STEAM NAVIGATION. 



The shaft of the screws is placed horizontally in the vessel 

 parallel to the keel, and directly above it. 



The faces of the paddle-wheels look sideways and are conse- 

 quently parallel to the keel. 



The faces of the screws look sternwards, and are consequently 

 at right angles to the keel. 



25. The paddle-wheels are in pairs one at each side of the vessel 

 and outside the hull, being supported on the projecting ends of 

 the paddle-shaft, and covered by large semi-cylindrical drums 

 called paddle-boxes. 



The screws are generally single wheels, within the vessel under 

 its hull, and placed near the stern. 



Only the lower parts of the paddle-wheels are immersed. The 

 screws are altogether submerged. 



26. The paddle-shaft being carried on each side beyond the 

 timbers of the vessel, the wheels supported by it and revolving 

 with it, are usually constructed like undershot water-wheels, 

 having attached to their rims a number of flat boards called 

 paddle-boards. As the wheels revolve, these paddle-boards strike 

 the water, driving it in a direction contrary to that in which it is 

 intended the vessel should be propelled. On the paddle-shaft 

 two cranks are constructed, similar to the crank on the axle of 

 the fly-wheel of a stationary engine. These cranks are generally 

 placed at right angles to each other, so that when either is in its 

 highest or lowest position the other shall be horizontal. They 

 are driven by two steam-engines, which are usually placed in the 

 hull of the vessel below the paddle-shaft. In the earlier steam- 

 boats a single steam-engine was used, and in that case the unequal 

 action of the engine on the crank was equalised by a fly-wheel. 

 This, however, has been long since abandoned in European vessels, 

 and the use of two engines is now almost universal. By the relative 

 position of the cranks it will be seen, that when either crank is at 

 its dead point the other will be in one of the positions most favour- 

 able to its action, and in all intermediate positions, the relative 

 efficiency of the cranks will be such as to 'render their combined 

 action very nearly uniform. 



The steam-engines used to impel vessels may be either con- 

 densing engines, similar to those of "Watt, and such as are used in 

 manufactures generally, or they may be non-condensing and high- 

 pressure engines, similar in principle to those used on railways. 

 Low-pressure condensing engines are, however, universally used for 

 marine purposes in Europe, and to a great extent in the United 

 States. In the latter country, however, high-pressure engines are 

 also used in some of the river steamers. 



27. The arrangement of the parts of a marine engine differs in 

 126 



