has settled it definitively in favour of iron. Ships properly constructed of that material, and properly treated during their service, suffer but little deterioration during long periods. Wood ships, on the contrary, even when constructed of well-selected and seasoned timber, and carefully used, are, as a rule, subject to comparatively rapid decay. Many examples may undoubtedly be found of great durability in wood ships, but these are exceptional cases; and, moreover, their occurrence has not put within the power of shipbuilders any means by which similar durability can be secured in other wood-built ships. For instance, the Sovereign of the Seas, built at Woolwich in 1635, is said to have been pulled to pieces forty-seven years later, the greater part of the materials having been found in such good condition as to be used in rebuilding her. Still more notable is the case of the Royal William, built about 1715, which remained on service for ninety-four years with only three slight repairs. Both these vessels were built of oak felled in the winter, and much importance was attached to this circumstance; but later experience in the Hawke sloop, built in 1793, threw some doubt upon the previous conclusion, the vessel having fallen into such a state of decay in ten years that she was taken to pieces.*

The very numerous schemes for preventing dry-rot and other kinds of decay in timber, which were proposed and tried prior to the introduction of iron ships, afford ample evidence that these cases of long-continued service were not common. These processes are now matters of history only. and will not be discussed; but there appears reason to believe that, on the whole, the best results, as to durability, were obtained with ships built of well-selected materials, which were allowed to season naturally, prior to being used in the ship, and after she was in frame.† This last-named condition

* See the remarks of Mr. Ambrose Bowden, quoted by Mr. Laslett at pages 68-70 of Timber

and Timber Trees.

† It may be interesting to mention that Lloyd's rules for wood merchant

of course involved slow progress with the construction of any ship, and was scarcely likely to have been fulfilled in the mercantile marine at any period; but in the Royal Navy, in the earlier half of the present century, it was frequently fulfilled, and some of the ships then built proved very durable.

With such varying conditions-depending upon the selection of the timber, the circumstances of its growth, the season when it was felled, the processes of seasoning, preservation, &c.—it will be readily understood that it is not an easy matter to assign the average durability of wood ships. Probably experience with ships of the Royal Navy prior to the general introduction of steam propulsion or the use of iron furnishes the best data for forming a just estimate; for the subsequent changes in matériel, from sailing to unarmoured steam ships, from these again to ironclads, and from wood hulls to iron, have all tended to introduce other conditions than those of fair wear and tear into the cessation of the service of wood-built ships. In 1841 Mr. Chatfield read a paper before the British Association, at Plymouth, in which he stated, as the result of careful examination, that thirteen years was the average time during which woodbuilt war ships remained efficient when employed on active service, and receiving ordinary repairs at intervals. Experience in the French navy points to a very similar term of service for wood ships. Moreover, the Rules for Wood Ships issued by the Committee of Lloyd's Register, and guiding the construction of by far the greater number of wood merchant ships, allow from twelve to fourteen years as the average period of durability to be assigned to the best descriptions of shipbuilding timber when properly seasoned and free from defects. Less satisfactory materials, used in subordinate parts of ships, or in vessels of inferior classes, have

ships strongly recommend the prac- of classification to vessels thus tice of "salting" the timbers, beams, treated.

&c.; and allow an additional year

considerably shorter periods assigned, ranging so low as from four to six years.

Under the most favourable conditions, therefore, the average durability on active service of well-built wood ships, fairly used and kept in good repair, may be taken at from twelve to sixteen years. It has been shown that in some cases much greater durability has been obtained; and, on the other hand, many instances might be cited where vessels hastily constructed of unseasoned or unsuitable timber have fallen into decay in half, or less than half, the average time of service named. It is, of course, understood that the period of service is considered to expire when the cost of the repairs would be so heavy, if they were thorough, as to make it more economical to replace the worn ship by a new one. In the United States navy, for example, many wood vessels, built with the greatest possible rapidity during the Civil War, have been condemned after only six or eight years of service; while others, on which work has been suspended, have actually rotted on the stocks, and will probably never be completed. The hurried construction, and use of any materials that could be procured, were undoubtedly the chief cause of the rapid decay; and on the other side of the picture may be placed the durability of the earlier screw frigates of the American Navy, which remained efficient for periods exceeding the average given above. Very similar results followed the hurried construction of the gunboats built for the Royal Navy during the Crimean War; they speedily fell out of service.

