The sheets are, generally galvanised before they are corrugated; but as in process of corrugation the sheets, especially the thicker ones, sometimes crack slightly on the surface (unless the iron is of the very highest quality), it is an advantage, with all sheets thicker than 20 gauge, to galvanise after corrnga-tion, so as to fill up with zinc any cracks that may hare occurred. As, however, a larger quantity of zinc adheres to the corrugated than to the flat sheets, they have, when so coated, a distinctly higher value. (Matheson.)

(2) Painting is an effectual method of preserving jron from oxidation, if the paint is good and properly applied, ami the iron in a proper condition to receive it. In order that the protection by painting may continue, the surface should be carefully examined from time to time, so that all rust may be removed. The paint may be renewed directly it is necessary.

Cast iron should be painted soon after it leaves the mould, before it has time to rust. The object of this is to preserve intact the hard skin which is formed upon the surface of the metal by the fusing of the sand in which it is cast. After this a second coat should be applied, and this should be renewed from time to time as required. In any case, all rust upon the surface of castings should be carefully removed before the paint is applied. Small castings are often japanned. Before painting wrought iron care must be taken to remove the 'hard skin of oxide formed upon the surface of the iron during the process of rolling, and which, by the formation of an almost imperceptible rust, becomes partly loose and detached from the iron itself. An attempt to prevent this rusting is sometimes made by dipping the iron, while still hot, in oil. This plan, however, is expensive, and not very successful. The scale is sometimes got rid of by " pickling," the iron being first dipped in dilute acid to remove the scale, and then washed in pure water.' If the trouble and expense were not a bar to its general adoption, this is the proper process for preparing wrought iron for paint, and it is exacted occasionally in very strict specifications.

But somewhat the same result may be obtained by allowing the iron work to rust, and then scraping off the scale preparatory to painting. If gome rust remains upon the iron, the paint should not be applied lightly to it, but by means of a hard brush should be mixed with the rust. Ordinary lead paints, especially red lead, are often used for protecting iron work, but they are often objected to on the ground that galvanic action is set up between the lead and the iron. Matheson recommends oxide of iron paints for iron work generally, and bituminous paints for the inside of pipes or for ironwork fixed under water. The ironwork for roofs, bridges, and similar structures, generally receives one coat of paint before it leaves the shops, and two or three more after it is fixed.

(3) Dr. Angus Smith's process is an admirable means for preventing corrosion in cast-iron pipes. The pipes having been thoroughly cleaned from mould, sand, and rust, are heated to about 700° F. They are then dipped vertically into a mixture consisting of coal-tar, pitch, about 5-6 per cent, of linseed oil, and sometimes a little resin, heated to about 300° F. After remaining in the mixture several minutes, long enough to acquire the temperature of 300° F., the pipes are gradually withdrawn and allowed to cool in a vertical position. Perfect cohesion should take place between the coating and the pipe, and the former should be free from blisters of any kind. In practice, the heating of the pipes before immersion is found to be very expensive, and is frequently omitted. However, many engineers consider it essential for really good work.

(4) All turned, fitted, and tooled surfaces should have a coating of tallow mixed with white-lead to deter its melting and running off. Dr. Percy recommends rosin melted with a little Gallipoli oil and spirit of turpentine, of such proportions as will make it adhere firmly without chipping off, yet admit of being easily detached by gentle scraping.

(5) Ventura Serra, after many years of experiment and observation, having noticed that knives used in cutting plants belonging to the family of Eu-phorbiaceas did not rust, is led to recommend for this purpose an alcoholic solution of gum (resin of) euphorbium. This when applied to steel, iron, or copper, forms a thin, uniform, and very adherent layer, which effectually protects the metal. Experiments with copper immersed in sea-water - a ship's sheathing - were followed by gratifying results.

(6) According to the 'Engineer,' J. Machabee has some little time since invented a composition to preserve iron from rust, applicable also to other materials, such as stone or wood, used in conjunction with metal. The following is the composition: - Virgin wax, 100 parts; Gallipoli, 125; Norwegian pitch, 200; grease, 100; bitumen of Judea, 100; gutta-percha, 235; red-lead, 120; white-lead, 200. These ingredients are mixed together in a boiler in the order above, the gutta-percha being cut up in small pieces, or rasped. The mixture is stirred at each addition, and poured into moulds. For iron, it is melted and laid on with a brush.

(7) Girders, angle-irons, and similar large masses of iron, are often placed in exposed situations, where damp air, steam, and acid vapours have access. If the iron be put up in the rough, it very speedily rusts, and under favouring conditions the corrosion soon reaches a dangerous point. Contractors generally agree to supply such irons painted in three coats of minium, which, if honestly done, to a certain extent protects the metal; but a novel mode of treating girders is to heat them until, if touched with oil or fat, they cause it to frizzle, and then plunge them into a vat of mixed oil and grease. This mode of treating cast iron is said to be superior to any "painting," as the oleaginous matter actually penetrates the pores, and prevents oxidation for a very long time, while it does not prevent painting, if desirable, afterwards.