In the naval service, the first rate is a ship of the line of one hundred guns or upwards, having three decks or tiers of guns; and the seventy-four is of the line but third rate, with two decks or tiers of guns. The titles of sloops, brigs, corvettes, and cutters, are applied to the smaller sizes of vessels. In the merchant ser vice, a ship has three masts, all square rigged ; a bark has also three masts, but carries square sails on the fore and main masts, and a fore-and-aft sail on the mi zen-mast. A brig has two masts, and is square rigged. A '1.igant.ine or hermaphro rite brig has two masts, and is square rigged on the fore, and fore-and-aft rig ged on the main-mast. A schooner has two masts, both rigged with fore-and-aft sails ; sometimes a topsail is added, and the title is changed accordingly to topsail schooner. A sloop has only one mast, and is fore-and-aft rigged.
If we compare the carcase of a ship to the skeleton of the human body, the keel may be considered as the back -bone, and the timbers as the ribs. It, therefore, supports and unites the whole fabric, since the stem and stem-post, which are elevated on its ends, are, in some mea sure, a continuation or the keel, and serve to connect and inclose the extremities of the sides by transoms; as the keel forms and unites the bottom by timbers. The keel is generally composed of several thick pieces, placed lengthways, which, after being scarfed together, are bolted and clinched upon the upper side. When these pieces cannot be procured long enough to afford a sufficient depth to the keel, there is a strong thick piece of tim ber bolted to the bottom, called the false keel.
A new era has taken place in the art of ship-building ; and we are indebted to Sir Robert Seppings for some of the most important improvements in marine amid teeture which have characterized the pre sent century. Several large ships have already been rebuilt at Chatham on his principle, and orders have been given for building several new ships.
1st. The frame of a 74 gun ship, used to be formed of more than BOO different timbers, placed at right angles to the keel, which may be considered as the back-bone of an animal, and the frame timbers its ribs. Each rib is composed of separate pieces, of the thickness of 14 inches, or thereabouts. Between the se veral divisions of the frame or ribs, is a space from 1 to 5 inches wide.
2dly. The whole exterior frame was covered with planks of different thick nesses, or, to carry on the figure, the ribs are covered by a skin of greater or less substance, from the extreme ends of them to the keel or back-bone. The in
side of the frame was also almost entirely lined with planks ; within which is an other partial range, as it were, of interior ribs, at a considerable distance from each other, termed riders.
idly. Across this frame were pieces of timber called beams, united together so as to be of sufficient length to reach from one side of the ship to the other.
From this account, it will be perceived that all the materials composing the fab ric of a ship, are disposed nearly at right angles to each other. And this disposi tion, which is well known to be the weak est, is particularly so in a ship, the im mense body of which, subject to violent action from impulses in every direction, is sustained by a greater pressure on the centre than the extremities, arising chief ly from the difference in the fore and af ter parts of the body, to that of the mid ship or middle part.
The length of a 74 gun ship being 170 feet or more, it requires but little know ledge of the strength of timber to per ceive that planking of that length, how ever thick, or in whatever way joined or' put together, must, under the present system, bend with its own weight. The fastenings, and, consequently, the con nection of several parts of the fabric, must therefore suffer from the want of stifness, and a change of form is the con see uence.
This may be shown by putting together four pieces of wood, and then securing them with iron pins in the form of a square ; which, on the least pressure, may be made to change its form to the rhombus ; but let another piece be fixed to it diagonally, and the figure of the frame will be found immovable. Place a bar in the middle, parallel to two of the sides, and secure it firmly by iron pins ; still the figure will easily be moved by the hand, like a parallel ruler, and assume the rhomboidal shape ; but apply to the frame what the carpenters term time brace, as in a common field-gate, and the figure will remain immovable. And if this brace or diagonal piece is not fixed to it, the outer part of the gate (or that part most dis tant from the hinges) will have a constant tendency downwards, until at length it will reach the ground.
The substitution of the triangle, or brace, for the rectangle, comprehends the principle of the new system.