450 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



force and velocity. We speak here of the principal disadvantage of 

 paddles which affects the progress of ships of all sorts. But in ships 

 of war they have a still greater drawback ; they reduce the offensive 

 power by taking up the room from the guns ; they reduce the 

 defensive power by exposing the propeller itself to the fire of the 

 enemy. 



The result has been that the transformation of the navy from 

 sailing to steamships was retarded, till a new propeller was invented 

 which was not subject to either of the two disadvantages we have just 

 mentioned, and which thus rendered possible a wide application of 

 steam power to ships of war. This new propeller is the screw, which, 

 like the paddle-wheels, steam itself, and many other mechanical and 

 physical inventions, &c., has been the object of a pretty numerous 

 series of trials and attempts before the true and decisive mark of 

 success, that of industrial or practical realization, was obtained. 



Although the system has not been extensively introduced, Euthven's 

 hydraulic propeller must be mentioned. In this the steam-engine 

 is used to eject two jets of water at high velocity, from nozzles at 

 the ship's side. From trials made by the Admiralty with the 

 Watenvitcli, this mode of propulsion held its own with the screw. 

 The nozzles turn in collars fitted to the ship's side, and can be pointed 

 ahead or astern. 



III. SCKEW STEAMERS. 



The screw is nothing else than an ordinary male- screw or frag- 

 ment of one, which, forming part of the ship, advances through the 

 water and propels the ship, forming the movable female-screw in the 

 water itself. The motion of rotation of the screw about the axis of 

 the propeller is produced by a steam-engine on board the ship. 



All that has been said on the propulsive actions of paddle-wheels 

 is applicable to the screw. Here also, by pressing on the movable 

 mass of water, and impressing on it a motion in a direction contrary 

 to that of the ship, the motion of the latter is produced. It is thus 

 inevitable that a considerable fraction of the moving force should 

 disappear as a pure loss. The advantages of the screw as compared 

 with paddle-wheels are of another kind, which we will shortly 

 mention. 



