Drainage by Gravitation, 1 1 



water to flow to its ultimate destination, the sea, is that due 

 to gravity, each particle of the fluid endeavouring to attain 

 the lowest leveL If the bottom of the drain be perfectly flat, 

 so long as there is any fall in the surface of the water the 

 particles at the higher end will continue to move until they have 

 arrived at the lower level ; and as every particle is free to move, 

 the whole mass of water in a channel, from the surface to the 

 bottom, will be in motion until a low-level horizontal surface 

 is attained. The action of a mass of water in a running 

 stream is not that of a body moving in a plane parallel with 

 the surface or bottom of the stream, but partakes more of a 

 rotary character, by which the particles of the water are con- 

 tinually being rolled round and round from the bottom to the 

 top, this action being increased where the bottom is irregular 

 and full of holes. The result of this rotary motion is indicated 

 by the numerous miniature whirlpools that are constantly 

 forming on the surface, and by the manner in which light 

 floating substances appear and disappear. It is, no doubt, 

 due to this centrifugal motion that the deep holes that are 

 found in running streams are maintained, the pebbles being 

 whirled round and round at the bottom, keeping the loose 

 soil continually in motion and allowing it to be carried away 

 by the running water. 



A very slight inclination in the surface of water, even the 

 fraction of an inch in a mile, will cause movement An in- 

 crease in the surface inclination rapidly increases the velocity 

 and the quantity discharged. An increase of four times the 

 fall in the surface of a stream doubles its discharge. 



Velocity of Water in Drains. — The velocity is 

 governed by the rate of inclination of the surface of the water, 

 and not of that at the bottom of the drain. The velocity of the 

 water due to gravity is checked and retarded by the friction 

 of the water against the rubbing surfaces with which it comes 

 in contact, this rubbing surface consisting of the sides and 

 bottom of the drain, weeds, sides of bridges, or other impedi- 



