course is not merely advantageous in preserving the structure, but has the effect of reducing the tendency to fouling. Neglect of precautions against corrosion has the effect of making fouling more rapid. Some persons even go so far as to affirm that if all rusting were prevented on the bottoms of iron ships, they would be free from fouling; and that if a smooth, clean surface could be maintained, the plants and animals would not attach themselves. Some serious objections to this view may be urged; but it is needless to dwell upon them, since the conditions laid down can never be fulfilled in practice on the bottom of an iron ship, subject to blows, abrasions, and all the wear and tear of service, besides being almost constantly immersed in corrosive sea-water. All iron ships with unsheathed bottoms become foul in a comparatively short time; and cases are on record where a few months in tropical waters have sufficed to produce such an amount of fouling as to reduce their speed by one-half. Under ordinary conditions, if an iron ship can be docked and have her bottom cleaned and re-coated once or twice a year, all goes well; but longer periods afloat induce an objectionable amount of fouling.

Hence it is that vessels intended for cruisers in the Royal Navy, as well as special vessels in the mercantile marine, intended to keep the sea for long periods and to maintain their speed, have been either constructed on the composite system, or else had their iron hulls sheathed over with wood planking and covered with some metallic sheathing, such as copper, Muntz metal, or zinc. The clippers which were formerly employed in the China tea-trade, and whose annual races home attracted so much notice, were built on the composite system, resembling iron ships in all respects except that they had wood planking, keels, stems, and sternposts, and had their bottoms copper-sheathed. These vessels could lie in the Chinese ports unharmed, under conditions which produced very objectionable fouling in iron ships. In the Royal Navy at the present time the composite system of construction is applied to vessels up to the size of corvettes;

the outside planking being worked in two thicknesses and the bottoms copper-sheathed. For larger and swifter cruisers, such as the Volage and Inconstant classes, the use of an iron skin becomes a necessity in connection with the provision of structural strength; and in most of these vessels copper sheathing has been adopted, two thicknesses of wood planking being interposed between the sheathing and the iron hull. Two of the ironclads of the Royal Navy, the Swiftsure and Triumph, have also been built on a similar plan. It has now been thoroughly tested during seven or eight years, and has proved satisfactory; but it involves some special dangers, and it is a very expensive method of construction, so that endeavours have been made to substitute zinc for copper, and one thickness of wood for the two formerly employed. The Audacious, at present flag-ship on the China station, was thus sheathed some six or seven years ago; and the experiment proved sufficiently successful to procure further trials of zinc sheathing in two or three other vessels, some of which are now on service. But the matter must still be regarded as in the experimental stage.

The anti-fouling properties of copper sheathing are due to the fact that the action of sea-water upon its surface produces oxychlorides and other salts which are readily soluble, and do not adhere strongly to the uncorroded copper beneath. Hence the salts, instead of forming incrustations, are continually being washed off or dissolved away, leaving the sheathing with a smooth, clean surface, and preventing the attachment of plants or animals. Some chemists have attached importance also to the poisonous character of the salts of copper in preventing fouling; but the foregoing is undoubtedly the more important feature, and is commonly termed "exfoliation" of the copper. The rate at which this wasting of the copper proceeds varies greatly under different circumstances, and with different descriptions of copper; and formerly this subject received much attention, the aim being to secure the minimum rate of wearing consistent with

