THE EARTH. 1?3 



becoming more level as it approaches towards the 

 sea. 



If this acquired velocity be quite spent, and 

 the plain through which the river passes is en- 

 tirely level, it will, notwithstanding, still continue 

 to run from the perpendicular pressure of the 

 water, which is always in exact proportion to the 

 depth. This perpendicular pressure is nothing 

 more than the weight of the upper waters press- 

 ing the lower out of their places, and conse- 

 quently driving them forward, as they cannot re- 

 cede against the stream. As this pressure is 

 greatest in the deepest parts of the river, so we 

 generally find the middle of the stream the most 

 rapid ; both because it has the greatest motion 

 thus communicated by the pressure, and the 

 fewest obstructions from the banks on either side* 



Rivers thus set into motion are almost always 

 found to make their own beds. Where they find 

 the bed elevated, they wear its substance away, 

 and deposit the sediment in the next hollow, so 

 as in time to make the bottom of their channels 

 even. On the other hand, the water is continu- 

 ally gnawing and eating away the banks on each 

 side ; and this with more force as the current 

 happens to strike more directly against them. 

 By these means it always has a tendency to ren- 

 der them more straight and parallel to its own 

 course. Thus it continues to rectify its banks, 

 and enlarge its bed, and, consequently, to dimi- 

 nish the force of its stream, till there becomes an 

 equilibrium between the force of the water and 

 the resistance of its banks, upon which both will 



