It has been stated that in no navy (except that of the United States) are there to be found iron ships of the pure construction. When iron ship-building was first introduced in England, two iron sloops were laid down and experiments were carried on with targets representing their sides, to find out the effect of shot upon them. The frames of these vessels were spaced only one foot apart, and it was found that a very serious amount of splintering took place when a shot penetrated. It was also found that for thicknesses beyond half an inch the projectile was broken to pieces on contact. This advantage for iron was, however, soon counterbalanced by the rapid increase of calibre, change to rifled guns and high velocities, and the introduction of steel and chilled projectiles. The great advantages offered by iron construction, however, rendered its introduction an absolute necessity when it became a question of speed. The rapid fouling of bottoms, with its consequent loss of speed, could only be overcome by coppering, and this necessity introduced another and far more serious difficulty, that of galvanic action. The introduction of the system of sheathing iron with wood is due directly to Mr. Grantham, an English builder. The main principles laid down by him were to have widely spaced transverse frames, and on the exterior of the iron skin to work a system of angle-irons which should in themselves bring up the strength of the hull to the standard, making up for the loss by wide spacing. The iron skin having been covered with pitch, a wooden filling was introduced between the angle-irons, wedged solid so as to form a complete wooden casing. Over this was placed a layer of tarred paper, and then the sheathing proper of the ship was fastened by brass screw-bolts to the wooden filling, completely insulating the iron hull. This sheathing could then be coppered. These exterior angle-iron frames, being worked as a support to the inner ones, take, as a rule, an opposite direction. Thus if the main inner frames are transverse, the exterior ones are longitudinal, and vice versa. Grantham’s method is in reality the one shown in the description of the Russian composite system. The English Admiralty method is somewhat different. Here the sheathing is in two thicknesses, the inner course being tap-bolted to the iron skin, the bolt-heads being sunk well into the planks and covered with pitch. The outer course shifts seams and butts with the inner one and is secured with brass screw-bolts. Over this course comes the copper. The Admiralty system is the one followed in France in the construction of the first and second rate fast cruisers. The difficulties of insulation are not yet solved by any means. Could the sheathing be absolutely excluded from moisture the system would be perfect, but as yet no means have been devised by which the wood can be prevented from becoming wet and thus serving the part of the porous jar in a voltaic cell.