Perhaps there is no feature of the steamship which is less suspected of being misunderstood than the propeller. To the average mind, its character is apparently so self-evident as barely to require any unusual consideration. But its introduction as a means of ship-propulsion has been the cause of a good deal of miscomprehension, and has set to work the keen brains of some of the most able mathematicians in order to determine the exact relation which it bears towards the ship and the manner in which it is capable of being used for the greatest good, and with the utmost economy. Here and there in the course of the narrative I have hinted at some of these problems, but in order not to break up the continuity of the story, I deemed it best to defer until now the fuller presentation of the subject. It is not necessary to remark that the propeller’s function is, by means of its revolutions, to drive the ship ahead, and to overcome the resistance which encounters the hull. Besides the skin friction, the eddy-making, and the wave-making, there is also the resistance of the air. Now let us suppose for a moment that instead of propelling itself ahead by its own engines and screws, a liner were to be taken in tow by a powerful tug-boat. It would follow then that the pull required to cause the liner to go through the water would be equal to those total entities of resistance which we have just enumerated. But let the tug be cast off, and allow the liner to start her engines and proceed by means of her propellers. The above resistance now becomes augmented by the resistance of the propellers. The reason is that the propeller causes a suction which tends to pull the ship back.