Strictly speaking, the Nemesis has no fixed keel, but the lower plate of iron, which connects the two sides of the ship together along its middle, is called the keel-plate. She is, therefore, almost perfectly flat-bottomed; and, in order to obviate, as much as possible, the disadvantages attendant upon this peculiar construction, there are two sliding or moveable keels, capable of being raised or lowered to the depth of five feet below the bottom of the vessel. Each of these keels is about seven feet in length, one being placed before and the other abaft the engine-room. They are each enclosed in a narrow case or tank, one foot wide, running from the bottom of the vessel up to the deck, and which, of course being open below, allows the water to rise in it, to the level of the sea on the outside of the vessel. In this, the keel, which is of wood, 4½ inches thick, works up and down by means of a small winch, and a strong chain which is attached to it. Thus it is evident, that either the foremost or the aftermost keel can be raised or lowered, independently of the other, if circumstances require it. As it would, however, be impossible to steer with accuracy, a vessel of this construction, with a rudder merely of the ordinary description, and which, from its shallowness, would, in a heavy sea, be in a great measure out of water, there is a contrivance by which a moveable or false rudder is attached to the lower part of the true or fixed rudder, and which descends to the same depth as the two false keels, and, like them, can be raised or lowered at pleasure. The main or true rudder was composed of wood, but the lower or false rudder was made of iron, and was so constructed as to grasp the lower part of the upper or fixed one, firmly on either side, but was bolted through in such a way as to be moveable, as if it were fastened by a hinge, so that, by means of a chain run up to the taffrail from its outer edge, it could be hauled up to any height required. The next striking peculiarity in the construction of the Nemesis was, that the entire vessel was divided into seven water-tight compartments, by means of iron bulkheads; so that, in fact, it somewhat resembled a number of iron tanks, cased over, so as to assume the external form of one connected vessel. By this means, the occurrence of any accident, such as striking on a rock, or shot-holes, &c., which might occasion a dangerous leak in one compartment, would have no effect upon any other part of the vessel. The advantages of this arrangement were often tested, during her three years' hard service; and, indeed, within a few days after her first departure from Liverpool, as will be presently related, this contrivance sufficed to save her from the almost certain destruction which would otherwise have awaited her.