by caulking to make it water-tight. In this construction everything is against the strength of the ship; and, in a large sea-going vessel it would be impossible for her to hold together unless united by longitudinal stays

Fig. 28.

and stringers of iron.

5. To ascertain the correct value of this construction a platform of wood sheathing of one-sixth the full dimensions was prepared and submitted to the test of experiment. It was composed of red pine, screwed to ribs four inches asunder, representing the frames in a ship in full size of two feet asunder, planking 2 inches by 1 inch, representing sheathing of 6 inches thick, 12 inches wide, and 12 feet long. The whole length of the model was 8 feet, and having attached cross-heads at each end, as shown in the drawing, fig. 28, it was suspended to the jib of a crane, and experimented upon as follows:

TABLE II.-TENSILE STRENGTH OF TIMBER FRAMES.

Length 8 feet, Ribs 4 inches apart. Joints 2 feet apart. Planking 2 x 1 inches. Sectional area= 99 x 7.95 inches = 7.87 square inches.

With the first weight, 991 lbs., the joints were slightly separated; with 1,528 lbs. considerable contortion was observed; and with 1,751 lbs. it separated at the joints in the form shown at a a a, fig. 27, or on a larger scale at fig. 29.

It is almost superfluous to make the comparison-if the above experiments are to be relied upon--as the disparity is so great as

to reduce the strength of a wood-clad ship on iron frames to less than that of the iron one. If, for example, the area of the

130

model be reduced one-fifth, it would then be the representative of 1.57 inch area of iron, which, taken at 30,000 on the square inch, would be 30000 x 1.57-47100 lbs., as the breaking weight at which iron sheathing of that proportion would be torn asunder. Now comparing this with 350-2 lbs., the breaking weight of the model sheathing, and the result is 47100: 350-2, or as 134-51; that is, iron is, in this position, 134 times stronger than wood.

In this comparison the sheathing of wood on iron ribs can only act as a water-tight covering attached to iron frames, as the tensile strength of the timber, in the absence of solid transverse joints, contributes only in a very small degree to resist the strains of the ship, and hence arises, in every description of timber-built vessels, the creaking noise and opening of the joints as the vessel plunges and labours in a heavy sea. It is however widely different with an iron-sheathed vessel, as in this case there are no joints, and the whole of the sides and hull present a powerfully resisting force to the strains and motion of the ship.

6. An exclusively timber-built ship has many advantages over that of the composite construction, as the ribs are nearly close together, and when properly fitted with wood or copper fastenings, as practised in Her Majesty's dockyards, the strengths are considerably increased, but still greatly inferior to a properly constructed iron-built ship.

From the extremely limited powers of resistance indicated by the model, it was considered necessary that the tensile strength of the timber, of which it was made, should be tested, but this had no effect on the strength of the platform, as the resistance in the construction does not apply to the timber, but to the pins or fastenings by which they are attached to the ribs. In

the case of the model, the screw nails were bent and torn asunder by a comparatively small weight. Doubtless, if the nails had been made double the strength, the results would have been more satisfactory, but the material, whatever it may be, is not calculated from position to meet the force of tension where the joints are numerous and unconnected.

The following tables give the strength of the timber of which the model sheathing was composed :—

No. of Experi

ment

TABLE III.-TENSILE STRENGTH OF TIMBER.

Sectional area of timber=98 x 1.04 inches = 1.01 square inches.

TABLE IV.-TENSILE STRENGTH OF TIMBER.

Sectional area of Timber=98 x 2:00 inches = 1.96 square inches.

7. To show the value of iron constructions as compared with those in combination with wood as exhibited in section figs. 40 and 41, it may be instructive to insert an extract from the experiments of 1838, or that part of them which relates to the resistance of wrought-iron plates and different sorts of timber to pressure, by a blunt hemispherical instrument, at right angles to the surface.

Irrespective of the experiments made to determine the strength of wrought-iron plates and the relative strength of the joints by which they are united, the investigation would be incomplete if we omitted another inquiry of equal importance, namely, the resistance offered by plates to a crushing force, such as exhibited in the injuries received by vessels when stranded on rocks or taking the ground in harbours where the surfaces

are uneven.

Almost every person connected with nautical affairs is acquainted with the nature of the injuries received by timberbuilt vessels when placed in circumstances affecting their stability, or when resting on hard and unequal ground, such as frequently occurs in tidal harbours at low water. Such a position is attended with danger under every circumstance; and in order to determine the relative values of the two materials, wood and iron, it was considered desirable to institute a similar class of experiments on both, and thus to afford the means of comparison between them. English oak, as the strongest and best material used for the construction of first-class wooden vessels, was selected for this purpose, and the results obtained from both are given, under circumstances as nearly similar as the nature of the experiment would admit. They are as follows:

8. In each of the experiments the plate was fastened upon a frame of cast iron, 1 foot square inside and 1 foot 6 inches outside, its breadth being 3 inches and thickness half an inch. The sides of the plates, when hot, were twisted round the frame, to which they were firmly bolted. The contraction, by cooling, caused it to be very tight, and the force to burst it was applied in the centre. This was done in order that the force might in some degree resemble that from a stone or other body with a blunt end pressing against the side or bottom of a vessel: a bolt of iron, terminating in a hemisphere 3 inches in diameter, had

thus its rounded end pressed perpendicularly to the plate in the middle. The results are given in the following tables :

TABLE V.-EXPERIMENTS TO DETERMINE THE RESISTANCE OF PLATES OF WROUGHT IRON TO A FORCE TENDING TO BURST THEM.

In figs. 30 and 31 will be found representations of the fractures of the plates experimented upon.

From the above we obtain the strength of plates to resist