the retention of anti-fouling properties. For this purpose Sir Humphry Davy suggested to the Admiralty the use of "protectors," formed of iron, zinc, or some metal electropositive to copper. When these protectors were put into metallic connection with the copper sheathing and immersed, galvanic action resulted, the protectors were worn away, and the rate of wearing of the copper was decreased in proportion to the ratio of the surface of the protectors to the surface of the sheathing. When the protector had about 10 of the surface of the sheathing, there was no wasting of the copper: with a smaller proportionate surface of the protectors the copper wasted somewhat; but even when the protectors had an area only Tooo part that of the sheathing, there was proved to be a sensible diminution in the rate of wearing. The limits of protection from fouling appeared to be reached when the surface of the protectors equalled part of the surface of the sheathing. After experience on actual ships it was found, however, that preservation of the copper by this means led to rapid fouling, and the plan was abandoned. Nor has any substitute been since found; the practice being to exercise great care in the manufacture of the copper, and to regard its wasting as the price paid for preventing fouling. Muntz metal-an alloy of copper and zinc in the proportions of about 2 to 3-has been used largely as a substitute for copper, especially in the ships of the mercantile marine, and appears to answer fairly well, being, of course, much cheaper than copper. Such alloys are supposed by some persons to have the advantage of not producing powerful galvanie action upon iron immersed in sea-water and metallically connected with them; but this property has not been definitely established. On the other hand, it appears that, after being long immersed, the alloy tends to alter in composition. Muntz metal sheets have been found to become brittle after being some time in use; and the explanation given is that, the zinc being electro-positive to the copper, galvanic action is established between the two metals in

the alloy, and part of the zinc removed. The introduction of a third metal, such as tin, is said to prevent this objectionable change, even when it is present in very small quantities.

Zinc is another material largely used for sheathing the bottoms of wood ships. When immersed in sea-water, the salts formed on the surface of a zinc sheet are very much more adherent to the uncorroded zinc than are the corresponding salts of copper, and are comparatively insoluble-or perhaps, we should say, are slowly soluble-by ordinary sea-water. Hence it appears that a coating of oxychloride of zinc, &c., is likely to form on the sheathing, not being washed away or removed like that on copper; and consequently zinc does not possess such good anti-fouling properties as copper, nor present such a smooth surface. But it answers fairly well, and lasts for a considerable time under ordinary conditions. In some waters, however, and those of the tropics especially, zinc sheathing has been found to perish very quickly, owing probably to such a composition of the water as favoured the rapid solution of the salts formed on the surface, the exposure of the uncorroded zinc, its rapid oxidation, and so on. John Hay records that, in the Trinculo, one commission on the African coast sufficed to strip the bottom of zinc and leave the wood exposed, fouling of course ensuing. Other cases are reported where zinc sheets inch thick have, under exceptional conditions, been worn through in the course of twelve months.

Sir

Under ordinary conditions, zinc sheathing is much more durable in fact, to increase its anti-fouling qualities, it is often put into communication with some metal, such as iron, which is electro-negative to itself, in order that the galvanic action which is produced may have the result of keeping the surface of the zinc freer from incrustations to which marine plants and animals can adhere. Apart from this, it may be interesting to give the relative losses sustained by copper, zinc, Muntz metal, iron, and steel, when suspended in the

sea for purposes of experiment by Dr. Calvert and Mr. Johnson.*

These results are open to some doubt when applied as units in estimating the probable loss occurring during long periods of immersion in sea-water of various qualities; but they are valuable for purposes of comparison between the metals, and between the case of immersion in a vessel of seawater, and in the sea itself, where there are many causes tending to remove the salts formed on the surfaces. The greatly different rates of wearing in different seas is a matter of common experience; and the experiments made by Mr. R. Mallet, F.R.S., furnish some valuable information on this head.† Iron boiler plates which lost from 0·007 lb. to 0·009 lb. per square foot per month in clear sea-water, lost about twice as much in foul sea-water. With steel, very similar results were obtained.

Wood ships are protected from fouling by nailing the metal sheathing directly upon the wood planking; iron ships cannot be protected in quite so simple a way, the metal sheathing having to be attached in a manner dependent upon its position in the galvanic scale relatively to iron, and

See the Transactions of the Literary and Philosophical Society of Manchester for 1865, quoted at page 199 of Shipbuilding, Theoretical and Practical.

† See reports of British Association, 1841-43; also vol. xiii. of the Transactions of the Institution of Naval Architects.