Drainage by Gravitation. 19 



water rolls along the bed of the channel, particles of material 

 the specific gravity of which is too great to allow of their 

 being raised above the bed of the channel. A stream running 

 with a velocity of 6 inches a second, or about one-third of a 

 mile an hour, will transport soft clay ; a velocity of half a mile 

 an hour will carry sand as large as linseed ; a velocity of two- 

 thirds of a mile will sweep along fine gravel ; while a current 

 moving at the rate of a mile and a half an hour will roll 

 along rounded pebbles ; and at the rate of two miles an hour 

 pebbles the size of a hen's tgg will be moved along the 

 bottom of the channel. 



In some rivers upwards of 2 per cent, in weight of the 

 total volume of water passing along their channels consists of 

 material carried in suspension. In the Tees, when the drain- 

 ing works were going on, the quantity of material in suspen- 

 sion in the water was as much as 5 oz, to a gallon, or -^^ of the 

 weight of the water. The proportion in the Durance and the 

 "Vistula in floods is -^-^ ; in the Garonne and the Rhine in 

 Holland, ^Jq- ; the Rhone, ^ J^ ; the Po, -^^ ; in the Humber, 

 ^|-^ ; in other rivers the proportion varies from the above as a 

 maximum to x^-J^^, as a dry weather flow. To give an illus- 

 tration of the quantity of material transported by a river, it 

 is stated that the Durance transports in one year 17,000,000 

 tons of earthy matter.* The river Witham, in Lincolnshire, 

 before the recent improvements were carried out, passed 

 through beds of shifting sands at its mouth. The tidal flow 

 was stopped by a sluice across the river about eight miles 

 above the mouth, and consequently the ebb was very sluggish 

 when there were no land floods running down, the tidal water 

 entering the confined portion of the river at the rate of 

 from 3 to 4 miles an hour. During the dry summer of 1868, 

 when there was no fresh water flow down the river, the 

 amount of sand brought up and deposited along the bed of 

 the channel was calculated to be one-and-a-half million tons. 



* * Iriigation in France,' Trans. Inst C. E., vol. li. 



