we have described and shown in fig. 46, and which, it must be admitted, is of inferior construction, we shall approximate nearly to the facts by treating it as a simple beam; actually a vessel is placed in this position, either when supported at each end by two waves or when rising on the crest of another wave, supported at the centre, with the stem and stern partially suspended. Now, in these positions the ship undergoes alternately a strain of compression and a strain of tension along the whole section of the deck, corresponding with equal strains of tension and compression along the whole section of the keel, the strains being reversed according as the vessel is supported at the ends or the centre. These are, in fact, the alternate strains to which every long vessel is exposed, particularly in seas where the distance between the crests of the waves does not exceed the length of the ship.

It is true that a vessel proportioned like the above section will continue for a number of voyages to resist the continuous strains to which she is subjected whilst resting in water. But supposing in stress of weather or from some other cause she is driven on a rock with her bows and stern suspended, in the position shown in the sketch, fig. 47, the probability is that she would break in two, separating at T from the insufficiency of the deck. This is the great source of weakness in wrought-iron vessels of this construction, as well as of wooden vessels when placed in similarly trying circumstances.

To prove this, let us give the vessel already described the full benefit of being considered a well-constructed beam, which indeed is more than we may expect; and applying the formula,

which would then be applicable, we shall find her powers of resistance comparatively small.

The sectional areas of wrought iron we shall find to be

To balance this area at the bottom, we have at the top, taking the sheathing-plates to a depth of six feet below the upper deck:

The deck-planking, which to a limited extent contributes to the resistance to a tensile strain, might be taken at one-sixth the resisting powers of iron, or equivalent to

but the entire absence of longitudinal joints reduces this resistance much farther, and we have, therefore, considered the resistance of the deck-planking to be equivalent to a section of 100 square inches of wrought iron.

Now if we apply the formula for beams, we have, for the constant, c=60, on account of the joints being only double riveted; a=400 square inches; d=the effective depth, which, since the side-plates have been taken for a distance of six feet from the deck, could not exceed 24 feet; l-length=300 feet; hence the centre-breaking load is equivalent to

or in other words, a weight of 960 tons suspended from

bow and stern, apart from the vessel's own weight, would cause her to break asunder.

We may verify this calculation by another, in which the strength is calculated from different data. If we substitute for the area of the top the whole midship area of the vessel, and for c=60 substitute C=18.3, we get,

which gives a close coincidence of result with the previous calculation.

If, however, the deck-beams were covered with iron plates throughout the whole length, on each side of the hatchways, so as, by a new construction, to render the area at the deck equal to that at the bottom, we should then have for the centre breaking weight

or nearly twice the strength given in the preceding case.

If we now consider the amount of displacement in tons in the vessel we have described, we shall find that the margin of strength is far from satisfactory. When loaded to a depth of 18 feet draught of water, the displacement would be about 177,000 cubic feet, which is equivalent to a weight of about 5000 tons for the ship and cargo. If we consider this weight as uniformly distributed, and compare it with the strength we have determined, we have,

Load uniformly distributed

Breaking-weight with load distributed, 1920 × 2

Leaving a deficiency or source of weakness equivalent to

or 4 of

so that it is evident that if laid high and dry, in the position shown in fig. 47, she would break with 3 the load which she actually carries. Under ordinar

cumstances, it is true that a vessel could never be placed in such a position, unless when stranded on a lee shore, or under circumstances where each receding wave would leave her with not more than six or eight feet of water over her keel, and in these conditions she must inevitably go to pieces.

I refer to these extreme cases because our iron constructions, in which we risk so much life and property, may be exposed to even this degree of danger, although circumstances so critical do not frequently occur. If we might suppose material added to the deck-section, either by iron plates under the planking or in any other form, so as to give an area of wrought iron equivalent to that of the bottom, or 604 square inches, the strength would be nearly doubled, but would still be short of an adequate margin for security to resist the force of impact, as the waves lifted the vessel and dashed her again on the rocks. may be urged that this is an extreme case; but it is an extreme such as we must guard against, and vessels ought in every case to be built of sufficient strength to secure them from failure in all the conditions in which it is possible for them to be placed.

It

Having shown the imperfect state of our constructions from an example selected from the earlier stages of iron shipbuilding, I would now direct attention to the most recent forms of iron vessels.

It will be observed from the following diagram, fig. 48, that considerable improvements have been effected, both as regards strength and the distribution of the material, since the infancy of iron shipbuilding, when the properties of the material and the results of its combination were very imperfectly known. It is now widely different; and though still far from perfection, it is nevertheless of a character which gives greatly increased security to life and property.