oak, the strength being as the depth or thickness of the plates in the first case, and as the squares of the depth in the second. The resistances are therefore in the ratio of 1: 12, the iron being 12 times stronger than the oak.

It will be noticed that the beams experimented upon in Tables VIII., IX. and X. are not of the strongest form of section. These defects, however, have long ago been remedied by the introduction of those composed of a web plate, to which double angle-irons, duly proportioned, are riveted, and form the top and bottom flanges of the girder, as shown in section in Table XII. Latterly beams are often rolled solid of the same form as that shown in Table XI.; but as we have fully investigated this subject in The Application of Iron to Building Purposes,' and is analytically considered in Chap. XIV. of this work, we may refer the reader desirous of gaining further information on the strength of beams to that volume, and to our concluding chapter on the strength of materials, than to pursue the subject more in detail.

19. From the foregoing experiments, relative to the weakness of jointed timber, and from the great difference which exists between the resisting powers of iron and wood, it is evident that any combination, however well executed, is not calculated to insure the requisite strength for sea-going vessels subject to severe strains. On the contrary, it appears obvious that a vessel constructed with iron frames and covered with wood sheathing is a decidedly weak and unsatisfactory structure. It is true, that a vessel of this kind may be strengthened, as before stated, by stringer plates and diagonal bracing, but to do this is not to adopt the form best calculated to give the required strength; it would be preferable and cheaper to make the ship entirely of iron, and dispense with the wood sheathing altogether. The longitudinal and cross sections, figs. 40 and 41, will clearly explain this mode of construction.

Different Methods of constructing Ships of Iron and Wood.

20. From the foregoing diagram, it will be observed that two wide longitudinal stringers, a, a, run the whole length of the ship on each side, and between these are a series of diagonal

plates or stays, e, e, e, &c., riveted on the exterior surface of the transverse frames in order to unite them in the shape of a side-girder equal in height to about two-thirds the depth of the ship. This plan of fastening the sides is certainly advantageous, and may be considered as an equivalent for those defects which we have noticed in regard to the very limited power of resistance obtainable from the exterior jointless covering of wood. As regards its powers of resistance, in retaining the ship in form, and in preventing her going to pieces in a heavy plunging sea, it must appear evident, that the lower stringer plate should have been much more extended, carried lower down the sides to render it available in stiffening the ship, and in affording that resistance to prevent her becoming hogged or tearing herself to pieces under strain.

In the lower part of the hull, the longitudinal keelson A, and the plates and angle-irons a, b, c, d, will materially assist in strengthening those parts, but they are decidedly imperfect when compared with an iron floor closely riveted, and a carefully connected exterior covering of iron plates.

21. Taking the vessel at 206 feet long, 32 feet beam, and 24 feet deep, it may be shown, by the usual methods of calculation, that the strength of this description of vessel is not one-half the strength of an iron ship of the ordinary construction, and only about one-third that of a well-proportioned and well-constructed iron vessel. These are facts entitled to consideration in this form of structure, as the vessel, of which we have given illustration, would be broken in two with one-half the weight of her displacement, or 1,200 tons, and one-fourth of that weight, or 600 tons, suspended from each end, when supported in the middle, would also produce the same effect.

22. It has been proposed, in order to strengthen vessels of this description, to have them double planked, as shown in fig. 42; but this would still be inadequate to meet the requirements of strength in a sea-going ship. This combination cannot, therefore, be recommended, and in every case, where safety is a primary consideration, there is no material so good, and no construction so perfect, as that of a well proportioned and a soundly constructed iron ship.

23. We might enlarge upon this subject if we had the power

Fig. 42.

to recommend it or any other modification of its construction. We are, however, convinced that the two materials are seldom united in the constructive arts with any degree of advantage, especially in cases where great strength is required, and more particularly in constructions which have to contend with forces calculated to tear them asunder. In these respects, the composite principle cannot be considered a good one; nevertheless, before dismissing the subject, it is necessary that we should notice Mr. M'Laine's (of Belfast) principle of uniting iron and wood for purposes connected with the navy.

Mr. M'Laine, in advocating this principle, states, 'that the principal strength of an iron vessel is located in the plating, and that the framing is of little value except to keep the plating in shape; that it is, therefore, a mistake to retain only the comparatively useless iron frame in composite vessels; and that it is much simpler, and more in accordance with correct principles, to retain the iron skin to give strength to the composite structure.' It is also stated, that the destruction of iron fastenings, in coppered vessels, is caused by the leakage, holding in solution

oxide of copper, chiefly taken up while filtering through the paper or felt underneath the copper sheathing; the leakage passes down the inner surface of outside planking, and if it come in contact with iron, the oxygen of the oxide of copper having a greater affinity for iron than copper, the atoms of copper are deposited, and oxide of iron is formed; it follows therefore, that any iron framing or plating coming in contact with the outside planking must be subject to serious deterioration where there is leakage, and that it would be highly desirable to prevent the leakage coming in contact with the iron of the structure.'

H

In order to accomplish the aforesaid desirable results, Mr. M'Laine proposes to construct vessels having keel, stem, sternpost, frame, and outer planking nearly the same as in an ordinary wooden vessel; but, instead of constructing the ceiling or inner planking of wood, he would construct it of iron, united all round at bottom and ends of vessel, and made thoroughly water-tight, forming a complete inner skin with beams, stringers, keelsons, bulkheads, platforms, &c., of iron, as shown in figs. 43 and 44, 45 and 46. The greater part of the wooden frame to be merely of dimensions sufficient for bolting wooden planking to, and inserted between (and fastened with fore-and-aft screw bolts to) angle-iron frames, riveted all round outside of the iron ceiling. He further states that the wooden floorings should be made deep in the throat, and stiffened with plates on each side, riveted to angle-iron frames, or iron floors to be fitted inside of iron ceiling to give requisite transverse strength. Apron, inner-post, and deadwood to be inserted between, and bolted to large angle-irons, riveted on iron ceiling, with extra bolts through ceiling, if requisite. Outer planking, within the influence of copper sheathing, to be fastened to wooden frames, with screw treenails or yellow metal bolts. Top timbers of frames to be, by preference, composed of teak. In wake of armour plates, spaces between frames to be filled in solid with teak, iron ceiling increased in thickness, and perpendicular angle-iron web stringers introduced at intervals, to resist shot, and strengthen the vessel.'

Owing to the iron ceiling being perfectly tight, no foreign matters could get into the spaces between the frames, to decompose in the bilge water and generate gases injurious to animal life and productive of decay in timber exposed to their influence; these spaces would, therefore, be eminently eligible for the introduction of a thorough system of ventilation through the vessel, by driving a current of fresh air, taken from above, through box-keelson, thence through apertures in bottom of keelson opening into each space between floors, then up through the spaces between frames, and out into 'tween-decks through apertures in iron ceiling, grated over and fitted with adjustable covers to regulate amount of ventilation. A three-horse engine would probably supply power sufficient for the ventilation of a