in Fig. 109 have come into general use as the most effective and easy of construction.

KEELS. This part of the vessel requires to be made exceedingly strong to resist the pressure or violent shocks to which it is subjected, when a vessel grounds. It is made in various ways, generally with a false keel, which is riveted on below the ribs by two angle-irons. The false keel is intended to receive the first shock in grounding; and is so arranged that it may even be carried away without material injury to the true keel. Fig. 111 shows a method in which it will be seen that the sheathing-plates a a are bent downwards so as to grasp the side of the keel, which consists of a massive plate of iron; whilst the angle-iron of the ribs is bent upwards at right angles, and is firmly riveted to the vertical keel plate.

Fig. 111.

DECKS.-The floorings are supported upon beams extending from one side of the vessel to the other, and attached at either end to the ribs or side frames. In the section, Fig. 108, the two upper decks are supported upon wooden beams, as in an ordinary Fig. 112. wooden vessel; but wrought-iron beams may be substituted for these with great advantage, as shown at g 9.

Fig 113.

These deck-beams have been made of various forms, the best of which for large vessels is probably that shown in Fig. 112, which consists of angle irons riveted to the top and to the bottom of a thin vertical plate. In some cases a vertical plate, with two angle-irons at the top and one at the bottom, is used, and has

the advantage of greater simplicity, though the material is not so well distributed. The box beam (Fig. 113) is employed for sup porting the shafts and paddle-boxes of steamers, etc.

RIVETING OF THE PLATES.-In all wrought-iron constructions, the mode of joining two plates together is the same. When the article can neither be produced at once from the rolling-mill nor the steam-hammer, and except in the comparatively few cases where parts are welded together, they are universally united by rivets. A series of holes being made through both pieces, a small bolt, with a head upon one side, is passed through each, and then quickly hammered down on the other side to another head, so as to grasp the parts tightly between them. These rivets are usually employed in a red-hot state, both because they are then more easily hammered down, and because in cooling they contract and draw the parts together with great force.

Since the introduction of this process, the greatest improvement has been the substitution of the riveting-machine, invented by Mr. Fairbairn; by means of which the object is secured in considerably less time and at less cost, and which completes the union of the plates with much greater perfection than could possibly be done by the hand. But this new and very superior process has not as yet been successfully applied to the riveting of plates for ships.

On comparing the strength of plates with their riveted joints, it will be necessary to examine the sectional areas taken in a line through the rivet-holes with the section of the plates themselves. It is perfectly obvious that in perforating a line of holes along the edge of a plate, we must reduce its strength; it is also clear that the plate so perforated will be to the plate itself, nearly as the areas of their respective sections, with a small deduction for the irregularities of the pressure of the rivets upon the plate; or, in other words, the joint will be reduced in strength somewhat more than in the ratio of its section through that line to the solid section of the plate. For example, suppose two plates, each two feet wide and three-eighths of an inch thick, to be riveted together with ten three-fourth inch rivets. It is evident that out of two feet, the length of the joint, the strength of the plates is reduced by perforation to the extent of seven and a half inches; and here the strength of the plates will be to that of the joint as 9: 6.187*, which is nearly the same as the respective areas of the solid plate and that through the rivet-holes; or as 24: 165t. From these facts it is evident that the rivets cannot add to the strength of the plates, their object being to keep the two surfaces of the lap in contact. It may be said that the pressure or adhesion of the two surfaces of the plates would add to the strength; but this is not found to be the case to any great extent, as in almost every instance the experiments indicate the resistance to be in the ratio of their sectional areas.

The ratio of the areas.

The ratio of the breadth of metal

Fig. 114.

When this great deterioration of strength at the joint is taken into account, it cannot but be of the greatest importance that in structures subjected to such violent strains as ships, the strongest method of riveting should be adopted. To ascertain this, a long series of experiments were undertaken by Mr. Fairbairn, some of the results of which will be of interest here. The joint ordinarily employed in ship-building is the lapjoint, shown in Figs. 114 and 115. The plates to be united are made to overlap, and the rivets are passed

through them, no covering-plates being required, except at the ends of the plate where

Fig. 115.

they butt against each other. It is also a common practice to countersink the rivet-heads on the exterior of the vessel, that the hull may present a smooth surface for her passage through the water. This system of riveting is shown in Fig. 111, where the rivets of the sheathing-plates are countersunk. This system of riveting is only used when smooth surfaces are required; under other circumstances their introduction would not be desirable, as they do not add to the strength of the joint, but to a certain extent reduce it. This reduction is not observable in the experiments; but the simple fact of sinking the head of the rivet into the plate, and cutting out a greater portion of metal, must of necessity lessen its strength, and render it weaker than the plain joint with raised heads.

