Trussing ships to prevent arching was used on the Continent as' early as 1759; and the Swedish architect Chapman, who describes the manner of placing the trusses, speaks of the practice as being generally followed. Tho method consisted in setting up three parallel rows of fir-pillars from one end of the ship to the other ; the middle row rested on the keelson, and the others on parallel longitudinal timbers, fastened by bolta to the ribs of the chip, one on each side of the keel ; on the heads of these pillars in each row, sod under the lower deck of the placedship, was placed a longitudinal timber like an architrave ; and diagonal braces were placed from the head of one pillar to the foot of the next in each of the three rows. It is evident that such a disposition of braces, if well executed, would serve to prevent or diminish the arching of the ship ; but it is far inferior to the method now followed, because of the interference with the loading, and the liability of the pillars to be displaced in consequence of a violent movement of the ship.
In order to understand the construction of a ship and Sir R. Sep pings's application of the braces, let fig. 5 represent a part, near the mainmast, of a longitudinal section through a large ship of war. Iu this figure e e is the top of the keelson ; h It and k k are timbers ex tending through the whole length of the ship above the ribs, which are supposed to be close together, and not covered by an interior planking; m m, &c., are the principal timbers of the braces; n n, &c., are the trusses to those timbers ; x x is a longitudinal shelf, on which rest the beams of the orlop deck, and the section shows how it is sup ported on the braces m m, &c. ; y y is a like shelf, on which rest the beams of the lower gun-deck. The beams which support the upper decks rest likewise upon longitudinal shelf-pieces, which appear at w to and z z; and these lie upon chocks or upon pillars placed against the sides of the ship at intervals between the decks : the same shelf-pieces appear at x, y, w, z, in fig. 4. Through these and through the ribs pass the bolts by which the iron knees r r r, fig. 4, are attached to the body of the ship. In small vessels the beams are not always fastened by iron knees, but are merely cooked and bolted to the shelf-pieces : two or more iron knees are however usually placed under the beam near the main and fore masts. The shelves just mentioned constitute as many hoops connecting the ribs of the fabric together longitudinally ; they are fastened by trenails and bolts to'the ribs and to the beams of the decks : e P, &c., fig. 5, are the ports, and the dispositions of the braces between them are shown at r &c.
The connection of the two sides of the ship with each other is effected by means of the beams which extend under the decks from aide to side. Tho tops of the iron knees are fastened to these, near
their extremities, by bolts passing through them ; and the lower parts of the knees are joined to the ship's side by bolts passing quite through the planking, the ribs, the chock-pieces, and the knees themselves. The beams of the principal decks in large ships are usually made in two pieces which abut end to end, and are connected together by having a strong middle piece scarfed and bolted to them. The planks of the gun and upper decks are recommended to be laid obliquely above the beams, and their outer extremities enter into a rebate formed near the interior side of the ship in certain longitudinal pieces which are channelled for the purpose of carrying off the water from the decks : the diagonal position is given to the deck planks, in order that they may increase tho strength of the ship in a transverse direction. The exterior planking of the ship is laid on the ribs in longitudinal directions as nearly as possible parallel to the surface of the water ; and at the extremities of the ship bolts pass obliquely through them and through the stem-post, the stern-post, and the adjacent rib timbers.
The oblique timbers, or diagonal braces mat, &c. (figs. 4 and 5), which are about six or seven feet asunder, cross the ribs at angles of about 45°, and are placed in contrary directions from the middle of each side towards each extremity of the ship. Their upper ends abut against the horizontal shelf under the lower gun-deck, and their heels or lower extremities against the keelson, or against the horizontal timber on each side of it. These braces are attached to the timbers of the ship by cylindrical cocks and bolts, and the lower ends arc con nected with the keelson by iron straps. The timbers hh, kk, which are of fir, are attached to the sides of the diagonal braces ; and the truss-timbers n n, &c., are placed diagonally across the rhomboidal space formed between the principal braces and the longitudinal pieces, in order to prevent the former from becoming bent by the compression which they may suffer endways from any strain which the ship may experience. The diagonal frames, together with the oblique trussing between the ports in the upper works, not only resist the tendency of the ship to become arched, but the former resist also any pressure which may take place externally against the bottom of the ship in the event of grounding ; at the mune time the exterior planking, the longitudinal timbers, and the oblique planking of the decks bind the whole fabric in one firm body. A close contact of the several parts of the diagonal frames is evidently of the utmost importance, as on it depends the security of the ship against a change of figure.