330 THE BEAUTIES OE NATURE chap. 



their valley, ■which again they gradually raise 

 above the rest. Hence, unless they are kept 

 in their own channels by human agency, 

 such rivers are continually changing their 

 course. 



If we imagine a river running down a 

 regularly inclined plane in a more or less 

 straight line ; any inequality or obstruction 

 would produce an oscillation, which when 

 once started would go on increasing until 

 the force of gravity drawing the water in a 

 straight line downwards equals that of the 

 force tending to divert its course. Hence the 

 radius of the curves will follow a regular law 

 depending on the volume .of water and the 

 angle of inclination of the bed. If the fall 

 is 10 feet per mile and the soil homogeneous, 

 the curves would be so much extended that 

 the course would appear almost straight. 

 With a fall of 1 foot per mile the length of 

 the curve is, according to Fergusson, about 

 six times the width of the river, so that a 

 river 1000 feet wide would oscillate once in 

 6000 feet. This is an important considera- 

 tion, and much labour has been lost in trying 



