422 On the Various Modes of Propelling Vessels. 



and great ingenuity has been devoted to the lessening or 

 removal of this inconvenience. The advantage derived from 

 feathering oars suggested the feathering of the float boards ; 

 but, besides that every contrivance for such a purpose must be 

 complicated, and therefore both expensive and liable to acci- 

 dents, most of the modes of feathering proposed are such* as 

 attain their object only when the vessel is at rest, the current 

 produced by her motion not being taken into account by their 

 inventors. The shock and consequent vibration caused by the 

 paddles, when entering and leaving the water, has, however, 

 been greatly lessened by breaking them into portions which 

 successively enter and leave the water. 



The unequal immersion of the paddle-wheels, on account of 

 variations of the water line, is another serious inconvenience ; 

 and plans have been employed for raising and lowering them, 

 so as to accommodate them to circumstances; but no con- 

 trivance for the purpose has been such as to merit its being 

 adopted. 



The short distance between the paddle shaft and the steam 

 cylinders would give rise to an obliquity of the connecting-rod, 

 with steam-engines of the ordinary form, that must cause a 

 great loss of power. Hence marine engines are peculiar in 

 their construction ; their stroke is shorter, the position of the 

 cylinders is varied according to circumstances ; in some cases a 

 connecting-rod is rendered unnecessary by the use of oscil- 

 lating cylinders, and in others a piston-rod, by the use of trunk 

 engines. 



Instead of the paddle-wheel, attempts have been made to 

 use, at each side of the vessel, a chain, having paddles attached 

 to it, and passing round two wheels, its lower part being near 

 the water line ; but it was found that the friction of a heavy 

 chain and the complication of the apparatus were fatal to it. 



To get rid of the inconvenience arising from the paddles 

 being too much or too little submerged, paddles entirely sub- 

 merged have been tried. They were laid on their sides, so 

 that their axes were vertical, one being at each side of the 

 vessel, which was indented, so that only a portion of the 

 paddles projected beyond its side. But, as may be supposed, 

 the arrangement did not answer. On the whole, the paddle- 

 wheel has retained, almost unchanged, the form given to it by 

 its first inventors. 



The screw propeller, which has nearly superseded the 

 paddle-wheel, has been used for ages in China. The operation 

 of sculling may have suggested it. Oars placed, at an angle, 

 on an axis which was made to revolve in a plane parallel 

 to the vessel's length, would be a screw propeller. It might 

 havo been suggested also by the windmill ; for a windmill with 



