water, the submarine.surveying apparatus will be used in addition to the ordinary mode of sweep- ing ; the stages (with the divers down the chain- ladders on their platforms) being drawn by a screw-steamer about half a-mile the hour, there being on either side of the chain-ladders the ordi- nary means of communication of air and for speaking.

The ship-lifting apparatus consists of two flat- bottomed iron vessels (with separating shores fore and aft, so as to admit the wreck coming up be- tween them), each fitted with a screw, and with powerful steam machinery available for working the screw, as also crab winches for the lifting process, and having a deck on which the machinery is placed and the lifting operations performed; a series of iron tubes are arranged in the central line of the same, passing from the deck to the bottom, and through these the chains are worked by powerful machinery, so that the lifting power is applied from the centre of the vessels without lurching or disturbing their vertical position, and having the advantage of direct action, it being a dead pull upon the wreck or subject to be lifted.

The encircling chain or chains are carried by the tug-boat, which is fitted on purpose to work those chains round the wreck, the divers pinning and shackling all the chains below water; the wreck, when raised to a sufficient elevation, and secured by chains passed under it, is then bodily carried to the beach or other required destination by aid of the propelling power of the screw at- tached to each lifting vessel; or, if it is desired to float the wreck,- or to lift it when stranded, it is then raised between the two lifting vessels by operations also from their centre brought to bear over their inner sides, a proportionate counter- balance of water (confined within a fore-and-aft compartment) being let in to the lifting vessels on their opposite or outer sides during this inner sidal strain.

As an illustration, take the case of a 120-gun ship, with its equipment, sunk in the channel.

In estimating the weight of a sunken vessel, and the power required to raise it, the following facts should be remembered:— That any solid body (whether large or small in bulk, in proportion to its weight), immersed in a fluid, displaces exactly Us own bulk of fluid, and the force with which the body is buoyed up is equal to the weight of the fluid which is displaced; hence any solid body of a greater density than water, when wholly immersed in that fluid, loses exactly as much of its weight as the weight of an equal bulk of the water, that is, of the water which it displaces. That the specific gravity of sea-water is 1-024 ;ozs. to the cubic foot. That there are 35 cubic feet of sea-water to the ton avoirdupois.

The total weight of a 120-gun ship out of the water is, by a careful analysis (set out in the prospectus) of the weight of the materials therein, extracted from the " Encyclopaedia Metropolitana," under the head of Naval Architecture, shown to be 4,609 tons.

To ascertain the displacement of water caused by the immersion of such a ship, take one item of the materials contained in the ship, viz., lead; of this the analysis shows there are 9 tons, or 20,160 Ibs. In a bulk of 9 tons of lead, the solid content of a cubic foot thereof is by the analysis shown to be 752 Ibs. By dividing the weight in pounds of the 9 tons of lead by the number of pounds in the cubic foot, we obtain the number of cubic feet of lead (26|), and hence its displace- ment of an equal bulk of water.

The aggregate of cubic feet of materials in the contents of such a ship is shown by the analysis to be 147,507, and this, divided by the number of cubic feet of sea-water in the ton (35), will give the specific gravity in tons of the displaced fluid, viz., 4,2144 tons ; and hence the difference of the specific gravities of the ship and the displaced fluid is the weight to be lifted.

Tons.

The specific gravity of a 120-guu ship is . 4,609 The specific gravity of an equal bulk of sea-water (at a calculation of 1,024 ozs.

to the cubic foot) is 4,2141 The weight to be lifted of a 120-gun ship sunk in the channel to restore its equili- brium, or float it, is therefore .... 395 The lifting power is that of steam, contained in the two flat-bottomed iron vessels. The available dimensions of each lifting vessel are—length, 120 feet; breadth, 30 feet; depth, 20 feet. The total weight of each lifting vessel with equipment is 455 tons.

By multiplying the length, breadth, and depth together, we obtain the number of cubic feet therein.

Tons.

120 x 30 X 20 = 72,000, and these, divided by 35 (the number of cubic feet of sea- water in a ton), will give in tons the gross buoyancy of each lifting vessel, viz. 2,057 Deduct the weight of lifting vessel with equipment 455 The floating power in tons of each lifting vessel is 1,602 A further deduction of l-7th must be made for the space above load-line, f. e., the water-line of the vessel when subjected to the greatest immersion consistent with flotation 223 The net available buoyancy in tons of each lifting vessel is 1,373 2 The capacity for applying power in the two lifting vessels is to the extent, in tons, of 2,746.