330 THE BEAUTIES OF 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 
