COMMON PADDLE-WHEEL. 



revolves. The wheel is immersed to the depth of the lowest paddle-board, 

 since a less degree of immersion would render a portion of the surface of 

 each paddle-board mechanically useless. In the position A, the whole 

 force of the paddle-board is efficient for propelling the vessel ; but as the 

 paddle enters the water in the position H, its action upon the water not 

 being horizontal, is only partially effective for propulsion : a part of the 

 force which drives the paddle is expended in depressing the water, and the 

 remainder in driving it contrary to the course of the vessel, and, therefore, 



Fig. 12. 



by its re-action producing a certain propelling effect. The tendency, how- 

 ever, of the paddle entering the water at H is to form a hollow or trough, 

 which the water, by its ordinary property, has a continual tendency to fill 

 up. After passing the lowest point A, as the paddle approaches the posi- 

 tion B, where it emerges from the water, its action again becomes oblique, 

 a part only having a propelling effect, and the remainder having a tendency 

 to raise the water, and throw up a wave and spray behind the paddle- 

 wheel. It is evident that the more deeply the paddle-wheel becomes 

 immersed, the greater will be the proportion of the propelling power thus 

 wasted in elevating and depressing the water ; and if the wheel were 

 immersed to its axis, the whole force of the paddle -boards, on entering and 

 leaving the water, would be lost, no part of it having a tendency to propel. 

 If a still deeper immersion take place, the paddle-boards above the axis 

 would have a tendency to retard the course of the vessel. When the vessel 

 is, therefore, in proper trim, the immersion should not exceed nor fall 

 short of the depth of the lowest paddle ; but for various reasons it is 

 impossible in practice to maintain this fixed immersion : the agitation of the 

 surface of the sea causing the vessel to roll, will necessarily produce a great 

 variation in the immersion of the paddle-wheels, one becoming frequently 

 immerse,d to its axle, while the other is raised altogether out of the Water. 

 Also the draught of water of the vessel is liable to change, by the variation 

 in the cargo ; this will necessarily happen in steamers which take long 

 voyages. At starting they are heavily laden with fuel, which as they 

 proceed is gradually consumed, whereby the vessel is lightened. 



151 



