You know, of course, that the ocean greyhound of to-day is a twin-screw ship--that is, that instead of being driven through the water by one propeller, she has two--one on each side of the end of her keel. Each screw is worked by its own set of engines. These engines are entirely independent of each other. The rudder is moved to one side or the other by steam or hydraulic power. Should the rudder become useless from any cause, it is possible to steer the ship by these screws. Most of you know that you can steer a row-boat by putting more force on one oar than on the other. If you want to turn sharply you back-water with one oar and row ahead with the other. So it is with these screws. By backing one screw and going ahead with the other, the ship can be turned around almost within her own length, as the phrase is. The ordinary vessel that loses her rudder is in a sad fix. The twin-screw ship simply needs a little extra care in handling. In fact, it has happened more than once that an ocean greyhound has been steered for more than a thousand miles straight into port while the rudder was useless. It is easy to appreciate the necessity for making fast time across the ocean when you remember that each idle moment means a loss of earning power. The vessel costs $2,000,000. She will be worn out, say, in ten years. Her value will be very small. So that every moment of her ten good years must be made to tell. Suppose her navigators should be so careless as to let her wander one hour's journey off her course. Another hour would be lost bringing her back. That would mean a clear loss of two hours. Mathematical experts could tell you exactly what that loss would amount to. All we know is that not one instant shall be thrown away. [Illustration: COALING.] Perhaps you have been aboard one of the largest ships coming up the bay from Sandy Hook to New York. Have you noticed the churned-up white water that flows away behind her? Watch it, and you will observe that now on one side, now on the other, the foam ceases to flow so thickly. This shows that one screw or the other has almost stopped for a moment. The ship-channel coming up the bay is so narrow and shallow that at certain low stages of the tide a great steamship drags the water along with her body, just as your own body can drag the water in a bath-tub. The result is that the rudder has very little effect in guiding the ship. Under such circumstances the screw on one side or the other is slowed so as to steer the vessel.