the case where an iron ship strikes upon a sharp pointed-like crag of rock or coral reef with considerable force, it frequently happens that a hole is made through the plate; but even when such an accident occurs the damage is generally _local_, the parts not immediately subject to the concussion remaining unhurt. No general leakage is, therefore, consequent on such an accident, as would be the case in all vessels built of wood. [Sidenote: Affords greater capacity for stowage.] As the hull of an iron ship is both thinner and considerably lighter[122] than that of a wooden ship, an iron ship of the same external dimensions as a wooden one has both greater capacity for stowage and greater power to support the weights which may be put into her. These differences vary in some degree with the dimensions and form of the ship, being greater in proportion to the increased dimensions of the ship. They may, of course, be determined by computation; but, in all cases, an iron ship will carry considerably more cargo than a wooden one of the same external dimensions. Again, the consideration of economy must not be omitted in any comparison of the merits of ships built entirely either of timber, or of iron. The economy begins with the construction, for the original cost of an iron ship is less than that of a wooden one, and, apart altogether from her superior capacity for cargo, it runs on with the course of the ship’s service as the result of several causes; as, for instance, the smaller amount and less expensive character of repairs: moreover, as it is not even yet known how long iron ships will last, the precise saving from their use cannot be estimated. On the other hand, the period of service of mercantile timber-built ships is defined. If they reach or exceed thirty years’ service, they must be ships of the very highest class as to quality[123], and must, indeed, within that period have undergone frequent and very expensive repairs. As iron ships are not subject to the same decay, at the same time that accidental damages are generally repaired at a much less cost, every item saved by the diminished charge for repairs is clear profit. [Sidenote: Admiralty slow to adopt iron for ships of war.]