214 OUTLINES OF NATURAL PHILOSOPHY. 



Breakers formed over a great extent of shore, are di- 

 stinguished by the name of Surf. The surf is 

 greatest in those parts of the earth where the wind 

 blows always nearly in the same direction. 



SECT. V. 



HYDRAULIC ENGINES. 



Engines moved by the IMPULSE of Water. 



305. W HEN a wheel is put in motion by a 

 stream of water striking against a float-board, the 

 action of the water diminishes, as the velocity of 

 the wheel increases, till at last the momentum of 

 the water, or of the accelerating force, is just 

 equal to the momentum of the resistance, or of the 

 retarding force. The motion of the wheel then be- 

 comes uniform. 



Mills with undershot wheels, or such as receive the 

 impulse of the water at right angles to their radii, 

 are machines of this kind. 





. 



306. A machine driven by the impulse of a 

 stream, produces the greatest effect, or does the 



most 



