The principle on which the diagonal braces act may be easily under stood from the following considerations :—Let • m, • e, h. 6, be two principal braces; r n and n c, 0 D and 11 the struts or trusses inserted between them : then if 0 be the point of support, and if the parts beyond N and It be those which have a tendency to sink by the weight of the superstructure, it will follow from the 'elements of mechanics, that in this case the braces A at and A x, and the longitudinal timber r o, will be subject to the strain of extension in the direction of their lengths, while the trusses a II and n o, r u and n c, as well as the longi tudinal timbers D B and m x, will be subject to the strain of com pression. The effect of braces disposed in the positions A at, A N, in midships. and of others corresponding to them towards the head and stern, will be that the strain arising from the weight of the extremities Of the ship, and by which arching is produced, is counteracted by the power of the trusses, in positions corresponding to r b and o 0, to resist being compressed. A contrary effect would take place if the principal braces were disposed in the directions cr and c 0, c being the punt supported ; for then the trusses between them, and the longitudinal pieces D B, would become disengaged, and would fall out of their places.
Sir Robert Seppings's diagonal braces were first introduced in 1810, and his paper on the application of them is contained in the 'Philo sophical Transactions' for 1814. They were first applied to the Tremendous; and the success Nrafi complete, for after three years' service it was found, by means of lines of sights along the ship, that the gun-deck had not arched in the least, and the upper deck only three fourths of an inch.
The introduction of iron knees and braces in wooden ships has not only strengthened them considerably, but is a great advantage in the saving of apace for stowage. The she:ad:ling of ships at first consisted of a second covering of planks applied, on the exterior of the first, over the bottom and sides as far as they were under water. [SneAsniso.] To save the first expense of copper, a mixed metal called 31untz's Metal, or yellow metal, and composed of 6 parts copper to 4 parts zinc, was in 1832 manufactured at Birmingham under patent, and its use has considerably increased in the merchant service. It has been found that there is a limit to the quantity of copper in this kind of sheathing, as unless the quantity of copper be between 50 and 63 per cent., and the zine between 50 and 37 per cent., the compound will not roll at a red heat—while too much copper renders the compound difficult to work, and too much zinc renders it too hard when cold. By this use of metal sheathing shells and sea-weeds are partially prevented from adhering to the sides and bottoms ; the friction of the water against them is diminished, and the damage which would be caused by worms is avoided.
As further details on the practice of wooden shipbuilding may be obtained from such excellent works as that of Mr. James I'eake, of ' II.al. Dockyard, Woolwich, we only add some information on the manner in which various subjects intimately affecting the whole question of naval architecture have been treated.
In 1814, Mr. Walters, a civil architect, took out a patent for a method of counteracting the arching of a ship by braces of iron, which he proposed to apply on each aide of the ship between the rib-frames and the exterior planking. The braces were to extend obliquely both ways from the upper-deck beam in miclehipa, in the direction of the line of shortest distance on the surface, towards the keel, where those which were on opposite sides of the ship were to meet under the bows and stern. lly this disposition the extremities of the vessel could not sink without putting all the Umberto between the trusses in a state of compression, and thus all their joints would be kept close together.
In the' Memoir() de la Stabilit4 des Corps Flottants; published in 1822 by the Baron Dupin, the stability of a ship as derived from differ ence of form was considered, and has proved useful to the shipbuilder.
In about 1827, some lepers, chiefly contributed by the students of the School of Naval Architecture at Portamouth Dockyard, were pub limbed by Messrs. Morgan and Creuze, formerly pupils of that insti tution ; and as these embodied several valuable translations from foreign authors an the laws of the resistance of fluids, they may still be read with considerable benefit. The Rev. Dr. Woolley, the late principal of that school, and our present great authority in naval mathematics, declared, in a paper read before the Institution of Naval Architects in 1860, that the experiments therein referred to were most carefully made, and entitled to rank among the most important contributions towards the determination of the theory which have ever appeared.
In 1851, Mr. Fincham, the tnastersehipwright at Portsmouth Dock yard, published a' History of Naval Architecture,' which throws much light on various branches of the science.
In 1852, Lord Robert Montagu endeavoured, in a small work pub lished by him on naval architecture, to influence the form of vessels by the dissemination of a new theory affecting the question of the molecular motion of water when disturbed by an advancing body. Something similar was also published at New York by Mr. Griffiths. But no permanent benefit to science seems to have been thereby pro duced ; for, as already mentioned, the theory of resistances is still in a very incomplete and unsatisfactory state. The nearest approach to any theory founded on the disturbance of the particles of water is that of the form assumed by the mass of water so displaced, and which constitutes the theory announced to the scientific world in 1842 by Mr. John Scott Russell. This, as a question of shipbuilding, is one of paramount interest, intimately connected as it is with that of speed and the various other dependent qualifications of a ship, as inffiteneing the ready defence of the country. Mr. Russell's theory has been already alluded to under RUSSELL, JOHN SCOTT, in the Bioo. Div. of this but an illustration under SDIPBUILDINO may be acceptable.