teel, and by it the
face of the plate is made high in carbon and very hard.

The system invented by Sir Joseph Whitworth, of Manchester, England,
consists in what might be called scale armor. A section of a sample of
the armor represents four plates. The outer layer, one inch thick, is
composed of steel of a tensile strength of 80 tons per square inch;
the second layer, one inch thick, of steel whose tensile strength is
40 tons per square inch; the third and fourth layers, each one-half
inch thickness, of mild steel. The outer layer is in small squares of
about ten inches on a side, and is fastened to the second layer by
bolts at the corners and one in the middle of each square. The surface
is flush. (See Fig. 9.) The end sought by the above system is to break
up the shot by the hard steel face and to restrict any starring or
cracking of the metal to the limit of the squares or scales struck.
The bolts are of high carbon and are extremely hard steel.

[Illustration: Fig. 9.]

Armor plates must often be bent or curved to single or double
curvature and sometimes to a warped surface to fit the form of the
ship. There are several methods of bending plates. One method employs
a cast iron slab of the required form, which is placed on the piston
of a hydraulic press. The armor plate is placed face down on this
slab, and on top of the plate are laid packing blocks of cast iron, of
such sizes and shapes as to conform to the required curve. These
blocks take against the upper table of the press, when the piston is
forced up, and the hot plate is thus dished to the proper form.

In the French method of bending, an anvil or bed plate of the required
curve is used, and the armor plate is forced to take the curve by
being hammered all over its upper surface with a specially designed
steam hammer.

The edges of the plate are trimmed by large, powerful slotting
machines or circular saws; the latter, however, operate in exactly the
same manner as a slotter, except that there is no return motion to the
tool. Each tooth of the saw is but a slotting tool, and these teeth
are, by screws, rendered capable of being nicely adjusted in the
circumference of the saw.

The plates are fastened to the hulls and backing by heavy bolts,
varying in size according to the weight of the individual plate. For
the 6,000 ton armored ships, these bolts are from 2.75 to 3.1 inches
in diameter and from 18.45 to 23 inches in length. They are tapped two
o