The Sun.
London, England, 1855-01-15 00:00:00

INSTITUTION OF CIVIL ENGINEERS.

Both evenings (Dec. 12 and Jan. 9), were entirely de-
voted to tho discussion of Mr. P. W. Barlow's paper " On
some peculiar features of the Water-bearing Strata of the
London Basin."

It was urged, that the term "water-bearing strata "
should be more clearly defined, and it should bo shown
whether it meant the chalk, through the fissures of which
the major portion of the water percolated, or the sands
which were entirely saturated with water.

The views expressed some time since, by the Rev. Mr.
Clutterbuck, in his communications to the institution in
1842 and 1843, were insisted on as having been confirmed
by subsequent investigations. Professor Ansted's paper
"On the absorbent properties of chalk," was also quoted
in aid of the proposition of its retentive properties, and it
was shown that the action was chiefly by capillary attrac-
tion, and existed to such an extent that chalk would contain
one-third of its bulk and one-fourth of its weight of water.
Hence it wag argued that nearly all the chalk was super-
saturated with water "in situ," but nevortheless it did not
follow that it could be drained, except through the fissures
and layers of flints.

It was admitted that the indications of Mr.
Dickenson's rain-gauge might not be critically ac-
curate ; but they were so far correct as that they
served to inform him what supply of water ho might
expect in the river. The rate of evaporation from the sur-
face, both by sun and wind, required to be watched care-
fully. If it were possible to evaporate down to about six
feet beneath the surface, very little, if any water, falling
as rain, would reach the lower beds. After a dry summer
the earth required 3 inches of rain to compensete for the
previous evaporation, and it was only after that quantity,
had fallen that there was any surplus moisture for re-
plenishing the springs.
The details were given of the experiments made and the
effect produced by the pumping at the well sunk by the
Grand Junction Canal Company, near Tring. The depth
of the well was 75 feet, with two bore-holes below that
level, down into the chalk, whence by means of engine
power and two pumps of 12 inches diameter and 6-feat
stroke, the water was intended to be pumped for the
use of the canal. The results of the experiments tended to
demonstrate, not only that the water in the adjacent wells
was reduced, but that the river was lowered and a pond in
the vicinity disappeared; but on arresting the pumping,
the water resumed its original level in all. It was urged
that the sources of the river were in the upper part of the
chalk, and therefore that if the well was paddled and
close-ateined, so as to exclude the top-water, no effect
would be produced ; this, however, on trial, was not found
to be the ease, as on the resumption of pumping a repeti-
tion of the same results was observed. It was also stated
that the well at the Croydon Waterworks, also sunk into
the chalk, had similarly affected the river Dandle.
The accuracy of this statement was impunged ;—it was
urged that the quantity of water pumped had not been
stated, and it had not been shown that there had been a
failure in the attempt to reader the steining of the well
impermeable. It was indispensable to make wells water-
tight at the upper portions, in order to prevent them from
injuring other wells of less depth in the vicinity. It was
stated to be practicable to obtain a large supply of water
from the chalk, by tie same system as had been employed
use of the dock. It was also stated that a large quantity
fur procuring a supply of fresh water at Grimsby for the
of water was obtained from the chalk, at Woolwich, for
the supply of that district, without acting detrimentally
upon the neighbouring wells.
The peculiar features of the absorption of water in va-
rious parts of the course of the Lea and the reappearance
of the springs at other spots were noticed, and analogous
instances quoted—such as the river Mole, which partially
disappeared at several places in its channel, and after a
subterranean course of some distance through the porous
strata and fissures again rose to the surface.
M any celebrated springs owed their origin to this cause ;
the water when once it was absorbed finding its way into
various open strata, or fissures and beds, and only again
appearing at some fault, or where, by denudation, or other
cause, the porous strata appeared at the surface.
The great variety in the quality of the chalk was in-
sisted on, as necessary to be considered, in assuming it
to be a useful water-nearing stratum. In reality the qua-
lity varied infinitely, from an open texture, broken up and
mixed with heterogeneous materials, to a close_ compact
substance, almost of the density of stone; its distance
from the surface and the thickness of the bed, varying
almost to the same extent ; its capacity for water-bearing,
or rather for water-yielding, would, it was argued, be also
as various and uncertain.
It was contended that the sands underlying the chalk
would be found the most certain sources of water supply.
The condition of the strata at Grirnbsy were different to
those of the London basin ; and in the latter locality, some
years ago, in any well of moderate depth, :eater was certainly
met with, whereas at present it was only found at consider-
able deptbs,and was only obtainable by great pumping power.
The water was no wchiefly found in the fissures and flint beds,
and but little in the solid chalk, beyond that held in
suspension, and which it was difficult to extract. Water
might be found in sufficient quantity, in the chalk, for the
supply of two, or three suburban parishes near the metro-
polis, but the conditions were very different from those of
London itself, and it was strongly contended that it was
illusory to trust to any such source for a large supply by
Artesian wells. The district to the south-east of London
wee eouipaLatively intact, but it could only be proved, by
experiment, to what extent it was capable of furnishing a
largo supply ; although it was admitte4 that a large quan-
tity of water was wasted by the springs rw.i.ing into or
rising in the river, which might probably be beneficially
ployod. The case of the well now being sunk at Hampstead
was quoted : that well had been sunk at a spot 172 feet above
