SHEATHING, in Naval Archileclare, a sort of covering nailed all over the outside of a ship's bottom, to protect the planks from the ravages of worms. Formerly, this sheathing consisted only of boards tarred and payed over; but now copper is resorted to, not merely as a substitute, but as an additional covering, and it has become almost universal, where the expense can be sustained; it is of especial utility in vessels making long voyages and into warm climates. The rapid corrosion of copper, by the action of sea 'Water, renders the frequent renewal of it a very serious item of expense to the ship-owner. In a patent which Mr. Robert Mushet took out a few years ago, for "certain means ,or processes for improving the quality of copper and alloyed copper, so as to render it more durable when employed as sheathing to ships bottoms," be states (what our experience confirms), that owing to some defect in the manu facture of the copper, the sheathing upon a ship is sometimes worn away by oxidation in a much shorter period than usual, lasting only one, two, or three voyages, instead of five, six, seven, or more (according to the time and distance, and thickness of the copper). The cause of this Mr. Mushet considers to arise, not simply from the impurity of the metal, but from the undue proportion of the alloy with which it may be mixed. He also states, that he has found that she purest copper, exposed to the action of sea-water, is not so tenacious as when alloyed in the manner he proposes in his specification, and on which he founds his patent. He directs, that to 100 lbs. of copper is to be added 2 oz. of the regulus of zinc, or 4 oz. of the reptile of antimony. or 8 oz. of the regulus of arsenic, or 2 oz. of grain tin. Instead of these alloys separately, they may be employed in conjunction in the following proportions : viz. to 100 lbs. of the copper add half an ounce each of the zinc and tin, 1 oz. of the antimony, and 2 oz. of the arsenic. By these mixtures, Mr. Musket states that the copper is rendered much more cohesive and fibrous in its texture, and that the corrosive effects of the sea-water will, in a great measure, be prevented.
Mr. Christopher Pope, of Bristol, took out a patent, in 1824, for the manu facture of metallic plates, chiefly for the purpose of sheathing of ships, in which copper is altogether discarded. These plates are composed of tin and zinc, or of tin, lead, and zinc. Mr. Pope says, to his specification,—" To unite tin and zinc, I take a certain quantity of zinc, as it is usually made, and melt it in an iron pot, and when melted, I add an equal quantity of tin ; and having stirred them together in a fluid state, I cast cakes of it in moulds of about 8 inches broad, 10 inches long, and t of an inch thick, which cakes are afterwards hammered or rolled out into sheathing. To unite tin, lead, and zinc 3 melt. certain quantity of lead, and add to it twice the quantity of tin. This composition I cast into small lumps, and having melted three times as much zinc as I had previously melted of lead, I then add the small lumps of tin and lead, and melt the whole together; whiclmethod I find to be the best." This mixture he casts into cakes of the size before mentioned, and then rolls them out into sheets; and he particularly enjoins, that no more heat be used than is just sufficient to compound the alloy, as the metal becomes hardened by an excess of heat ; and that it is advisable, in rolling out the cakes, to heat them to the temperature of boiling water, by which he says that "they will roll or hammer softer than when cold." This metallic sheathing has, we are informed, been more extensively employed for covering of the tops of houses, than the bottoms of ships.
A few years ago, some very favourable accounts wee. published of the patent Indian rubber sheathing, which consisted of a coarse fabric of fibrous material, saturated with a solution of the elastic resin, together with pitch and tar. The price of the sheathing was ten pence per sheet, of the size of 34 inches by 20. It wag found to be a complete protection against the worm, and must, at the least, we think, form an improved substitute for the felt in general use.