306 



OF STEAM NAVIGATION. 



[SECT. x. 



parabola, the effect again decreases. I was led to examine this point, by observing 

 the form of those fins of fish which are used for impelling in a similar manner. 

 The lines A D E F show the size of a square paddle capable of producing the same 

 effect. It strikes the water at once with its whole breadth, as at G. The parabolic 

 one strikes a little sooner, and gradually acquires its full hold of the water. 



635. The best position for the paddles appears to be in a plane, passing 

 through the axis, as represented in the figure ; if they be in a plane which does 

 not coincide with the axis, they must either strike more obliquely on the fluid on 

 entering, or lift a considerable quantity in quitting it. 



In the direction of the breadth of the paddle, it is evident the form should be 

 such that the resistance to its motion should be the greatest possible, and the 

 pressure behind it the least possible. These conditions appeared to be fulfilled, in 

 a high degree, by making it a plane in this direction also. A flat curve has been 

 used, the concave surface to strike the fluid, and perhaps with a very small increase 

 of power. To set the paddles at any other than a right angle, must obviously be 

 a defect ; for the resistance to motion becomes less when the surface strikes the 

 water obliquely, whereas the greater this resistance, the greater the effect in 

 impelling the vessel. 



636. It is desirable that the action of the paddles should be as equable and 

 continuous as possible. But in attempting to render the action of the paddles 

 equable, their number ought not to be increased more than can be avoided, 

 because the construction is more expensive, and the time for the water to flow 

 between them so as to afford a proper quantity of reaction is reduced ; neither do 

 they clear themselves so well in quitting the water. If we suppose A E to be the 

 line the water would assume when at rest, the most favourable arrangement with 

 the smallest number of paddles appears to be to make a paddle at A, just entering, 



FIG. 29. 



when one at B is in a vertical position, and the one E quitting the water : if a 

 smaller number were employed, there would be a short interval, during which 



