plates are simply rollers, so that the boom can swing round like a 

 crane. To illustrate its action, place a row of marbles all round a dinner- 

 plate, in the groove or depression near its edge ; put another plate, bottom up- 

 wards, on the top of the marbles ; the top plate can then be revolved around 

 its own centre with very slight friction, under a great weight. The boom, or 

 cross, is a girder of wrought iron, 120 feet long, projecting 60 feet each way 

 from the pier, and weighs 80 tons. Surmounting the boom is a king-post, or 

 cylinder of wrought iron, 60 feet high, 7 feet in diameter, and weighing 60 

 tons. From its top to the ends of the boom extend massive iron cables. The 

 end of the boom over the deck is fastened directly to the deck by cables 

 when mere lifting and no swinging is required. But when weights are to be 

 transferred from the sea to the deck, or thence to the dock, these cables from 

 the boom are fastened to a car which runs on the underside of a circular rail- 

 way extending round and fastened to the legs of the tripod or pier. As the 

 boom swings, the car rolls with it. Suspended in a row from the other end 

 of the boom are 10 sets of gigantic, iron four-fold pulleys. The chain cables 

 working in them pass along the boom over fixed rollers down to the deck, 

 and under similar rollers along the deck, to their ten respective " crabs," 

 which stand in a row on the side opposite to the derrick, and by which they 

 are hauled in. All or a part of the pulleys or lifters may be worked at once, 

 as circumstances require. The crab is a massive frame, holding a windlass 

 consisting of a shaft attachable by a clutch to the main shaft from the en- 

 gine, and two other shafts so geared to the first that they move more slowly. 

 The two last shafts are simply grooved rollers, a foot in diameter, standing 

 say four feet apart. The pulley-chain passes around the pair of rollers (not 

 each roller separately) several times, and thence into the hold below, but is 

 not fastened to the rollers. The chain has such great friction on the rollers 

 that it will either raise the weight, stop the engine, or break, it cannot slip. 

 If the ordinary windlass were used, it Avould have to be of immense diameter 

 to hold several hundred feet of chain, while, if the chain were to wind upon 

 it in two or more layers, it would jerk, bind, and give trouble generally. The 

 two rollers revolve with and upon the periphery of the wheel which drives 

 them, so that their rolling friction is reduced to the minimum. Altogethei-, 

 these crabs are extremely well designed and adapted. In the middle of the 

 vessel, and attached to the shaft moving all the crabs, are two oscillating 

 steam engines of thirty-horse power, each with Barran's cup-surface boilers, 

 the latter being under the deck. 



" So far, we have a tractive power of 1000 tons on the 10 sets of pulleys. 

 But suppose them to be attached to a sunken ship of that weight. After a 



