206 OUTLINES QF NATURAL PHILOSOPHY. 



tenacity of the soil, or the hardness of the rock, af- 

 ford a sufficient resistance to the force of the water. 

 The bed of the river then changes only by insensible 

 degrees, and, in the ordinary language of Hydrau- 

 lics, is said to be permanent, though in strictness 

 this epithet is not applicable to the course of any ri- 

 ver. 



SECT. III. 



PERCUSSION AND RESISTANCE OF FLUIDS. 



297. IF the sections of two streams be tbe same, 

 the forces with which they strike on planes directly 

 opposed to them, are as the squares of their velo- 

 cities. 



For the force of a stream must be as the force of each 

 particle, and as the number of particles that strike 

 in a given time. Now, the force of each particle is 

 as the velocity of the fluid, and the number of par- 

 ticles that strike in a given time, the section being 

 given, is also as that velocity. Therefore the whole 

 force of the stream must be as the square of its ve- 

 locity. 



Hence if v be the velocity of any stream, c? the area 



of the section, f the* force, and m a constant co- 



2 efficient 



