Under-Drainage. 
570 
present paper, in order that I might attract the attention of the 
highest practical authorities to the subject ; and there was only one 
opinion expressed upon the effect of under-drainage on the arterial 
system : which was, that the floods were more quickly precipitated into 
the valleys, in proportion to the extent of under-drainage in th;; 
various river basins, and that the continuance of flood depended upon 
the capability of the main outfall. Reference was particularly made 
to valleys into which the tributary water-courses had been opened for 
the outlet of upland drainage water, and it was stated that heavy 
rainfalls now produce a flood in two days, whereas it formerly took a 
week or more to do the same thing. The question whether under- 
drainage increases the volume of flood waters was not discussed, 
though the prevailing impression was, that — contrary to the theory of 
drainage, which would lead to the expectation that the rain would 
pass thi'ough the drained soil less rapidly than it had hitherto passed 
off the saturated siu'face — the quantity discharged at flood times was 
not diminished. In a paper now under discussion at the same Insti- 
tution, by the Rev. J. C. Clutterbuck, it is stated as the result of a 
generiil inquiry into the peculiarities of the Thames basin, that the 
floods are advanced twenty-four hours in seventy-two hours within 
the last twenty years. That the hastening of floods, not so much 
from any positive increase in the quantity of water, but from its con- 
centration, is the general effect of under-drainage, I do not think any 
one will deny, nor that in proportion to the number of under-draius 
will be the suddenness with which the discharge is effected from the 
land into the tributary channels. Doubt was expressed by several 
eminent engineers whether the effect of imder-drainage of the fi'ee 
soils could possibly increase the perennial supply of v/ater to the 
rivers ; but as the subject was new to the Institution in the light in 
which it was introduced (viz., by separating the retentive soils from 
the percolative, water-bearing soils, and showing that the two are 
susceptible of distinctly different treatment and results), I was not 
surprised to find an indisposition to confirm the deductions to which 
my experience had brought me ; and I attributed this to the circum- 
stance that the free soils have hitherto for the most pai't been drained 
in the same way as the clays, and have rather increased the evil of 
sudden flood than been made the means of regulating and of improving 
the supply in summer. So important do I deem it that this view of 
the question should be well understood, that I venture to exhibit the 
plans of two works of free-soil drainage which have been carried out. 
One is in Wales, in the Silurian district, and the other is in the South 
of England, on the Bagshot sand foimUtion. In both cases parallel 
drainage was originally intended by the owners, though their inten- 
tions gave way to occasional or test-hole di'ainage. In the first 
instance a contract was entered into in accordance with the plan A, 
and fortunately abandoned, by which a parallel system of one uniform 
depth and interval was to be carried out, and the number of rods per 
acre would have been 132, and the cost IIZ. 10s. per acre. If that 
drainage had been adopted, there is little doubt but that some benefit 
would have been derived. The numerous drains would have pre- 