*

Recent experience with the wood ships of the Royal Navy may be quoted in support of the views expressed.† Taking the unarmoured wood ships, from frigates downwards, it appears that after ten to fifteen years of service they have reached such a condition as to render it impolitic to repair them.

* See published reports of the Secretary of the Navy.

† See Parliamentary Paper (No.

297) of 1876, of Vessels Launched, Broken up, Sold, &c., from 1855.

Special requirements have kept a few such vessels on service for longer periods; but no injustice is done to the class in fixing sixteen years as the general upper limit of durability for sea-going wood ships.

Ironclad wood-built ships are no longer-lived; in fact the conditions in these ships are, on the whole, less favourable to durability than they are in unarmoured ships. Nearly all the converted ironclads of the Royal Navy (Caledonia class) dating from 1861, but not actually on service until two or three years later, are now either on the Harbour Service List or else in such a condition as to render their repair inexpedient. So also is the Lord Clyde, which is about two years younger. In the French navy, also, very similar steps have been taken, the earlier wood-built ironclads having been struck off the effective list. The Italian navy furnishes still further examples, and so does the Austrian; but it is unnecessary to multiply illustrations of the comparatively speedy decay of wood ships. Even when all possible care has been taken in their construction, hidden sources of decay may exist in the structure, and sooner or later produce serious results. No certain length of service can be guaranteed under these conditions to any wood ship; and not unfrequently it happens that, in the examination of some apparently trifling defect, the discovery is made of much more serious and unsuspected decay, leading in some cases to the condemnation of the ship as unfit for further service. With iron ships the conditions are quite different, as we will now proceed to show.

Iron is not subject to those internal sources of decay to which timber is liable: nor is it subject to the attacks of worms or marine animals which can penetrate the comparatively soft planking; nor is it liable to rot in consequence of imperfect ventilation or other causes. Moreover, in a well-built iron ship there ought not to be any sensible working; whereas in wood ships, however carefully constructed, the connections and fastenings must, as we have shown, be less satisfactory; the entire prevention of working is practically impossible, and in such working is found a fruitful

source of weakness or decay. Corrosion or rusting of the surfaces is the special danger requiring to be carefully guarded against in iron ships; and it is by no means insignificant in its character. Both outside and inside, an iron ship is constantly exposed to conditions tending to promote corrosive action. The above-water parts of the hull are the least likely to suffer; but even these, on the outside, have to resist the effects of air, water, and weather, and in the inside are exposed to changes of temperature, the condensation of vapour, and other circumstances productive of rust, if left unchecked. The under-water parts of the hull are much less favourably situated. Outside, the bottom plating is immersed in corrosive sea-water; and inside, the plating, frames, &c. are to some extent exposed to bilge-water, often very corrosive in its character, to the chemical action of coal or other substances carried in the hold as cargo, and not unfrequently to galvanic action produced by metallic connection with pipes, &c., of copper, brass, or lead, immersed in the same bilge-water as the iron. Moreover, in steamers there are the great alternations of temperature in the parts adjacent to the boilers and engine-room, the condensation of steam upon the surfaces of the iron, and the production of gases more or less effective in aiding corrosion. Adding to these extraneous causes the generally admitted facts that in iron, such as is used for shipbuilding, the want of homogeneity in the various parts of the same plate or bar may cause corrosion to begin, or accelerate its progress; and that when rust has once formed it tends to propagate itself, eating deeper and deeper into the iron affected, it will be evident that watchfulness and precaution are needed to insure the preservation of iron ships. Their durability, in short, is not a result to be assumed as an intrinsic quality; but they differ from wood ships in this important feature:-with care and proper treatment they can, at moderate expense, be maintained in a sound and efficient state for very many years; whereas wood ships cannot be so maintained without an unwise outlay. The causes of decay in the iron ship lie upon