brasure gave way, and it became apparent that the accuracy of rifled guns rendered splayed embrasures inadmissible in Buch positions. About this time, too, experiments with plates of various thicknesses in different positions-vertical, and inclined at angles varying from 10 to 60° with the horizon-showed amongst other results that a given mass of armour plates in a vertical position offered as much resist ance as the same mass disposed over the same vertical height in an inclined position. In all these experiments the backinga and fastenings occupied important positions. Targeta were accordingly made with elastic backings, without elastic backings, and with compound backings of various kinds; but the general result of trials against these targets was to establish the advantage of elastic backings. The armour plates were fitted mechanically without bolts or rivets, were fastened with bolts, and were fastened with continuous rivetting round their edges; the result of the experiments established the superiority of bolts as fastenings for armour plates. Many more experiments had, however, to be made before the proper form of bolt was settled; these were by no means the least interesting of the experiments made in working out this subject, but space does not avail to give the history of them here; and it may be sufficient to say that a bolt has been adopted about 3 inches in diameter in the shank, or rather less than the lesser diameter of the screw, which has a rounded thread (about 6 to the inch) and a shallow rounded cut; that the bolt holes are made one inch larger in diameter than the bolts; that the edges of the holes are coned, and that cupped washers receive the nut on the inner end, which is cupshaped (as is the washer of the front of the bolt) as shown Plate VIII. in Plate VIII.; and that various minor arrangements have been made for inducing gradual action, and so relieving suddenuess of strain upon the bolt. Finally these bolts are made of special iron of a fibrous nature, and elongating from 35 to 45 per cent. under pressure.

The law of the resistance of armour plates received early investigation during the course of these experiments. Sir W. Fairbairn, supposing that the resistance of armour plates to the impact of a shot was analogous to the resistance of plates to a punch, deduced this general formula (C being made equal to 3,374,900):

ᎳᎤ

Thickness of plate perforated = D

(Sir W. Armstrong endeavoured to ascertain the resist ance by the application of the dynamic theory of heat. Captain Noble, R.A., from many experiments, concluded that the penetration of steel shot is directly proportional to the work in the shot, whether a heavy shot with a low velocity or a light shot with a high velocity be employed; and that the resistance of armour to steel shot varied as the power of its thickness. But Captain English, R.E., who appears to have treated this subject most scientifically, considers that the energy absorbed by a plate in resisting a shot is made up of the energy expended in enlarging an indefinitely small hole through the plate, and of the energy absorbed in frictional resistance, and is equal to the cube of the diameter of the shot multiplied by a constant-a separate constant being required for each proportion to the diameter of the shot, of depth of indent in the one case, and thickness of plate in the other.

Having obtained a law which can be relied upon for thick armour plates (it is not, however, applicable to very thin plates), the mode in which the iron in an armourplated wall should be disposed, i.e., in one solid plate or in two or three thicknesses of plates, was shortly settled. A comparison of experiments against 7 inches of iron armour plate disposed in one, two, and three thicknesses gave resistances in the proportion of 100, 96, and 89. Further experiments

against single and double plates of 5 inches showed that the double plate offered nearly three times the resistance of the single plate; and finally a comparison between a 15-inch plate and three 6-inch plates showed that though the solid plate offered slightly more resistance to a single blow it broke up sooner under repeated blows than did the three plates; but as, of course, there is a limit in manufacture to the mass of an armour plate, it is evident that the thicker the plate the more numerous will be the joints in a given area, and if the armour be in one thickness, all these joints are lines of weak resistance. The disposal of the joints, then, is a matter of first importance in designing iron structures, and in this respect the arrangement in several thicknesses, or the plate upon plate system, as it is called, offers the greatest advantage, for the plates can be so arranged that no joint goes through more than one-third the thickness; moreover, this system is more easy and more economical in construction, and can be strengthened with greater facility at any future time. In order to put the conclusions arrived at to practical test, a shield for a casemate opening 12 feet long and 8 fect high was constructed of three 5-inch plates placed at inter vals of 5 inches, the intervals being filled with iron concrete, the plates were whole, each covering the entire opening, and each was bolted to the plate next to it, the inner plate being bolted to the shield frame in rear; the front plate was secured by 10 bolts, the others by d bolts each, the total weight of the shield was 56 tons 8 cwt. It was struck with 16 shot from 25-ton, 18-ton, and 14-5-ton muzzleloading rifled guns at 200 yards distance; the average energy of each blow was 5321 foot tons, and the aggregate 90,000 foot tons,-upwards of 1000 foot tons per foot superficial of the sbield's face; but the shield was not driver out of shape, and its back or interior face was uninjured

This experiment was so satisfactory in its results that henceforward there was no hesitation with respect to the broad principles of a mour construction, and it only remained to adapt these principles to the different cases which presented themselves. These are sufficiently illustrated by the drawings in Plates VIII. and IX. The first is an Plates open battery shield for an opening 9 feet wide and 7 feet VIII high; the second is an ordinary casemate shield 12 feet and IX wide and 9 feet high, and the third is a portion of an iso lated sea fort with an exterior iron wall. The application of iron has facilitated improvements in the construction of the roofs of bomb-proof buildings, and the form of roof shown in the sea fort may serve as a model of construction of this kind. It is composed of arch plates of 4-inch iron curved to a radius of 5 feet 6 inches, springing from iron girders 1 foot 9 inches deep and 1 foot 2 inches wide. The arch plates are covered with brick arches, which are again covered with 3 or 4 feet of Portland cement concrete. This roof is equal to a load of one half ton per foot superficial, with a tensile strain upon the iron not exceeding 5 tons per square inch. Such a roof covered with 5 feet of earth was tested by vertical fire of 13-inch mortar shells at 1000 yards. The few rounds which struck it produced no effect, and after the 298th round practice was discontinued; but the earth was removed, and the bared concrete was struck by a shell, which only produced an indentation of 8 inches.

The cost of iron structures is, however, a drawback to their more general employment,- -so much so that only wealthy nations can afford to erect them, and but in limited numbers. The cost of an open battery shield may be stated at £800, its weight 10 tons; of a casemated shield, £1800, its weight 40 tons of an iron-walled fort, £5550 per gun, its weight 125 tons per gun.

It is, however, to be observed that, in special cases where the forts are constructed in very deep water, and where, consequently, the increase in the quantity of masonry in the substructure becomes very great for every additional foot