The arrangement of the materials in the triangular mode is such, that the pieces disposed horizontally are acted upon as ropes by a strain of the fibre, whilst the other parts, composing a series of triangles, are pressed upon as pillars ; in other words, the pressure acts in the direction of the fibres of the wood whereas, upon the rectangular or old plan, the fibres are acted upon transverse ly, or across the grain, in the same man ner as a stick is when placed across the knee, and pressed by the hands at each end, which first belids, and then breaks.
To prevent any transverse action upon the fibre of the timber is one of the ben efits arising from the new system, and to impede a longitudinal extension of the structure, is another. In a word, the sys tem of triangles is so constructed, in con junction with the planking of the ship, as conjointly to possess that property of a triangle already explained, viz., that its are is as unalterable as the compression or extension of the fibre of timber will ad mit it to be.
There is considerable difference in the details of ship-building as carried out in the United States compared with that of England. Even the technical terms of the art differ in the two countries. Few vessels here are built from the draught. In Europe, the line of flotation, or the in scribed line at the surface of the water, is called the first water line, or load line, and as they descend the figures increase. In this country, the lowest water line is denominated the first, and the numbers increase as we ascend.
Mr. naval constructor at Charles town, Mass., has discovered an ingenious mode of determining the capacity of ves sels, which is of ready application, and is adapted to all descriptions of freighting vessels ; sharp vessels, and our ocean steamers are exceptions, however, to its application. The rule is ; from 90° de duct the angle of the floor, or the degrees of dead rise ; multiply by -0075, the quo tient is the decimal for capacity. Multiply the length by the breadth, and thatpro duct by the depth, from the bottom of the garboard to the load line, and the last pro duct by the decimal of capacity, and divide by 35, the quotient is the capacity in tons.
The navigation laws of this .have a very injurious effect upon ship building, by building for tonnage rather than for quick sailing. A short time since it was thought necessary for a ship to draw more water aft than forward, in order to obey the helm readily, and this was made to appear on the water lines by the latter dipping considerably lower than the for mer. This practice has grown obselete,
and parallel water lines are used. Mr. Griffiths, in his work on Marine and Nitval Architecture, remarks, that the idea would have been regarded as preposterous of building a ship deeper forward than aft, but such is the present practice of New York, where it was first introduced, and the results have proved most satifaetory, ships having been built in this city hav ing from 3 to 5 feet of difference in depth at their ends, which adds greatly to their appearance us well as their performance. Stability of vessels in water is desirable, but to what this property is due is not yet decided. It is in part due to the breadth of the vessel compared with her general dimensions. Thus, increasing the beam, or a less proportion of depth, increases the stability. The steamer Georgia is the widest vessel of her class, except the Great Britain, but yet is one of the easiest vessels in her motions that floats. She is 3 feet wider than the Ohio, and wider than any of the Collins line, which are much larger than the Georgia. The Cunard steamers are narrower, though longer and deeper. The America and Europa have but 38 feet of moulded beam, and the Canada but 891 feet, while the Georgia, with ten more feet of beam, has more practical than any European steamer. What is true of steamers is true of ships. They may be built so long and so wide that the motion of the sea willMot be felt, that is, they will neither roll nor pitch.
An easy or light draught of water is essential to speed, and therefore to river navigation; it is generally looked for in shape, whereas it probably should be sought in the nature of the material and in the density ; very light draught iron boats are superior to wood. Other cir cumstances concur to favor the use of iron, such as the rapid rotting of the tim ber of the West Indies, South America, and even of that of our Southern States ; hence, for low latitudes, where ventila tion is not perfectly effected, iron ships are more durable. The cost of iron is, however, 25 per cent. more than that of wood. But the saving afterwards effect ed is an ample compensation. The wa ter tight bulk-heads would be a valuable adjunct in Mississippi vessels. These prevent the boat from sinking, even though a part be snagged and full of wa ter. The "Reindeer," on the Hudson River, is a model of speed even among the fast sailing vessels of that river.