There are two kinds of lap-joints, those said to be single-riveted (Fig. 114) and those which are double-riveted (Fig. 115). At first the former were almost universally employed, but the greater strength of the latter has since led to their general adoption in the larger descriptions of vessels. The reason of the superiority is evident. A riveted joint gives way either by shearing off the rivets in the middle of their length, or by tearing through one of the plates in the line of the rivets. In a perfect joint the rivets should be on the point of shearing just as the plates were about to tear. but in practice the rivets are usually made slightly too strong. Hence it is an established rule to employ a certain number of rivets per lineal foot. If these are placed in a single row, the rivet-holes so nearly approach each other that the strength of the plates is much reduced; but if they are arranged in two lines, a greater number may be used, and yet more space left between the holes, and greater strength and stiffness imparted to the plates at the joint.

The results of Mr. Fairbairn's experiments upon the two forms of joint are given in the following summary:

From the above it will be seen that the single-riveted joints have lost one-fifth of the actual strength of the plates, whilst the doubleriveted have retained their resisting powers unimpaired. These are important and convincing proofs of the superior value of the double joint; and in all cases where strength is required, this description of joint should invariably be used.

Comparing these results with those of a former analysis, we have

which in practice we may safely assume as the correct value of each. Exclusive of this difference, we must, however, deduct thirty per cent. for the loss of metal actually punched out for the reception of the rivets; and the absolute strength of the plates will then be, to that of the riveted joints, as the numbers 100, 68, 46. In some cases, where the rivets are wider apart, the loss sustained is, however, not so great; but in boilers and similar vessels where the rivets require to be close to each other, the edges of the plates are weakened to that extent. Taking into consideration the various circumstances affecting the experimental results, we may fairly

*The cause of the increase of strength in the double-riveted plates may be attributed to the riveted specimens being made of best iron; whereas the mean strength of the plates is taken from all the irons experimented upon, some of inferior quality, which will account for the high value of the doubleriveted joint.

assume the following relative strengths as the value of plates with their riveted joints.

Taking the strength of the plate at

100

56

The strength of the riveted joint would then be . 70 And the strength of the single riveted joint WOOD AND IRON AS MATERIALS FOR SHIP-BUILDING.-We shall consider this point under three heads—

STRENGTH,

DURABILITY,
ECONOMY.

To ascertain the superiority of iron over wood in regard to strength, let us consider the strains to which a vessel is subjected. Let us take, for example, a vessel of similar dimensions to the "Great Western" (the first steamer that successfully crossed the Atlantic), 212 feet long between the perpendiculars, 35 feet beam, and 23 feet from the surface of the main deck to the bottom of the sheathing attached to the keel. Now, considering a vessel of this

Fig. 116.

magnitude, with its machinery and cargo, to weigh 3000 tons, including her own weight; and supposing, in the first instance, that she is suspended upon two points, A and B, resting on the bow and stern, at a distance of 210 feet, as shown in Fig. 116; we should then have to calculate, from some formula yet to be determined by experiment, the ultimate strength of the ship.

To determine this formula with accuracy is a work of research. In the meantime, we are fortunate in having before us that which applies with so much certainty to tubular bridges and tubular girders; and all that is required in this case will be to ascertain the correct sectional area of the plates, to prevent the tearing asunder of the bottom, and the quantity of material necessary to resist the crushing force along the line of the upper-deck on the top. It is true that the necessary data have yet to be determined; but the iron ship-builder cannot be far wrong if he assumes the weight W in the middle (Fig. 116) to be equal to the united weights of the ship and cargo. This, in the case before us, would give an ultimate power of resistance of 3000 tons in the middle, or 6000 tons equally distributed along the ship, with her keel downwards.

Assuming these tests, or the calculation derived therefrom, to be correct, let us now bring the vessel into a totally different po