MECHANICS 75 



may (whatever its length) be made to deliver consider- 

 ably more water if its first three inches and the last 

 yard of its length, be enlarged and given a conical 

 shape. 



The motion of water in the bed of a river would, but 

 for the resistance offered by its sides and its bed, go on 

 continually accelerating from its source to its mouth, 

 like a solid body falling by gravity. In that case 

 enormous destruction would be produced in the lower 

 lands while the upper parts would be deprived of all 

 moisture. But the adherence of the particles of water 

 together, and the friction against the sides and bed 

 of the river, produce a resistance which increases with 

 the velocity of the current, till it equals the accelera- 

 tive force of the descent, and so a uniform motion becomes 

 established. 



Irregularities in the sides and beds of rivers often 

 produce currents setting obliquely, or eddies, and of 

 course the steeper the descent of the river's bed the 

 greater the velocity and force of the current. 



When a liquid has any part of its surface raised above, 

 or depressed beneath, the rest, it will, as has been said, 

 return to the general level. But in so doing it, like 

 a pendulum, acquires a velocity which carries it beyond 

 the position of equilibrium, arid thus it oscillates, com- 

 municating similar oscillatory motions to the adjoining 

 portion of the surface of the liquid, and that to the 

 next and so on. But as all these communications of 

 motion are not simultaneous but successive, an appearance 

 is produced of an elevation, or of a series of elevations, 

 travelling along the surface in other words, we have 

 what is called a wave or a series of waves. 



Each wave contains particles of the liquid in all 

 degrees of oscillation, elevation and depression, and the 



