record where, without pitting, iron plates on the bottoms of ships have worn much more rapidly. In the Megæra, for example, when fifteen years old, many plates were found to have become reduced inch from their original thickness; and if this rate of wear had been maintained, the loss in a century would have been not much less than thrice as great as that given by the laboratory experiments. It is, of course, quite conceivable that under other conditions the wear in the Megæra might have agreed with the laboratory experiments; but neither such experiments nor actual results on ships can furnish any general law for the rate of

corrosion.

The Regulations issued by the Admiralty for the preservation of iron ships contain the best summary of the precautions necessary for that purpose with which we are acquainted. As the circulars on this subject are generally accessible, it will be sufficient to summarise the main points. Galvanic action of copper, brass, or lead upon the iron hull is to be prevented by making the lower pieces of suction-pipes, &c., which are immersed in the bilge-water, of iron or zinc or zincked iron wherever that is possible. Where copper or brass pipes are unavoidable, they are to be well painted or varnished and covered with canvas in order to reduce their action on the iron. The gun-metal screw-propellers are also to be painted for the same reason, and bands of zinc, termed "protectors," are to be fitted near them, in order to concentrate the galvanic action of the propellers upon the protectors and save the bottom plating: this plan has answered admirably. In order to preserve the inner surfaces of the bottom plating below the bilge from the injurious effects of the wash of corrosive bilge-water from side to side as the ship rolls, cement is used, and has proved of great advantage to both merchant and war ships. Other surfaces of plates and bars in the interior are protected by suitable paints or compositions. All parts of the hull are ordered to be made as accessible as possible for inspection and repairs. In cases where parts are necessarily

inaccessible under ordinary circumstances-such as under the boilers or engines, &c.-careful records are to be kept of them; and when opportunity offers, as during a thorough repair at a dockyard, all such parts are to be opened up and inspected. When a ship is in the reserve or on service, all accessible parts are to be inspected once a quarter, cleaned and painted when necessary. Annually a more thorough survey is to be made, by dockyard officers when possible; and then the only parts to be left unvisited are those which cannot be reached without great difficulty-as, for instance, spaces which can only be attained by lifting the boilers or machinery. The use of double bottoms facilitates a thorough examination; especially of the inner surface of the outer plating, and all the parts of the inner plating underneath engines and boilers. The outer surface of the bottom plating is to be sighted at least once a year; it is protected by some anti-corrosive paint or composition, and if the annual examination shows it to be necessary, this protective material is renewed.

Such are the main points in the Admiralty Regulations. Conformity to them must prevent any serious corrosion taking place for rusting ought to be detected in its earlier stages, and the surfaces, being frequently cleaned and coated, ought not to suffer greatly. The system has now been in force for some years, and has worked most satisfactorily. In a modified form it is applied also to the preservation of the ironwork in the composite ships of the Royal Navy.

Thirdly, iron ships gain upon wood in being more easily and cheaply built and repaired. Upon this division of the subject but few remarks will be necessary, although it has great practical importance.

Timber is only obtainable by the shipbuilder in pieces of which the forms and dimensions are limited by causes beyond his control; and the greatest care has to be bestowed upon the "conversion" of the logs, in order to get out of

them the best possible finished timbers. For some parts of a ship where the curvature is considerable-as, for instance, the ribs—it is not unfrequently a matter of difficulty to procure suitable timber. Even when a good choice has been possible, considerable labour and skill have to be expended on fashioning the pieces; and we have shown how difficult it often is to effect a good combination of piece with piece. Manual labour is, moreover, almost a necessity in the greater part of the work of building a wood ship.

