CHAP. VIIL] 



STEAM NAVIGATION. 



451 



The screw is placed behind the ship, in a rectangular framework 

 which opens near the stern-post. The axis or driving shaft which 

 supports it is parallel to the keel. It rests at the front end against 

 the buttress, a sort of block solidly fixed in the hold, and behind it 

 passes through the hull by a stuffing box. The engine sets this 

 shaft and the screw in motion either directly by cranks and knee- 

 joints, or indirectly by wheel-gearing. 



This propeller is always immersed, and at such a depth that 

 the disturbing motions of the sea have no action upon it. It 

 is not therefore subject like the paddle-wheels to inequalities of 

 action. In addition to this, the screw is almost entirely pro- 

 tected from shot and so are the engines required to move it, 

 since they are fixed like the screw 

 below the water-line. Lastly a 

 consideration of the highest interest 

 for steamships of war the fighting 

 decks are not in the least obstructed 

 by them. 



In general, the screw has this 

 further advantage over paddles, that it 

 leaves the ship quite free for working 

 the sails so that screw-steamers may 

 be rigged to take advantage of the 

 wind when it is favourable, which is a 

 great advantage from an economical 

 point of view, ships with sails and 

 paddle-boxes are, on the contrary, 

 difficult to manage. 



We will pass in review as briefly as may be the history of the 

 invention of the screw and its application to steam navigation. 



As in the case of the paddle-wheels, the first attempt was to 

 move the screw by living forces, either of men or animals. Duquest 

 (1727) availed himself of the currents of rivers for rowing boats, 

 by using Archimedes' screw. Paucton (1768) employed a helicoid 

 of four branches which he turned by the hand. 



In 1803, the engineer Dallery took out a patent for the propeller 

 moved by steam and composed of two screws one with a movable 

 axis, placed in front, serving for rudder, and the other placed behind, 



G G 2 



FIG. 



, -Framework of screw 

 behind a ship. 



