From Companies 16,200,000,000 gals.
„ Artesian Wells 1,920,000,000 „
„ Land Spring Pumps 1,095,000,000 „
--------------
Total yearly 19,215,000,000 „
Hence it would appear that the rain falling in London in the course of
the year is _rather more than double that of the entire quantity of
water annually supplied to the metropolis by mechanical means_, the
rain-water being to the other as 2·005 to 1·000.
Now, in order to ascertain what proportion of the entire volume of rain
comes under the denomination of street surface-water, we must first
deduct from the gross quantity falling the amount said to be caught,
and which, in contradistinction to that mechanically _supplied_ to
the houses of the metropolis is termed, “catch.” This is estimated at
1,000,000 gallons per diem, or 365,000,000 gallons yearly.
But we must also subtract from the gross quantity of rain-water that
which falls on the roofs as well as on the “back premises” and yards
of houses, and is carried off directly to the drains without appearing
in the streets. This must be a considerable proportion of the whole,
since the streets themselves, allowing them to be ten yards wide on an
average, would seem to occupy only about one-tenth part of the entire
metropolitan area, so that the rain falling _directly_ upon the public
thoroughfares will be but a tithe of the aggregate quantity. But the
surface-water of the streets is increased largely by tributary shoots
from courts and drainless houses, and hence we may fairly assume
the _natural_ supply to be doubled by such means. At this rate the
volume of rain-water annually poured into and upon the metropolitan
thoroughfares by natural means, will be between five and six thousand
millions of gallons, or one hundred times the quantity that is daily
supplied to the houses of the metropolis by mechanical agency.
Still only a part of this quantity appears in the form of
surface-water, for a considerable portion of it is absorbed by the
ground on which it falls--especially in dry weather--serving either to
“lay the dust,” or to convert it into mud. Due regard, therefore, being
had to all these considerations, we cannot, consistently with that
caution which is necessary in all statistical inquiries, estimate the
surface-water of the London streets at more than one thousand millions
of gallons per annum, or twenty times the daily mechanical supply to
the houses of the entire metropolis, and which it has been asserted is
sufficient to exhaust a lake covering the area of St. James’s-park, 30
inches in depth.
The quantity of water annually poured upon the streets in the process
of what is termed “watering” amounts, according to the returns of the
Board of Health, to 275,000,000 gallons per annum! But as this seldom
or never assumes the form of street surface-water, it need form no part
of the present estimate.
What proportion of the thousand million gallons of “slop dirt” produced
annually in the London streets is carried off down the drains, and
what proportion is ladled up by the scavengers, I have no means of
ascertaining, but that vast quantities run away into the sewers and
there form large deposits of mud, everything tends to prove.
Mr. Lovick, on being asked, “How many loads of deposit have been
removed in any one week in the Surrey and Kent district? What is the
total quantity of deposit removed in any one week in the whole of the
metropolitan district?” replied:
“It is difficult, if not impossible, to ascertain correctly the
quantity removed, owing to the variety of forms of sewers and the
ever-varying forms assumed by the deposit from the action of varying
volumes of water; but I have had observations made on the rate of
accumulation, from which I have been enabled roughly to approximate
it. In one week, in the Surrey and Kent district, about 1000 yards
were removed. In one week, in the whole of the metropolitan districts,
including the Surrey and Kent district, between 4000 and 5000 yards
were removed; but in portions of the districts these operations were
not in progress.”
It is not here stated of what the deposit consisted, but there is no
doubt that “mac” from the streets formed a great portion of it. Neither
is it stated what period of time had sufficed for the accumulation; but
it is evident enough that such deposits in the course of a year must
be very great.
The street surface-water has been analyzed by Professor Way, and found
to yield different constituents according to the different pavements
from which it has been discharged. The results are as follows:--
“_Examination of Samples of Water from Street Drainage, taken from the
Gullies in the Sewers during the rain of 6th May, 1850._
“The waters were all more or less turbid, and some of them gave off
very noxious odours, due principally to the escape of sulphuretted
hydrogen gas.
“Some of them were alkaline to test-paper, but the majority were
neutral.
“The following table exhibits the quantity of matter (both in solution
and in solid state) contained in an imperial gallon of each specimen.
“STREET WATERS.
-------+--------------------+----------+---------+--------------------------------
Number | | Quality | Quality | Residue in an Imperial Gallon.
of | NAME OF STREET. | of | of +---------+----------+-----------
Bottle.| | Paving. |Traffic. |Soluble. |Insoluble.| Both.
-------+--------------------+----------+---------+---------+----------+-----------
| | | | Grains. | Grains. | Grains.
