When ships have to be repaired they are brought into the repairing dock, which has a pair of gates shutting it off from the river; when they are closed (at low water) the water is pumped out from the dock, and the repairs done; when finished the water is let in, and the ship floats out. In small vessels it is sometimes sufficient to haul them on shore at high tide, so that when the tide is down they may be left high and dry and repaired, and when the tide is at the highest, hauled off again. Steam ships are constructed to be propelled either by paddle wheels having flat boards fixed to their circumferences, which on being turned round, take a great hold in the water, and so cause the motion of the ship; or by the screw-propeller, which has been described. Illustration: PADDLE AND SAILING SHIP. Illustration: BREAKING UP. The iron ships, which of late have almost superseded those of wood, are made of plates of wrought iron, rolled, while red hot, between rollers to the thickness required, which is generally half-an-inch; these plates have holes punched all round them by machinery, and are united by rivets placed in the holes red-hot and rivetted by heavy hammers. The most magnificent specimen of iron shipbuilding ever attempted is the Leviathan. This ship is 680 feet long, and is not made of one thickness or case of iron plates, but is upon a new principle called the “cellular,” consisting of an outer and inner casing of iron plates held together by partitions of iron so as to separate them into square compartments or “cells.” The objects gained by this arrangement are greater strength, and greater safety, for in case of injury to the outer portion, the water would enter only between the two in that compartment where the injury happened to be, and so fill only that small portion with water. The whole ship is also divided into compartments by means of double screens of iron, making it like a fire-proof box, and even if a fire should occur in one of these, the others would be preserved from its effects. The masts and yards of this great ship are also of hollow wrought iron plates rivetted together, and are both stronger and lighter than they would be of wood. A machine is placed at the lower part of each mast which by compressing, can crush it up, and cause it to break off and fall over the side of the vessel in case such a thing should be required, as in a very violent storm; and the standing rigging, which is of wire-rope, can be let loose in a few minutes so as to completely free the ship of the mast. This great ship is constructed to be propelled both by screw and paddles; the screw engines are four in number, and are each of 1,600 horse power, the paddle engines are also four, of 1000 horse power each, being 10,800 altogether. They will require about 180 tons of coal a day to work them; 12,000 tons of which are capable of being carried. The Leviathan will have six masts, and be able to spread 6,500 yards of sail, will accommodate