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 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.

Fig. 116.

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 position, as in Fig. 117, having the same weight of cargo on board, and supported by a wave, which, for the sake of illustration, we may consider as supporting the vessel upon a single point m the middle.

Fig. 117.

In this position we find the strain reversed; and in the place of the lower part of the hull of a ship being in a state of tension, it is, on the contrary, in a state of compression, and the whole of those parts below the neutral axis are subjected to that strain. On the other hand, the upper part is in a state of tension; and that tension, as well as the compressive strain below, will be found to vary in degree in the ratio of the distances from the centre of the neutral axis a (Fig. 117), round which the forces of tension and compression revolve. In this supposed position we may venture to calculate the strengths, in order to ascertain the limit or maximum of security, and act as if the vessel were placed in trying circumstances, - either contending with the rolling seas of a hurricane, or suffering the actual suspension of either portion when taking the ground. In these critical positions, we arrive at the conclusion, that calculations founded upon the formula for wrought-iron tubular beams will determine the strength and resisting powers of an iron ship, and that under every contingency and every circumstance in which the vessel can be placed. Moreover, it will give a wide margin of security under all those forms and conditions of peril to which every vessel navigating the ocean is exposed. We are fully aware that many thousand vessels are now afloat that would not stand one-third of the tests which we have taken; but that is no reason why we should not endeavor to effect more judicious distribution of the material, in order to attain the maximum strength, where human life and the fortunes of the public are at stake.

To show that we have not selected tests which no vessel would stand, we append the following incidents:

In hauling an iron steamer of nearly 400 tons burthen out of a temporary basin, she grounded on the extreme end of the bank, and was left, as the tide receded, with forty feet of her stern entirely without support, and her bow buried in the opposite bank. On the return of the tide, the vessel floated, and immediately afterwards she proceeded on her voyage.

A large steamer, the "Vanguard," ran foul of a reef of rocks on the west coast of Ireland, and continued exposed to the swell of the Atlantic beating her upon them for several days, with comparatively little injury, excepting only the corrugation of the plates along her bottom. She appears to have rested upon a number of small hard rocks from the stem to the full part of the vessel just under the paddle-wheels, and from that part to the stern to have been quite unsupported, except at one place where the keel was broken. Mr. Clark, who went to examine her, states that "although she was beating hard for so many days, no part of her engines was deranged. Her engines were kept constantly at work, and, in his opinion, are now in as permanent working order as ever they were. Had the 'Vanguard' been built of wood instead of iron, she could not have been saved."

"The ' Royal George,' one of the iron steamers running between Liverpool and Glasgow - a vessel of unusual length in proportion to her beam - got on a rock near Greenock at high water, when loaded with about 150 tons of dead weight besides her engines and coals, and was left there high and dry during a whole tide without sustaining any injury. She rested nearly on her centre; and all who saw her were of opinion that no timber vessel could have remained in that position without breaking her back." *

We might adduce numerous other instances in which iron vessels have, without material injury, stood the strains which must have caused a timber vessel to go to pieces. An iron ship is united by riveting into a single firm mass; whilst a wooden vessel is composed of an innumerable number of pieces, all imperfectly joined together, but which are, nevertheless, dependent on each other for support, - so that if any one gives way, the stability of all the rest is endangered.

In his paper on iron as a material for ship-building, Mr. Fail bairn gives the following results of some experiments on the comparative strength of wood and iron, when subjected to pressure from a blunt instrument placed at right angles to the surface of the plate. It will be seen that, in these experiments, an endeavor was made to place the material in circumstances similar to those mentioned above, where the vessel is beating upon hard and unequal ground. In these experiments, the wrought-iron plates were fastened upon a frame of cast-iron, one foot square inside, and one foot six inches outside. The sides of the plate, when hot, were twisted round the frame, to which they were firmly bolted. The force to burst it was applied in the centre by a bolt of iron, terminating in a hemisphere three inches in diameter.

* Grantham "On Iron as a Material for Ship-Building."

Here the strengths are as the depths, a half-inch plate requiring double the weight to produce fracture that had previously burst a quarter-inch plate.

The experiments on wood were made upon good English oak, of the same width as the iron plates. The specimens were laid upon solid planks twelve inches asunder, and by the same apparatus the rounded end of the three-inch pin was forced through them.

Here the strength to resist crushing follows the ratio of the square of the depth, as is found to be the case in the transverse fracture of rectangular bodies of constant breadth and span. The experiments show conclusively the superiority of iron in ordinary cases.