HYDRAULICS. 



155 



deeply, while the other wheel, on the other side may be lifted entirely out of 



water. 



349. To remedy in some degree these causes of inefBcienoy 

 and waste, the submerged propelling-wheel, known as the 

 screw- iwopelltr, has been introduced within the last few years. 

 The screw-propeller consists of a wheel resembUng in its form 

 the threads of a screw, and rotating on an axle. It is placed 



in the stem of the vessel, below the water-line, immediately in front of the 



rudder. Fig. 144 represents one form of the screw-propeller, and its locatioa 



in reference to the other parts of the vesseL 



Describe the 

 construction 

 and .Tction of 

 the scruw-pro- 

 pcller. 



Fig. 144. 



The manner in which the screw-propeller acts in impelling the vessel for- 

 ward, may be understood by supposing the wheel to be an ordinary screw, 

 and the water surrounding it a solid substance. By turning the screw in ono 

 du'ection or the other, it would move through the water, carrying the vessel 

 with it, and the space through which it would move in each revolution would 

 be equal to the distance between two contiguous threads of the screw. In 

 fact, the water would act as a fixed nut, in which the screw would turn. 

 But the water, although not fixed in its position as a solid nut, yet offers a 

 considerable resistance to the motion of the screw-wheel ; and as the wheel 

 turns, driving the water backward, the reaction of the water gives a propul- 

 sion to the vessel in a contrary direction, or forward. 



The great advantage of the screw-propeller is, that its ac- 

 tion on the water will be the same, no matter to what degree 

 it may be inunersed in it, or how the position of the vessel 

 on the surface of the water may be changed. 



3.50. The application of the force of water in motion for im- 

 pelling machiuery, is most extensive and familiar. The sim- 

 plest method of applying this force as a mechanical agent, i3 

 by means of wheels, which are caused to revolve by tha 



Wliat Is the 

 preat advan- 

 tatre of thR 

 screw-propelliT 

 over the pad- 

 dle-wheel ? 



What is the 

 simplest meth- 

 od of using 

 water as a mo- 

 tive power f 