Iron, on the contrary, is obtainable by the builder from the manufacturer almost of the sizes and forms required, the dimensions of the pieces and their sectional forms being limited only by the powers of the manufacturer, which continually increase as the demand increases. The progress already made is most remarkable, and there are yet no signs of the limit having been reached. Less than twenty years ago an armour plate which weighed 5 tons was considered heavy; now plates are commonly made weighing 20 or 30 tons, and plates of 40 or 50 tons can be produced if desired. Another example is furnished by the manufacture of wrought-iron beams. Formerly the sectional form ƒ in Fig. 116, page 385, was largely used, and the section e was made with difficulty by a special process: now e can be rolled easily, even in the largest sizes. The section c also has replaced, to a large extent, a girder formed by a plate with a single angle-iron on each edge. But it is needless to further illustrate a well-known fact: the progress of the iron manufacture tends towards the production of finished sectional forms, and the avoidance of cost and labour in combining plates and angles to produce such forms.

In building an iron ship, less work is also required in fashioning and combining the pieces than is the case with wood. Beams, for instance, in the iron ship are given to the builder in one length: costly scarphs like those in Figs. 109 and 110 are unnecessary. Bending takes the place of the costly fashioning required for the curved pieces of a wood

ship. Welding, lapping, and butt-strapping replace scarphing. And, what is no less important, machinery can be, and is, extensively employed in the preparation of the parts of an iron ship.

Any one who has witnessed the rapid progress on the framing of an ordinary iron ship, as compared with that on the erection of the ribs of a wood ship, cannot fail to have noticed the much greater simplicity of the operations required in the iron ship. And although in a vessel built on the longitudinal system of framing (see Fig. 104, page 331) the operations of construction are less simple than those in an ordinary iron ship, yet even here all that has been said above applies; individual pieces are procured of the forms and dimensions desired, they are combined simply, and the work admits of being pushed on rapidly.

Iron ships are also much more easily repaired. All, or nearly all, the surfaces of the skin-plating, as well as those of the transverse and longitudinal framing, in these ships may be, and should be, made easily accessible for inspection: for which purpose it is highly desirable that the inside planking (or "ceiling ") should be arranged in such a manner as to be readily removed. In case of damage, therefore, the injured parts can usually be reached, examined, and replaced without any great difficulty. Wood ships, on the contrary, are not so readily examined or repaired. The various parts are so closely associated, interlaced, overlapped, and fastened, as to render a considerable disturbance unavoidable if any considerable repair is needed. It is, for example, a task of some difficulty and expense to replace a rotten timber in the framework by a sound one, and when a vessel has been aground and had her bottom seriously damaged, the cost and difficulty of the repair must be considerable.

From many notable examples of the ease with which the repairs of iron ships may be effected, a few may be selected. The Great Britain was for many months ashore in Dundrum Bay, and although the bottom was battered by beating upon the rocks, and the boilers were forced up about 15 inches,

yet the damage was almost confined to the lower part of the hull, her form remained unaltered, and she was got off and repaired. The Tyne, an iron steamer, ran ashore on the south coast, and remained for several months in an exposed position; but she too was ultimately floated and repaired, being made as strong and sound as ever, although a large portion of her keel had been torn off and her floor much injured.* The Great Eastern furnishes still further proof of the ease with which an iron ship can be again made efficient after serious damage to her bottom; † and in the Royal Navy one meets with similar cases. The Agincourt was easily repaired after running on to the Pearl Rock; and the Bellerophon and Northumberland were again restored to efficiency without large expenditure after being injured by collision. Still more remarkable are the cases, of which several have been brought to our knowledge, where iron ships which have grounded and broken in two, have subsequently been floated, the separated parts reunited, and the ships again employed successfully. We regret that limited space prevents any details being given of these

occurrences.

Further, iron ships, under the ordinary conditions of service, require much less expenditure on repairs than wood ships, in order to meet wear and tear. This is a matter not admitting of question. It is, of course, difficult to speak with certainty as to the comparative costs; but probably it is within the truth to say that, on an average, the deterioration in a wood ship is not far from twice as great as that in an iron ship, in equal times, and under similar conditions of service. The usual allowance for wood ships is that in from twelve to fifteen years the casual repairs to

* Mentioned by Mr. Grantham in his work on Iron Shipbuilding. Much interesting information respecting the accidents to the Great Britain and Great Eastern will be

found in the Life of Mr. I. K Brunel.

See the remarks on page 33 as to the accident to that ship.