upon its anti-fouling properties. Copper sheathing, for example, may produce serious galvanic action upon the iron hull, or portions of the hull, if there is intimate metallic connection between the sheathing and the iron; and even a very indirect metallic connection will suffice to produce some action. Muntz metal, again, is electro-negative to iron, and therefore requires to be insulated. Zinc, on the contrary, being electro-positive to iron, need not be insulated from it; but since the rate of wasting required to prevent fouling of the zinc is practically governed by the amount of galvanic action set up on its surface by the iron, considerable care is needed in adjusting the relative surfaces of the two materials subjected to galvanic action. A brief description will suffice to show what has been done in practice to overcome these various difficulties.

Ships of the Royal Navy built of iron, or on the composite principle, and copper-sheathed, have two thicknesses of wood planking interposed between the copper and the iron portions of the hull. The inner thickness is bolted to the skin-plating or to the iron frames; the outer thickness is bolted to the inner, the bolts not being allowed to come into contact with the iron of the hull, nor with the bolts of the inner thickness. Wood stems and sternposts are fitted in many of the composite vessels; but in the swift cruisers and ironclads brass stems and sternposts are employed. The copper sheathing is not brought into contact with the metal stems or sternposts, nor with the metal kingston-valves, &c., passing through the bottom; and by these means it is endeavoured to insulate the copper from the iron hull. Doubts have been expressed as to the sufficiency of these precautions, it being supposed that there must be some metallic connection between the hull and the copper, resulting in corrosion of the iron. It will suffice to say in reply that the precautions taken at least prevent any powerful local action, such as might otherwise take place in the neighbourhood of the fastenings. In fact, after seven or eight years' experience with the sheathed ships of the

Inconstant and Volage classes, including service on very distant stations and in tropical waters, no signs of serious galvanic action or corrosion have been discovered upon careful examination. Further, the copper sheathing has well maintained its anti-fouling properties, which it could scarcely have done if it were causing much galvanic action on the iron hull.

One special danger is necessarily incurred by such ships, and ought not to be passed over. Any damage to the bottom, which stripped off the bottom planking and exposed a portion of the iron skin, would necessarily place that portion of the skin within the influence of powerful galvanic action for it would be immersed in the same sea-water as the copper sheathing, be almost certainly in metallic connection therewith, and have concentrated upon its comparatively small area the action of the very large surface of copper sheathing. The result must be very rapid corrosion of the iron skin, and possibly its perforation by holes. Such an accident, capable of stripping off wood planking 6 inches thick, firmly attached to an iron hull, must of course be exceptional in severity, and of very rare occurrence. No such case has yet occurred: but the Admiralty Regulations provide against the contingency, the commanding officer being ordered to have his ship examined and repaired with the least possible delay.

Allusion has already been made to the dangers attendant on galvanic action of the kind described, where some metal valve or pipe, connected with the iron skin and immersed in the same sea or bilge water, has produced local corrosion of a very serious and rapid character. The case of her Majesty's store-ship Supply illustrated this, and in the Megæra also there was reason to believe that galvanic action had taken place.* To prevent such galvanic action on the iron skin, very stringent rules are, as was shown above, laid down for the guidance of officers charged with the construction or care of iron ships in the Royal Navy.

See the report of the royal commission on the loss of the Megara.

To illustrate the greatly increased rate of corrosion of iron, incidental to galvanic action, a few examples may be taken from the results of the experiments recorded by Mr. Mallet. An iron plate immersed alone in clear sea-water was found to lose during a certain period a quantity which we will denote by unity: it was then immersed for an equal time in clear sea-water with an equal surface of the following metals electro-negative to it, and the corrosion increased as follows:

Other laboratory experiments, made on an extensive scale, have given different results for the relative intensities of the action of the various metals on the iron; but they fully confirm the fact that a greatly increased rate of corrosion results from galvanic action. The first two materials, copper and brass, are those of which the shipbuilder has need to take most heed in arranging the sheathing or fittings of iron ships.

