but not so simply or satisfactorily as those of iron ships, the difference being one inherent in the materials. To make the spur more efficient, it is usually armed with a sheath of metal or iron. Massive longitudinal and diagonal timbers are bolted inside the frames, and associated with iron crutches or breasthooks, to prevent the stem from being driven in or twisted when a ram attack is made. But even when all possible care is taken in fitting and fastening these strengthenings, the combination can scarcely be considered satisfactory. Weakness, working, and decay must affect it, as they do all other parts of a wooden structure. Repairs to such a bow must also prove difficult and expensive, as compared with the corresponding work in an iron-built ram, where all the parts are easy of access, and easily replaced. These are, however, matters of detail requiring no further consideration here, although they have great practical importance.

The superior strength of the bows of iron ships has been illustrated frequently in the mercantile marine, as well as in war-ships. Commonly, when collisions take place between two iron ships, the vessel struck is seriously damaged, perhaps founders, while the striking vessel escapes with little damage to her bows. Mr. Grantham quotes one case of special interest.* Nearly twenty years ago, when the Persia, the first iron-built Transatlantic steamer, was on her first voyage, she closely followed the Pacific, a wood steamer, and both are reported to have fallen in with large ice-floes. The Pacific was lost with all on board; the Persia ran against a small iceberg at full speed and shattered it, but sustained no serious damage.

The last class of local strains to be mentioned are those incidental to propulsion. Some of these have already been alluded to, viz. the strains connected with propulsion by sails, and those resulting from the attachment of the thrust-bearer to the hull of a screw-steamer. To these may be added

In his work on Iron Shipbuilding.

the strains produced by the moving parts of an engine, through the bearers to which they are secured; vibration or working at the stern of screw-steamers; strains in wake of the shafts of paddle-steamers; and many others. The whole subject is, however, one of detail, requiring to be dealt with during the construction of the vessel by her builder and the maker of the engines. Here again the general principle of distribution of strain underlies all the arrangements made. The parts upon which the strains are primarily impressed must be succoured by other parts of the structure,

with which they must be connected as rigidly as possible. Changes in the relative positions of the various parts cannot occur so long as the connections are efficient, and without such changes working cannot take place. Iron is a far better material than wood for making the connections, and it has been employed very generally for the purpose, even in wood ships, with great success. Vibration may, of course, occur without any absolute working in the structure; for either the ship as a whole may vibrate to and fro, or the observer may be deceived as to motion in

the structure by movements in platforms, or minor fittings forming no part of the structure regarded as a whole, and incapable of resisting strains or transmitting them. This distinction is especially important in vessels of great engine-power and high speed, wherein vibration, either real or apparent, may be considerable, whereas there is absolutely no working.

A single illustration of the usefulness of iron strengthenings in resisting local strains due to propulsion must suffice. Figs. 96-98 contain the details of one of the best examples that could be chosen; representing the arrangements at

the stern of one of the wood-hulled ironclads of the Royal Navy. Similar strengthenings have been extensively used in unarmoured wood ships. They were introduced in consequence of the serious working and weakness not unfrequently experienced at the sterns of the earlier screw steam-ships with good engine-power; and by their use these objectionable results have been altogether prevented. Inside the ship (see Fig. 96) the upper parts of the two sternposts are cased with iron plates; the heads of the posts are secured to iron plating (cc) worked on the upper beams. Between the two posts an iron knee (bb) is fitted, and strongly secured to the posts and to the counter of the ship.

With a lifting screw, this knee could not be fitted, but the screw-well might then be made an efficient strengthener. Partial bulkheads of iron are built across the stern at the fore side of the rudder-post and the aft side of the bodypost. The construction of these is shown in Figs. 97 and 98; their upper edges are secured to the deck-plating (cc), while their outer edges are bolted to the sides of the ship. Change of form is thus rendered practically impossible at those two sections. Change in the angle between the counter and the rudder-post is rendered difficult by the external metal knee a, Fig. 96, bolted to the post and the counter. Formerly these counter-knees constituted the main strengthening at the sterns of wood ships, and they were very frequently broken in the " throat" by the working of the post produced by the action of the propeller; now such accidents are scarcely known in the Royal Navy. The body-post is also strongly connected to the hull by the iron plating (dd, Fig. 96) under the lower-deck beams, and the brackets (ee). By these comparatively light and simple additions of iron strengthenings, what had been previously found an almost insoluble problem has been satisfactorily dealt with. This is but one example from the many which any reader interested in the subject will discover on investigating the details of construction in various classes of ships.

CHAPTER IX.

THE STRUCTURAL STRENGTH OF SHIPS.

The

THE structural arrangements now adopted in various classes of ships are the results of long continued development. Their origin is lost in antiquity, and many of the succeeding steps cannot be traced. During long periods, under the same conditions, methods of construction have remained unchanged; but altered circumstances and fresh requirements have produced great and rapid changes. From the canoe hollowed out of a single tree, or the coracle with its light frame and flexible water-tight skin, on to the enormous floating structures of the present time is a very remarkable advance; but the steps have been gradual, and not unfrequently unintentional, the full value of a new feature not being recognised until long after its introduction. history of this gradual change and improvement, culminating in the wonderful progress of the last half-century -into which have been crowded the development of ocean steam navigation, the introduction of iron sea-going ships, and the use of armoured war-ships-constitutes a most interesting field of study; but in the present work it cannot be touched. Nor can the structural arrangements of existing types of ships receive any detailed illustration, for which the reader must turn to strictly technical treatises on shipbuilding. It will be our endeavour-bearing in mind what has been already said respecting the causes and character of the principal strains to which ships are subjected-to make clear the general principles governing