STEAM NAVIGATION. 



the intermediate shaft for working the piston of the air-pump E ; p, p are 



the slide-valves, by which 



g> n ' the admission of the steam 



to the cylinders is regu- 

 lated ; G, G are double 

 eccentrics on the inter- 

 mediate shaft, whereby 

 the valves F, F are moved ; 

 H is a handle, whereby the 

 engines may be stopped, 

 started, or reversed ; I, I 

 are the steam-pipes lead- 

 ing to the steam -trunnions 

 K, K, on which, and on 

 other trunnions, connected 

 with the pipe M, the cylin- 

 ders oscillate ; N, N are 

 pumps, the pistons of 

 which are attached to the 

 trunnions, and are worked 

 by the oscillation of the 

 cylinders ; o is the waste- 

 water pipe, through which 



the water which has accomplished the function of condensing the steam is 

 ejected over-board. The same letters refer to the same parts in the two figures. 



54. To obtain from the moving power its full amount of mecha- 

 nical effect in propelling the vessel, it would be necessary that it 

 should constantly act against the water in a horizontal direction, 

 and with a motion contrary to the course of the vessel. No system 

 of propellers has, however, yet been contrived capable of perfectly 

 accomplishing this. Patents have been granted for many 

 ingenious mechanical combinations to impart to the propelling 

 surfaces such angles as appeared to the respective contrivers most 

 advantageous. In most of these the mechanical complexity has 

 formed a fatal objection. No part of the machinery of a steam- 

 vessel is so liable to become deranged at sea as the propellers ; 

 and, therefore, that simplicity of construction which is compatible 

 with those repairs which are possible on such emergencies is quite 

 essential for safe practical use. 



55. The ordinary paddle-wheel, as has been already stated, is a wheel 

 revolving upon a shaft driven by the engine, and carrying upon its cir- 

 cumference a number of flat boards, called paddle-boards, which are 

 secured by nuts and braces in a fixed position ; and that position is such 

 that the planes of the paddle-boards diverge from the centre of the shaft 

 on which the wheel turns. The consequence of this arrangement is that each 

 paddle-board can only act in that direction which is most advantageous for 

 the propulsion of the vessel when it arrives at the lowest point of the wheel. 

 In fig. 12, let o be the shaft on which the common paddle-wheel revolves ; 

 the positions of the paddle-boards are represented at A, B, c, &c. ; x T 

 represents the water-line, the course of the vessel being supposed to be 

 from x to y ; the arrows represent the direction in which the paddle-wheel 



150 