Trinity H. W. mark, with the intention of procuring a
supply of water from the chalk, which was found at a
depth of 320 feet from the surface. The chalk was com-
pact, hard, free from fissures, and comparatively dry ; and
the flints were distributed through the mass, and not in
layers. After sinking 218 feet in the chalk boring was
commenced, and had proceeled until a total depth of 1,13')
feet had been arrived at, traversing a thickness of 540 feet
of chalk, the subjacent gault, and into the red clay, pre-
sumed to overlay the lowes green sand, in which it was
anticipated a supply of water would be found. Little or
no water had, however, hitherto been yielded in all this
deep boring.
The influence of swallow-holes, in admitting streams into
permeable strata, was noticed, as also that the lower
green sand must evidently be the source of many of the
springs, having the appearance of arising from chalk ; as
the rain was rapidly absorbed at the denuded out-crop of
those sands, and the water frequently passed from them
through the fissures in the chalk, and appeared to proceed
from the latter stratum.
It was contended, that the anticipated quantity of water
to be derived from the chalk, and even from the green
sands, was exaggerated ; the supply might for the first few
years be considerable, but by degrees the super-saturation
would be reduced, and the supply would depend on the
percolation of the annual rain-fall. The best accredited
experiments, upon various soils, showed that not more
than four inches of rain infiltrated through the strata and
flowed outwards in the shape of springs. Beyond that
quantity then any supply must be derived, by pumping,
from the constant quantity held in suspension in the
various strata, and that source of supply, it was argued,
would be soon diminished in certain localities.
It was admitted that water was obtained from chalk,
chiefly, if not entirely, by infiltration through the fissures,
and the layers of flints; and that although chalk was a
rapid absorbent of moisture, it did not readily part
with it again, nor in its normal condition would it
afford a ft ee passage for water; therefore it
could not be correctly denominated a water-bearing
stratum ; whereas sand would tee correctly so termed.
in ceftain districts so intersected
excessive
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numerous layers of flint that
bearing stratum was ob-
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pumping from one well
would affect all around, as also the sources of the streams.
In other distr'..cts the chalk possessed such a density that
all hope of obtaining water from it was vain. The quality
of the 'fnalk under London was as various as the depths
etw:nich it was reached from the surface. The same pe-
cediaritiea exhibited themselves at Paris, where in one
situation water was reached at 574 feet, whilst at the
Puits de Granola it had Leen necessary to go as deep as
1,800 feet.
Sand was, on the contrary, essentially a water-bearing
stratum ; the facility of yielding moisture depending gene-
rally on the size, and therefore the collective compactness,
or openness, of the grains. It was as susceptible of satu-
ration as the chalk, but not as capable of retention of mois-
ture. In some districts the lower tertiary sands were
mixed with clay, which interrupted the flow of water,
whilst in others they afforded a free passage.
The intermittent character of the flow from the well at
Grenolle was mentioned, and was shown to arise chiefly
from stoppages of the pipe by sand, although the quantity
of water delivered was undoubtedly influenced by the fall
of rain on the out-crop of the tertiary strata, into the basin
of which the well was sunk. Artesian wells were not
sufficiently universal in their application, and could scarcely
yet afford correct data as to the character of the flow, but
instances were cited of one well of that kind, stated to
have been sunk in the 12th century, and another about a
century old, the supply from both of which was now, ap-
parently, undiminished in quantity and unchanged in
quality.
It was contended that the dry character of the chalk in
certain localities was an indication of its being really a
water-hearing stratum, as it was submitted that its layers
of flints, and its numerous fissures, afforded the most rapid
means of percolation.
In reasoning from results obtained by gauges it was
necessary to observe the character of the rain-fall ;if the
rain fell gently, the evaporation might be so great that
none would percolate ; or during a storm it might all run
off the surface ; therefore the question could only be solved
by observation for a long period, and over an extensive
area of out-crop. It was also necessary to observe the
swallow holes, and numerous depressions in the chalk dis-
tricts, as by them great quantities of water were conveyed
to the lower strata.
It was mentioned that the lower green sand stratum had
been reached in the Hampstead well at a depth of 1,150
feet from the surface, and a very short time would now
demonstrate the correctness or the fallacy of the views as
to the quantity of water to be obtained from those strata.
There was great difference in the condition of chalk, with
or without flints, and it was contended that in some cases
the latter contained a larger amount of water than the
former. The lower beds appeared to be saturated with
water which had percolated from rain or melted snow, and
was only arrested by the clay, and hence the accumulation
in the chalk, until the water rose to such a height as to
flow Out as springs.
The wells under London did not now yield more than
eight millions of gallons per day, and the water-level had
been gradually sinking. In the year 1824 the water, when
not affected by pumping, rose to within 10 feet of high-
water mark, but now it did not reach to within 60 feet of
the same level.

Ther© could not be any doubt of the south-eastern dis-
trict, mentioned in the paper, being very full of water,
but it might be anticipated that it was there, as in other
districts, an accumulated store, which, when heavily drawn
upon and not adequately replenished, would after a time
foil as other stores had done. Few questions were more
difficult to treat than that under consideration, and it Was
hoped that the results of investigation and of observation
1"141d he annually laid Wore the meting 4.