On the other hand, we have mentioned that the screw has its drawbacks owing to the possibility of its suffering injury. It was therefore decided that this could be avoided by placing it in a tunnel some distance forward of the stern, and thus protected against all likely damage. (A similar method is also employed in the steam fire-boats which are used by the London Fire Brigade on the Thames, and are summoned whenever a river-side warehouse or factory gets ablaze.) If reference is made to the illustration on page 257, this tunnel will be discernible. In order to leave nothing to chance a water-tight hatch is placed in the cock-pit floor just over the propeller, through which any pieces of sea-weed, rope, or other undesirable matter can easily be removed without having to beach the craft first. These little ships measure about 50 feet long, and about 15 feet wide; they are driven by direct-acting, compound, surface-condensing engines, which give to them a speed of about nine knots. In certain parts of the world where the rivers are shallow, either at their banks or in mid-stream, steam navigation is only possible by means of “stern-wheelers.” Such instances occur on the West Coast of Africa, and also in America. In general idea, though not in detail, this method is a reversion to the antiquated ship already discussed in Hulls’ idea for a tow-boat. The stern of these steamships to which we are referring is not ended in the same continuous straight line, but is raised slightly upwards at an angle so that the paddle-wheel is able to revolve freely without requiring such a draught of water as otherwise it would have needed if placed on the ship’s side in the usual manner. This will be seen on examining the stern of the _Inez Clarke_, illustrated opposite this page. This stern-wheeler was built as far back as 1879, but the points on which we are insisting are here well demonstrated. The draught of the ship, notwithstanding the weight of her engines, was only 15 inches, so that she was enabled to go into the very shallowest water, where even a bottle could float. Nevertheless her stern-wheel was sufficiently powerful to send her along at 15 miles per hour. Her measurements are 130 feet long, and 28 feet wide. Steamboats possessing a similar principle to that exhibited in the _Inez Clarke_, but much different in the arrangement, are to-day in use on the Ohio and Mississippi Rivers, being used as tugs to tow along a large fleet of flat-boats containing coal. As much as fifty to sixty thousand tons are taken in tow at one time. [Illustration: THE “INEZ CLARKE.” _From the Model in the Victoria and Albert Museum._] [Illustration: THE “NATCHEZ” AND THE “ECLIPSE” (1855).] [Illustration: THE “EMPIRE.” _From the Model in the Victoria and Albert Museum._]