Vertical and Inclined Armor.  In England, in 1860, a target composed of 4-1/2-inch plates backed with wood and set at 38 deg. from the horizon was injured about one-half as much by round 68-pounder shot as vertical plates of the same thickness would have been.  In 1861, a 3-1/4 plate at 45 deg. was more injured by elongated 100-pounder shot than a 4-1/2 vertical plate, both plates having the same backing and the weights of iron being equal for the same vertical height.  When set at practicable angles, inclined armor does not glance flat-fronted projectiles.  Its greater cost, and especially the waste of room it occasions in a ship, are practically considered in England to be fatal objections.  The result of Mr. Stevens’s experiments is, substantially, that a given thickness of iron, measured on the line of fire, offers about equal resistance to shot, whether it is vertical or inclined.  Flat-fronted or punch shot will be glanced by armor set at about 12 deg. from the horizon.  A hard surface on the armor increases this effect; and to this end, experiments with Franklinite are in progress.  The inconvenience of inclined armor, especially in sea-going vessels, although its weight is better situated than that of vertical armor, is likely to limit its use generally.

Fastening Armor.  A series of thin plates not only strengthen the whole vessel, but fasten each other.  All methods of giving continuity to thick plates, such as tonguing and grooving, besides being very costly, have proved too weak to stand shot, and are generally abandoned.  The fastenings must therefore be stronger, as each plate depends solely on its own; and the resistance of plates must be decreased, either by more or larger bolt-holes.  The working of the thick plates of the European vessels Warrior and La Gloire, in a sea-way, is an acknowledged defect.  There are various practicable plans of fastening bolts to the backs of plates, and of holding plates between angle-irons, to avoid boring them through.  It is believed that plates will ultimately be welded.  Boiler-joints have been welded rapidly and uniformly by means of light furnaces moving along the joint, blowing a jet of flame upon it, and closely followed by hammers to close it up.  The surfaces do not oxidize when enveloped in flame, and the weld is likely to be as strong as the solid plate.  Large plates prove stronger than small plates of equally good material.  English 4-1/2-inch armor-plates are generally 3-1/2 feet wide and to 24 feet long.  American 4-1/2-inch plates are from 2 to 3 feet wide and rarely exceed 12 feet in length.  Armor composed of light bars, like that of the Galena, is very defective, as each bar, deriving little strength from adjacent, offers only the resistance of its own small section.  The cheapness of such armor, however, and the facility with which it can be attached, may compensate for the greater amount required, when weight is not objectionable.  The 14-inch and 10-inch targets, constructed, without backing, on this principle, and tested in England in 1859 and 1860, were little damaged by 68-and 100-pounders.