a7id the Storage of Water. 
25 
In the Fen districts this subsidence has been found to amount 
to as much as 2 and 3 feet, and even in extreme cases, as in 
Whittlesea Mere, as much as 8 feet, involving the lowering ot 
the drainage wheels and the cills of sluices. 
Impeovement of Natural Channels. 
Regulating Weirs — Catchwater Drains — Bridges. — In cases 
where the natural gradient of the country is very steep, and 
it is desirable to hold up the water in the streams for the 
use of the cattle and to prevent the drying up of the soil in 
summer, or where the soil is so loose and friable as to be easily 
washed away, it may become necessary to regulate the fall by 
the use of steps or weirs. This process was adopted in the 
drainage of Hainault Forest, as described by Mr. Grantham in 
his paper on " Arterial Drainage."* The natural fall of the valley 
at the surface of the land was so great, that the velocity of the 
water would have destroyed the sides and bottom of the drain. 
Overfalls built of brick were therefore put in, varying from 
10 feet to 3 feet in width of opening, and rising from 3 to 5 feet. 
These overfalls were so constructed that boards could be inserted 
in grooves to hold up and store the water in summer for cattle 
or other purposes, and for irrigating the land on both sides. 
Where the district to be drained is flat and surrounded by 
land rising at a steep inclination, especially where mechanical 
power has to be resorted to for raising the water, it may be 
advisable to adopt the system of catch-water drains used by 
Mr. Rennie in the Fens. A drain was there cut skirting the 
low land, for the purpose of collecting the water from the higher 
level and carrying it off to an outfall above the point of dis- 
charge of the lowland water. The Fen drains have thus only to 
contend with the rain falling on the level. By the use of regu- 
lating sluices at the end, and at intervals along the catch-water 
drains, these serve as reservoirs for the storage of water which is 
admitted to the drains on the low level during the summer, so as 
to fill the ditches and supply water for the stock. Water supplied 
in this way from the high-land streams bordering on the Fens 
is highly prized, and considered of incalculable advantage. 
The arrangement of these high-level drains is often such as to 
involve the necessity of carrying the low-level drains across and 
consequently underneath their bed. This is simply and easily 
accomplished by syphons or " sunken tunnels," which consist 
merely of cast- or wrought-iron pipes or a wooden tunnel. It 
is hardly necessary to say that, if it is required, these may be 
* 'Trans. Instit. Civil Engineers,' vol. xix., Grantham on " Arterial Drainage." 
