CHAP, viii,] STEAM NAVIGATION. 449 



solidly attached to it by iron rods and fellies (see further on, Fig. 311,) 

 are rectangular plates, which when set in motion by the rotation of 

 the driving shaft, successively plunge into the water, and pressing 

 against it push forward the boat in the direction opposite to that 

 of their own motion. The wheels are always two in number, for the 

 sake of symmetry and equilibrium ; they are mounted on the same 

 axis or shaft which crosses the ship perpendicularly to its length, and 

 when they are immersed vertically in the water, their upper border 

 ought to be covered to a height of '10 to -20 metres. 



The mechanical work of paddles on the water resembles that 

 of oars. They produce no useful effect: in pushing the boat forward 

 except by pushing the water backward. This last motion, without 

 which the first, which is the reaction from it, would not exist, is 

 called the recoil. It absorbs a considerable quantity of the work of 

 the steam independently of the losses occasioned by friction. As an 

 example of this division of moving work, we will quote the w r ords of 

 M. Sonnet, as the result of experiments made on the steamboat. 

 Castor working between Honfleur and Havre. " Of 100 horse-power 

 furnished by the engine 33'9 are employed to overcome the resistance 

 of the water on the ship, which constitutes the useful work ; 58 -2 are 

 consumed by the recoil, that is to say, to put the water in motion ; 

 friction uses up only 7'9." 



The successive blows of the paddles on the water at their entrance 

 and exit produce a series of troublesome and fatiguing tremblings in 

 the ship which is much diminished by giving a slight inclination to 

 the paddle-boards in the direction of their length. One extremity 

 then enters after the other, or, if desired, the immersion may be 

 continuous over the whole length of the paddle-board. By this 

 means the blows and the tremblings resulting from them are almost 

 insensible. 



On waters with a calm surface, where the ships can preserve a 

 nearly horizontal position of equilibrium, paddle-wheels do excellent 

 service. But it is not the same on the sea, where the action of the 

 rollers makes the ship incline to the right or left, and this inclination 

 prevents the axis of the wheels from remaining horizontal. The two 

 wheels are then immersed unequally in the water and their action on 

 the water and their propelling motion becomes unequal. The result 

 is a dangerous deviation in the course of the ship as well as a loss of 



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