fourths of our ironclads, including all the ships added to the Navy during the last ten years, have iron hulls; and it is a significant fact that not a single wood ship is now being constructed for the Navy.

Iron shipbuilding originated in this country; has here received its most important developments; and has been the source of very great advantage. It has rendered us practically independent of foreign supplies of shipbuilding materials; which were becoming more and more important in the later days of the supremacy of wood shipbuilding, when the supplies of home-grown timber were quite inadequate to home requirements. Such supplies from abroad were liable to interruption in time of war; and during peace they placed English builders at a great disadvantage, as compared with builders in countries where shipbuilding timbers were abundant and cheap. The United States, Canada, France, and Italy, all furnished ample supplies of suitable timber; and the shipbuilding trade-so peculiarly British-seemed about to pass away into other hands, when the use of iron instead of wood once more restored the balance, and enabled us to regain our former national position.

But more than this: the use of iron ships has been the source of world-wide advantage. Had wood remained in use, ocean steam navigation could never have attained its present wonderful development, and international communication must have remained less regular and frequent. Without iron hulls, the ironclad reconstruction could never have been carried to its present position; nor could the swift cruisers have been built. Moreover, iron shipbuilding has done very much to encourage progress in the manufacture of wrought iron for all structural purposes, and thus has indirectly benefited other departments of work. In short, the experience of forty years fully confirms the wisdom of the change from wood to iron, and proves that, although iron has some drawbacks, it possesses a considerable balance of advantage. Other nations, endowed with a wealth of ship

building timber, have not failed to realise this: in France, Italy, and still more noteworthy in the United States, iron is rapidly gaining ground, and English models are being imitated or improved upon.

A better appreciation of the great increase in the sizes and proportions of ships which has accompanied the use of iron hulls in both the Royal Navy and the mercantile marine will be obtained from a few typical examples. Taking the Royal Navy first, the following tabular statement will suffice:

This increase in size has not merely been associated with the special strains due to the use of armour, but with the adoption of proportionately more powerful engines, and the attainment of higher speeds. The best of the screw line-ofbattle ships of the old type attained from 12 to 13 knots at full speed; this latter speed was also the maximum of the finest wood frigates. But now the armoured battle-ship has a speed of 14 to 15 knots; and the swift cruiser class have speeds of from 15 to 16 knots. Wood hulls could scarcely be expected to meet satisfactorily these greatly changed conditions; but iron hulls have answered the purpose, and there is no reason to think that the limits of the capabilities

of the material have been reached, even in the largest and swiftest ships afloat. Great engine-power in wood-built ships is very trying and injurious to the structures; but no similar wear and tear occurs with iron. The Orlando and her sister frigate, the Mersey, were, when constructed, experiments in the direction of applying large engine-power and great proportions of length to breadth in wood ships; but the results were anything but satisfactory. These vessels required considerable repairs during their brief period of service, and rapidly fell out of use. Against this failure to sustain successfully the strains incidental to screw propulsion, set the case of the iron-built Inconstant, which is longer than the Orlando, of less beam, three knots faster with 80 per cent. greater engine-power, and yet, thanks to her iron hull, displays no signs of working or weakness.

In the United States the attempt was made to build swift cruisers, the famous Wampanoag class, of wood. Without entering into any details of the controversy respecting this class, it may be stated that, on all hands, it is now admitted that the wood hulls were not well suited for the great enginepower put into the ships. The fact that several of the class have been left unfinished or unemployed after trial shows the estimation in which the vessels are held by the authorities of the American Navy. Further, it is interesting to note that American shipbuilders are, at length, devoting themselves energetically to the development of iron ship construction. Several small iron vessels have been recently added to their navy; and iron has been used for the hulls of many large fast steamships for ocean navigation. French designers have also acknowledged the superiority of iron to wood, by building their swift cruisers on the model of the Inconstant, and their ironclads on the bracket-frame system illustrated in Fig. 104, page 331.

In the mercantile marine, scarcely less remarkable changes have been made in the sizes and proportions of ocean steamers. Take, for example, vessels on the Transatlantic service. Not quite forty years ago, the wood-built Great

Western was considered a remarkably fine vessel; her dimensions were, length 210 feet, breadth 35 feet, tonnage (B.O.M) 1340 tons, load displacement 2300 tons. She was followed, in 1840, by the Great Britain, built of iron, of which the dimensions were, length 290 feet, breadth 51 feet, tonnage 3270 tons (register), original load displacement 3000 tons. These dimensions were then considered extravagant, if not unsafe; but the ship was quite recently at work, on the Australian line, although thirty-five years old. The changes made since her construction are still more remarkable. The largest Transatlantic steamers now at work are 490 feet long by 44 feet beam, and about 5500 tons (register), their displacement, when fully laden, being from 9000 to 10,000 tons. No one can for a moment suppose that such sizes and proportions could have been achieved with wood as the material, in conjunction with very powerful engines and extremely high speeds. Finally, as a crowning example of what may be done with iron, take the Great Eastern, 680 feet long, 83 feet broad, of 22,500 tons (register), and load displacement 27,400 tons, which after some fifteen years' service still remains strong and efficient, having meanwhile performed most arduous work in laying various submarine telegraph cables.

Iron ships are proved to be superior to wood in the following important particulars:-(1) Lightness combined with strength; (2) durability, when properly treated; (3) ease and cheapness of construction and repair; (4) safety, when properly constructed and subdivided. On the other hand, iron ships are inferior to wood in-(1) easy penetrability of the bottom by rocks or other hard pointed substances; (2) fouling of the bottom, and consequent loss of speed, after being afloat for some time. Compass correction in iron ships is now so satisfactorily performed that there is no need to refer to a matter which at the outset had great practical importance. Taking these points in the order ir which they have been named, each of them will be illustrated briefly; and after concluding these remarks, a few will be

added on the subject of the use of iron hulls in unarmoured ships of war.

First, as to lightness combined with strength. In woodbuilt ships of the Royal Navy it is found that about onehalf the total weight is required for the hull; in similar ships of the mercantile marine the hulls are somewhat lighter in proportion to the displacement. In iron merchant ships the hull frequently weighs only one-third of the total weight, high authorities agreeing that the change from wood to iron effects a saving of from 30 to 40 per cent. on the weight of the hull. The iron ships of the Royal Navy are not, as a rule, so lightly built as iron merchant ships, the difference being due to differences of form and the special requirements of their service. In some of the earlier iron vessels of the Navy, both armoured and unarmoured, the hulls were as heavy as, or even heavier than, the hulls of wood ships, in proportion to the displacements. But as the principles of iron ship construction have become better understood, considerable savings in weight of hull have been effected simultaneously with an increase in structural strength, and now it is not uncommon to find the weight of hull only 30 to 40 per cent. of the total displacement, in vessels carrying the thickest armour and heaviest guns. This expression of the weight of hull as a fraction of the displacement, or total weight, of the ship is by no means a complete view of the comparison of wood and iron ships. It takes no cognisance of the fact, to be hereafter illustrated, that forms, sizes, and proportions are now commonly adopted that could never have been used with wood as the material; and it does not recognise the variations which, for similar methods of construction, have to be made in the ratio of the weight of hull to the displacement, in order to secure equal structural strength in vessels of different sizes. It is, however, a sufficiently accurate mode of comparison for our present purpose, and is very commonly used. The following tabular statement will show at a glance the advantages in point of lightness possessed by iron ships of various classes;