Having given the general outline of the present im proved practice of building ships, the reader is refer red for more minute instructions to " The Elements and Practice of Naval Architecture, 3d edit." When the fabric of the ship is finished, it then be comes necessary to put up magazines, cabins, and store rooms, as internal accommodations; the number, and generally the situation of these, depend upon the size of the ship, the service on which she is to be employ ed, and the quantity of stores and provisions necessa ry for the men to navigate her, and to fight her guns.
The internal conveniences and decorations are also to be attended to, such as the cathead for the anchors, the boomkins for the foretack, the channels for the reception of the dead-eyes for the security of shrouds and backstays, the stern galleries and headrails.
During the progress of building a ship, a variety of measures are resorted to for the purpose of preserv ing the materials from early decay, such as painting the surfaces of the timbers and planks which come in contact, injecting tar and lime into the bottom; but for these and other methods the reader is referred to a recent work, "Knowles on Preserving the Navy." When the ship is ready to be launched into the wa ter, sliding planks are laid upon blocks of wood on each side, at an inclination of of an inch to every foot of their length, or at an angle of newly 10'; these planks are usually laid straight, but a slight curvature is preferred by some builders. The distance of these sliding planks from the keel on each side de pends upon the form of the body; but as a general principle, it may he considered as one-sixth the ex treme breadth of the ship. Ribbands are fitted on the outer edges of the sliding planks to keep the cradles in their places. A combination of large pieces of timber, called bilgeways, is then placed upon the sliding planks, and a cradle fitted thereon to the form of the ship. This cradle is attached to the slip by a shore on each side, called clog-shores, lying at a small angle of inclination. A short time before the ship is to be launched, the bilgeways are canted out, and the sliding planks payed over with a composition made of soft soap, oil, and tallow. When the cradle is replaced and properly fixed, the shores against the sides and stern of the ship are taken away, the blocks under the keel split out, and the dog-shores removed; and thus, being left without any impediment, the ship glides clown the inclined plane into the water.
Soon after the introduction of the diagonal system, considerable doubts were entertained by many prac tical men respecting the proper disposition of the braces and trusses; some asserting that the arrange ment given to them by Sir Robert Seppings was just the reverse of that which ought to have been. To
obviate any objection on this head, we extract from a paper in the Philosophical Transactions for 1818, an account of an interesting experiment, performed by Sir Robert Seppings, to prove the truth of his me chanical arrangement of the trusses and braces.
'' Early," says he, "in the year 1817, the Justitia, an old Danish seventy-four gun ship, was ordered to be broken up on account of her defective state; and hav ing observed her to be considerably arched, or hogg ed, I determined, notwithstanding her age and defec tive state, to apply the trussing principle to a certain extent, with a view to observe what effect it would produce on a fabric reduced to so weak and shaken a condition.
The officers of the yard were directed to place sights on the lower and upper gun decks prior to her being taken into dock; and to ascertain, when she grounded on the blocks, how much she had altered from the state in which she was when afloat. They were then to place a certain number of trusses in the hold, some in the forepart of the vessel inclined for ward at about an angle of forty-five degrees, and others in the after part of the vessel inclining (fit at the same angle. Others were also to be placed at right angles to the former, and so as to act against the beams of the deck. In the ports also, other truss es were introduced, those in the ports forward, in clining forward in an angle of 40°, and those in the midships aft, at the same angles, but in an opposite direction. (A drawing of the arrangement of the trusses, S:c. may be seen in the part of the Philoso phical Transactions referred to.) As it was uncertain where the centre of fracture would take place, a few of the port holes about the centre of the ship had trusses introduced into them in both directions. Wedges were applied to the heels of the trusses to set them tight. The ship being thus partially trussed, the water was let into the dock, and the ship floated out of it into the basin, where she was to lay one hour, when a committee was to examine the sights, and ascertain how much the ship had altered; and again, what change had taken place in twenty-four hours after floating. This being done, the trusses were to be disengaged in as short a time as possible, in order to observe whether the effect of their removal would be instantaneous or gradual." The following is an extract from the report of the Committee.