It was for some time supposed that Sir H. Davy bad discovered a remedy against the rapid oxidation of the copper. Regarding the action of the sea water upon copper as of a galvanic nature, that great chemist considered that, by the addition of small pieces of tin and zinc, the copper would be rendered negatively electrical, and oxidation prevented. Ships were sheathed on this prin ciple, and sent to sea; but they proved such bad sailers, from the foulness of their bottoms, that a negative was soon put upon the scheme. It is true that the copper was thus protected by the zinc and tin, but the barnacles (shell-fish) attached themselves so much to the protectors, as to introduce a greater evil than they were calculated to remedy. Sanguine hopes were entertained of the success of this plan, and the disappointment consequent upon failure, wail, of course, extensively felt. Founded uponthe same theory of the galvanic influ ence, a patent was, a few years ago, obtained by Professor Pattison, for making use of iron plates, protected by zinc, which, it was asserted, entirely prevented the oxidation of the iron : and that a ship sheathed with iron and little bits of zinc had been two years at sea, and returned home with a clean and bright sur face. The specification of the patent states that the iron plates may be of the usual dimensions of the copper plates, and for each area of 100 inches in the iron, a plate of zinc of from oue-eighth to one-fourth of an inch thick, equal to five inches in area, is attached to the lower extremity of the sheet, so that in sheathing the vessel from the upper part downwards, each succeeding sheet of iron shall be in contact, by lapping over, with the zinc piste of the sheet imme diately above it. Plates of zinc must also be attached to the inside of the sheet
of iron, bearing a proportion in area to those on the outside, of 3 to 5. The spikes and bolts by which the sheathing is fastened to the vessel. are each to be furnished with a disc, or washer, of zinc, fitting closely to the bead; 'and it is recommended that they be driven well home to insure perfect contact. The nails employed are to be made concave under the head, and the cavity is to be filled with melted zinc. The proportions of five square inches of zinc to one hundred of the iron, is not insisted on ; any greater proportion will be equally effectual, and the zinc may be alloyed with copper, tin, or lead, in the propor tion of from 3 to 10 per cent. By this mode of sheathing vessels, it is asserted, in the specification, that the corrosion or oxidation of the metal will be,nearly, if not entirely, prevented.