The method employed by Wilson in making compound plate is to first make a good wrought iron plate. To the surface of this and along each side of the length of the plate are fixed two small channel irons, as shown in Fig. 5. The plate is then raised to a welding heat in a gas furnace, and transferred to an iron flask or mould. Wedges are driven in between the back of the plate and the side of the mould, thus forcing the channel irons up snug against the opposite side of the mould. Moulding sand is then packed around the back and sides of the plate (see Fig. 6). The mould is lowered in a vertical position into a pit. Molten steel, manufactured by either the Siemens-Martin or Bessemer process, is then poured in through a trough that forms several streams, and forms the hard face of the plate. The molten steel as it runs down cleans the face of the wrought iron plate, scoring it in places, and, being of much higher temperature, the excessive heat carbonates the iron to a depth of one-eighth to three-sixteenths of an inch, forming a zone of mild steel between the hard steel and soft iron. The mould is placed in a vertical position to insure closeness of structure and the forcing of gases out of the steel. After solidifying, the whole plate is pressed, and passed through the rolls to obtain thorough welding. It is then bent, planed, fitted, tempered, and annealed to remove internal strains. [Illustration: Fig. 6.] In 1887, Wilson took out a patent for improvements in his process of making compound plates. In this method of manufacture he takes a wrought iron, fibrous plate, fifteen inches thick, built up from a number of thin plates. While hot from the forging press, he places this plate in an iron mould (see Fig. 7) about 28 inches deep, and upon it runs "ingot iron" or very mild steel to a depth of thirteen inches. In this form of mould the plate rests on brickwork, and is held in place by two grooved side clamps or strips which are caused to grip the plate by means of screws which extend through the sides of the mould. After solidifying, the plate, which is twenty-eight inches thick, is reheated and rolled down to eighteen inches. This is the iron backing of the finished plate, and it is again put in the iron mould and heated, when a layer of hard steel is run on the exposed surface of the original wrought iron plate to a depth of eight inches. This makes a plate about twenty-eight inches thick. It is taken from the mould, reheated, rolled, hammered or pressed down to twenty inches. After cooling, it is bent, planed, and fitted as desired, then tempered and annealed to relieve internal strains. [Illustration: Fig. 7.]