34 Artei'ial Drainage 
Fig.- 5. — Centrifugal Pump Engine and Iron Cylinder. 
Scale : 8 feet to an inch. 
and H, would let tlie water down to the porous strata below, 
and lessen the quantity falling into the water-course at H. 
An instance is given bj Mr. Homersham, in which the chalk was 
covered with clay 18 feet in thickness, and effective drainage 
obtained by sinking dumb wells through the clay and filling 
them with flint stones. The rain, instead of flowing off by 
surface channels into the rivers and causing floods, was ab- 
sorbed into the chalk, and escaped underground to the sea.* On 
Lord Dillon's estate in Oxfordshire, Mr. Bailey Denton brought 
the drainage of several hundred acres to a shaft 3 feet in difftneter, 
sunk from 20 to 30 feet into the oolite, and thus disposed of the 
whole of the water.f The basin of the Colne, a tributary of the 
Thames, has no outlet for flood waters excepting by " swallows, ' 
the soil of the district being drift clay, gravels, sands, with chalk 
beneath at considerable depths, the beds of which dip away from 
the valley.J In the oolitic limestone the waters from ditches may 
be frequently seen, when running a full stream from 18 inches 
to 2 feet deep, to disappear from the surface and be absorbed -by 
swallow holes. 
Outfalls. — The consideration of the improvement of main out- 
* ' Trans. Instit. Civil Engineers,' vol. xxii., " On the Upper Thames." 
t Bailey Denton,' Evidence, House of Lords' Committee on Conservancy Boards,' 
1877 ; QQ. 24C1, 2471, 2473. I Beardmore's ' Manual of Hydrology.' 