The increased cost of copper-sheathed iron ships is considerable, and in composite ships of the merchant fleet the use of the two thicknesses of planking is by no means common, doubtless because of the additional outlay required. With a single thickness of planking there is, of course, much greater risk of galvanic action, but in merchant ships Muntzmetal sheathing is commonly used, and its action on iron is supposed to be comparatively feeble. It has been asserted that no great difficulty would be encountered in making sheathing of such an alloy of copper and zinc as would be electro-neutral to iron, and have no galvanic action upon it when immersed in sea-water. We are unaware, however, that

any such sheathing has been tried, and nothing but experience could show whether or not it would be effective against fouling. Zinc sheathing has, however, been substituted for copper in many recent ships of the Royal Navy, and if it proves an efficient anti-fouling material, it will be much less costly than copper, and can under no circumstances produce anything but beneficial action on the iron hull.

Various plans have been tried for attaching zinc sheathing to iron hulls; that now commonly used in the Royal Navy is as follows:-A single thickness of planks (3-inch to 4-inch) is bolted outside the skin plating; to this the zinc sheets are nailed: the strakes of planking are not caulked, but the water which finds its way under the sheathing can pass freely through the seams to the iron skin. Iron stems and stern-posts are employed; and by various means a certain amount of metallic connection is made between the zinc and the iron hull, such connection, as explained previously, being desirable in order to keep the surface of the zinc freer from incrustation. Hitherto the practical difficulty has been to adjust the relative amounts of the surfaces of iron and zinc, contributing to galvanic action on the latter, in such a manner as to prevent too rapid or too local wearing of the zinc, without interfering with its antifouling properties. In fact, the present condition of this question bears a considerable resemblance to that previously existing, when iron protectors were under trial with copper sheathing. On wood ships, zinc lasts for a considerable time, but is not very successful in preventing fouling: there it has but little metallic contact to produce galvanic action. On some merchant ships where the zinc has been laid almost directly upon the iron skin, with felt or some similar material interposed, its rate of wear has been so quickened that a single voyage has sufficed to destroy it. Between these two conditions must lie the practically useful method of attachment, and upon this experience with actual ships can alone decide. Before many years the question will probably be in a much more settled condition; and although

there is little hope that zinc can ever be made to equal copper in its anti-fouling qualities and smoothness of surface, yet, if it be made fairly successful in this respect, it will greatly reduce the cost of construction, and the risks of accident or collision. Already it seems certain that a very great improvement upon the condition of unsheathed iron ships can be secured by the use of zinc sheathing.

At present, therefore, the question of preventing foulness of bottom in iron ships stands as follows:-By far the greater number of ships have their bottoms coated with some antifouling composition, and are docked for cleaning and recoating once or twice a year; when that is practicable, no serious loss of speed ensues. The annual cost for coating large ships may be taken as from £100 to £400 a year; and this is not a large outlay. A few iron ships, designed for distant voyages, and in which the power of keeping the sea, without serious loss of speed, during long periods is of great importance, have copper or zinc sheathing. Copper can be made to answer well as regards anti-fouling, but it involves a large additional outlay, and is open to the charge of possible damage to the iron hull in case of accident. Zinc has not been long enough in use to be regarded as having passed out of the region of experiment; but it promises to be fairly successful.

Many persons who admit the superiority of iron to wood hulls in vessels of the mercantile marine question the desirability of using iron hulls for war ships, unless they are ironclads. Unarmoured fighting ships, it is still urged, should be wood-built. A few remarks on this matter will not, therefore, be out of place.

Nearly forty years ago two iron steamers, the Nemesis and Phlegethon, were built for the East India Company, and successfully employed in the Chinese war of 1842. A few years later several iron frigates were ordered to be built for the Royal Navy; but these were ultimately converted into troopships, the Simoom and Megara amongst the number.