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stratum. Cut off from the general gravel-bed it will, of course, be unaffected by the
variations in the amount of water contained in this bed, and it will also be secure, so
long as the integrity of the clay-basin is unimpaired, from being dried by operations
which may drain the gravel-bed. It is only by tapping the lower part of the
basin that its character of a reservoir can be destroyed. Unless it be so tapped, which
may happen in the course of excavations for houses or sewers, the portions of ground
overlying it will, in all probability, be imperfectly drained; they will remain
damp, and apt to generate miasm and noxious emanations, arising from the decomposition
of the animal and vegetable matters on the surface, or carried in solution into
the soil. A pump connnected with such a well will constitute a useful means of
drainage. It is important however, to bear in mind that a pump of this description
should be regarded truly as a drain, and not as a source of water fit for human
consumption.
Section B. has been constructed in order to show the course, depth, and probable
effect of the projected "Northern Middle Level Intercepting" Sewer of Mr.
Bazalgette. The line of this sewer will run at a general depth of about 24 feet below
the surface. It will, consequently, run, for the most part, through the gravel-stratum,
which it will in many places bottom, having a diameter of from 9ft. Gin. to 10ft.; it
will occupy in its course a considerable proportion of the entire thickness of the
gravel. If the gravel-stratum were evenly disposed throughout the district, the effect
of this would be, that, owing to the easily-permeable property of gravel, the whole
stratum would be drained, even to a considerable distance. All the surface-wells
would be dried. But the peculiar arrangement of the clay in basins, to which I have
adverted, will greatly prevent this, to my mind, most desirable tendency. If this
great sewer should also serve as a great land-drain, two objects of the utmost importance
in their influence upon health would be accomplished:
First, the general dryness of the surface would be promoted,—withdrawal of
moisture is the removal of one essential element in the production of miasm and pestiferous
vapours.
Secondly, the drainage of the surface-wells would cut off a source of water
laden with salts, the results of decomposition, and with organic matter dissolved from
the putrifying filth of the pest-stratum. The analyses of some examples of water taken
from these surface-gravels will abundantly demonstrate their utter unfitness for human
consumption.
I cannot, therefore, but anticipate the most beneficial results from the drainageaction,
so far as it may extend, of the proposed great sewer of Mr. Bazalgette.