Drainage by Gravitation. 17 



When land drained by gravitation discharges its water into a 

 tidal stream, or, if drained by steam, the pumps do not work 

 at night, the area of the drains will require to be larger than 

 where the discharge is constant, as a reservoir has to be pro- 

 vided for the accumulation of water during the time the 

 doors are closed by the tide, or the pumps are not at work 



Cleaning Drains and Removal of Weeds. — Drains 

 running through fens and flat districts, where the current is 

 never very rapid, and where generally in summer-time there is 

 no current at all, are liable to become choked with weeds. The 

 earthy matter carried by the water in suspension in floods is 

 arrested by these weeds, and gradually a deposit accumulates 

 at the bottom of the drains, in which more weeds grow, and so 

 accretion goes on. The uniform depth of the channel is thus 

 deranged, the bed of the river rises and consequently the 

 water-way and the discharging capacity of the drain is 

 diminished. The weeds themselves also prove a great ob- 

 struction to the flow of the water. Accumulation of deposit 

 also takes place across the main drains at the places where 

 the lateral drains come into them. 



It is generally the practice to cut the weeds twice or three 

 times a year. The ordinary method is by an implement 

 resembling a number of scythe blades joined together, which 

 is drawn backwards and forwards across the drain by men 

 stationed on either side, and working upwards against the 

 stream. The weeds as cut are drawn out by rakes, and placed 

 above the highest flood level. The cost of this work in the 

 fen district, where it is termed " roding,'* is about 20^. a mile 

 for drains from 12 to 20 feet wide, and 30^*. for larger drains. 



A more effectual plan, and one which also at the same 

 time removes shoals and accumulations of deposit, is by 

 loosening and breaking up the deposit by means of a revolv- 

 ing implement drawn along the bottom of the drain at the 

 time when a current is running down. By this means not 

 only the soil of the shoals is loosened and carried away in 



c 



