We shall conclude this section by a notice of what are called Twin Boats, of which there have been several constructed both in this country and America. These vessels are composed of two floating bodies placed side by side, but some distance asunder, and connected together by a deck extending over both, and the paddle-wheel is ordinarily single, and placed in the middle. The chief object proposed in this construction is to obtain great speed by employing vessels of very great length and comparatively small transverse section, and, therefore, calculated to experience but little resistance from the water, and at the same time to obtain increased lateral stability. The plan, however, has not, upon the whole, been found to succeed: one objection we understand to be, that the water, not being able to escape laterally, accumulates in the space or channel between the vessels, and materially increases the resistance. Twin steam boats were employed for several years on the Tay ferries, but have since been superseded by vessels of the ordinary construction. The largest twin steam vessel ever built was the Great American Steam, Raft, which was formed of two separate hollow trunks, or spindles, with a deck laid over them.

These trunks were 300 feet long, and in the centre, or thickest part, were 8 feet in diameter, tapering in a regular parabolic curve to a point at each end. The paddle-wheel, which was 30 feet in diameter, worked in the space between the trunks. We do not know what was the power of her engines. The marginal figure is a cross section of one of the trunks, aaa are the staves, 26 in number, and 3 1/2 inches thick, to each of which is attached an iron bolt, b b, 26 inches long, passing through the staves,and countersunk on the outside of them. These bolts pass through an iron ring, c, on the inside of which they are screwed up by nuts, dd so that the staves are brought into very close contact; sufficient room is left in the centre for a man to enter and pass fore and aft, and turn the nuts, if necessary.

The annexed figure shows the plan of the beams connecting these two spindles, or trunks, upon which the decks are built: a a the trunks or spindles; b the paddle-wheel; cc c the boiler; dd the beams which connect it with the outside guard; ee,ff, the braces. This vessel was built at New York; was destined to ply between that city and Troy, hour; the greatest authenticated speed at sea was, however, only 14 miles per hour. At anchor her draught of water was 24 inches, but when running at her greatest speed it was stated to have decreased to 17 inches. After making a few voyages she was unfortunately lost by running upon a bank off the city of New York, where she speedily went to pieces. In 1837, Mr. Neil Snodgrass, of Glasgow, obtained a patent in this country for an invention similar to that of the American Steam Raft, hut with improvements in the details. His plan is to form the buoyant vessels, which support the superstructure in which is contained the cabins and machinery, of sheet iron, the form of the buoyant vessel to be cylindrical at the middle portion of the length, and at about one-third of the length from each end to taper off conically.

To prevent the loss of buoyancy which might arise from leakage, each of these vessels is divided transversely into separate compartments, of about four feet each in length, made air and water-tight, by forming them at first in separate lengths, and riveting them together on hoops of angle iron. To stiffen the entire vessel, four malleable iron bars are riveted thereto equidistantly and longitudinally their whole length, and along the upper side of each vessel is a beam of wood, which extends the whole length of the lower deck, and to which the whole of the joistingand beams for supporting the decks and machinery are bolted. The paddle-wheel to be placed in the channel between the floating vessels, and at about five feet before the centre of the vessels' length. In order to give a clearer idea of the minor details, we extract the marginal figure from his specification: it is a plan of the vessel on the lower deck;distant about 160 miles up the Hudson. It is said, that on her first trip she ran 21 miles in 61 minutes, and that her average speed was about 20 miles per

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At a a are the conical vessels; 6 b, the lower decks laid upon transverse joisting; c is the chief cabin; d the second; e e promenade surrounding the cabins; f f four cylindrical boilers, which are all connected to one steam pipe, which is provided with a large safety valve at g, from whence there is a large tube to convey the waste steam into the water reservoir below. At the lower part of each boiler is a pipe provided with a two-way cock, either to feed or to blow off each boiler; and in the general steam pipe above are four valves, through which the steam passes from short vertical pipes, leading from each boiler; by shutting any one of these valves, and the feedcock beneath, appertaining thereto, such boiler may be detached, if required; h is the steam engine, placed horizontally, and connected by the rod, k, to the crank, in, on the end of the shaft of the paddle-wheel; n o the funnel; p p rudders hung to the stern of each of the floating vessels. On the head of each rudder is fitted a yoke or crosshead, from which a rod or chain proceeds forward along the under side of the lower deck, where it is connected by any suitable means to the pilot's steering wheel, fitted on the upper decks, and in front cf the boat.

The annexed figure shows an end view of the boat. a a the floating cylinders; b b the lower deck, supported upon joists, crossing the sleepers, cc, and also by the diagonal says d d, proceeding from the cylinders, a a; c, the cabins; f the paddle-wheel; g the upper deck, surrounded by a railing; h the funnel.

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A small vessel on Mr. Snodgrass's plan, nearly resembling that shown in the figure, was constructed at Glasgow. We do not know whether she was.finally successful, but we believe the first trials did not altogether realize the expectations which were formed of her performances. This might, in part, arise from the floating cylinder not being of sufficient dimensions to carry the upper works, for with merely the machinery on board, the cylinders were fully one half immersed.

About the same time Mr. Gemmell of Glasgow, obtained a patent for improvements in steam vessels, part of which refer to the arrangements of the machinery on twin or double boats. Vessels of this description are generally propelled by a single wheel placed between the two boats, but Mr. Gemmell considered, that, owing to the rapidity of the current through this channel when the vessel is in motion, the paddles do not obtain sufficient reaction or resistance from the water to propel the vessel; he therefore proposed to employ two wheels, placing one at the outer side of each boat. The annexed figure is a plan of the boat and machinery: - a a are the twin hulls or boats, which are firmly united together by the beams bb, and the trusses c c. To avoid theliability, towhich twin-boats are peculiarly subject, of having other vessels or objects running between the hulls, or of otherwise becoming entangled together, from the bows of the two boats is projected a guard, dd, which also serves greatly to strengthen the parts therein included, e e are the boilers; ffthe engines; and g g the paddle-wheel.

We are not aware that any vessel has been constructed on Mr. Gemmell's plan, nor can we discover in it anything to recommend it. By placing the wheels on the outside of the vessel instead of the central channel, the extreme breadth of the vessel is augmented by the amount of the breadth of the channel, which, in the case of vessels plying on rivers, is a serious objection; and further, the paddle-wheels are no longer protected from collision with other vessels, but form the same dangerous and unsightly projections as in steamers of the ordinary construction.

It is somewhat singular that the first vessel propelled by steam in this country was a twin-boat, Mr. Miller's experiments being made in vessels of this description, which he had previously constructed without reference to the employment of steam as the propelling. power.

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