I have spoken above of laminated armor.  To secure the full benefit of this kind, the plates must be neatly fitted to each other; the surfaces must make close contact.  This requires accurate machining, and hence is expensive.  To overcome this point sandwiched armor was suggested.  This consists in placing a layer of wood between the laminations, as shown in Fig. 2.  It was found that laminated and sandwiched armor gave very much less resisting power than solid rolled plates of the same thickness.  Wrought iron armor is made under the hammer or under the rolls, in the ordinary manner of making plates, and has been exhaustively studied and experimented with—­more so than any other form of armor.

[Illustration:  Fig. 2.]

Chilled cast iron armor is manufactured by Gruson, in Germany, and is used in sea coast defense forts of Europe.

In 1867 several compound plates were made by Chas. Cammell & Co., of Sheffield, England, and were tested at Shoeburyness, in England, and at Tegel, in Russia.  These plates were made by welding slabs of steel to iron; but the difficulties were so great that the idea was abandoned for the time.

[Illustration:  Fig. 3.]

[Illustration:  FIG. 4.]

Compound armor, as now manufactured, is of two types:  Wilson’s patent, a backing of rolled iron, faced with Bessemer steel; Ellis’ patent, a backing of rolled iron, faced with a plate of hard rolled steel, cemented with a layer of Bessemer steel.  Both these kinds are manufactured in England and France in sizes up to fifty tons weight.  The Wilson process is used at the works of Messrs. Cammell & Co., of Sheffield, England, and the Ellis process at the Atlas Works of Sir John Brown & Co., of the same place.  These are the two leading manufacturers of compound plate.

[Illustration:  Fig. 5.]

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.