1 |Duke-street, | | | | |
| Manchester-square | Macadam |Middling | 92·80 | 105·95 | 198·75
7 |Foley-street | | | | |
| (upper part) | „ | Little | 95·13 | 116·30 | 211·43
5 |Gower-street | Granite |Middling | 126·00 | 168·30 | 294·30
12 |Norton-street | „ | Little | 123·87 | 3·00 | 126·87
3 |Hampstead-road | | | | |
| (above the canal) |Ballasted | Great | 96·00 | 84·00 | 180·00
4 |Ferdinand-street | „ |Middling | 44·00 | 48·30 | 92·30
2 |Ferdinand-place | „ | Little | 50·80 | 34·30 | 85·10
10 |Oxford-street | Granite | Great | 276·23 | 537·10 | 813·33
6 | „ | Macadam | „ | 194·62 | 390·30 | 584·92
11 | „ | Wood | „ | 34·00 | 5·00 | 39·00
-------+--------------------+----------+---------+---------+----------+-----------
“The influence of the quality of the paving on the composition of the
drainage water,” says Professor Way, “is well seen in the specimens
Nos. 10, 6, and 11, all of them from Oxford-street, the traffic being
described as ‘Great.’
“The quantity of soluble salts is here found to be greatest from the
granite matter from the macadamized road, and very inconsiderable from
the wood pavement.
“The same relation between the granite and macadam pavement seems
to hold good in the other instances; the granite for any quality of
traffic affording more soluble salts to the water than the macadam.
“The ballasted pavement holds a position intermediate between the
macadam and the wood, giving more soluble salts than the wood, but
less than the macadam.
“The quantity of solid (insoluble) matter in the different samples of
water, _which is a measure of the mechanical waste of the different
kinds of pavement_, appears also to follow the same relation as that
of the soluble salts; that is to say, granite greatest, next macadam,
then ballasted, and, lastly, wood pavement, which affords a quantity
of solid deposit almost too small to deserve notice.
“The influence of the quality of traffic on the composition of the
different specimens of drainage is well marked in nearly all cases;
the greatest amount of matter both insoluble and soluble being found
in the water obtained from the streets of great traffic.
“The following table shows the composition of the soluble salts of
four specimens, two of them being from the granite, and two from the
macadam pavement.
“It appears from the table that the granite furnishes little or
no magnesia to the water, whilst the quantity from the macadam is
considerable.
“On the other hand, the quantity of potash is far greatest in the
water derived from the granite.
“The traffic, as was before seen, has a very great influence on the
quantity of the soluble salts. It seems also to influence their
composition, for we find no carbonates either in the water from the
granite, or that from the macadam, where the traffic is little;
whereas, when it is great, carbonates of lime and potash are found
in the water in large quantity, a circumstance which is no doubt
attributable to the action of decaying organic matter on the mineral
substances of the pavement.
“ANALYSIS OF THE SOLUBLE MATTER IN DIFFERENT SPECIMENS OF STREET
DRAINAGE WATER.
-----------------------------------------+-----------------------------------
| Grains in an Imperial Gallon.
+-----------------+-----------------
| Great Traffic. | Little Traffic.
+--------+--------+--------+--------
|Granite.|Macadam.|Granite.|Macadam.
| No. 10.| No. 6. | No. 12.| No. 7.
-----------------------------------------+--------+--------+--------+--------
Water of combination and some soluble | | | |
organic matter | 77·56 | 29·07 | 22·72 | 13·73
Silica | ·51 | 2·81 | ... | ...
Carbonic Acid | 15·84 | 12·23 | None | None
Sulphuric Acid | 36·49 | 38·23 | 46·48 | 34·08
Lime | 6·65 | 13·38 | 25·90 | 16·10
Magnesia | None | 23·51 | Trace | 3·50
Oxide of Iron and Alumina, with a little | | | |
Phosphate of Lime | 2·58 | 1·25 | ... | ...
Chloride of Potassium | None | 10·99 | None | 2·79
„ Sodium | 53·84 | 44·88 | 18·44 | 19·70
Potash | 82·76 | 18·27 | 8·75 | 5·23
Soda | ... | ... | 1·58 | ...
-----------------------------------------+--------+--------+--------+--------
| 276·23 | 194·62 | 123·87 | 95·13
-----------------------------------------+--------+--------+--------+--------
“The insoluble matter in the waters consists of the comminuted
material of the road itself, with small fragments of straw and broken
dung.
“The quantity of soluble salts (especially of salts of potash) in
many of these samples of water is quite as great, and in some cases
greater, than that found in the samples of sewer-water that have been
examined; and it is open to question and further inquiry, whether the
water obtained from the street-drainage of a crowded city might not
often be of nearly equal value as liquid manure with the sewer-water
with which it is at present allowed to mix.”
With regard to the “ballasted pavement” mentioned by Professor Way, I
may observe that it cannot be considered a _street_-pavement, unless
exceptionally. It is formed principally of Thames ballast mixed with
gravel, and is used in the construction of what are usually private or
pleasure walks, such as the “gravel walks” in the inclosures of some of
the parks, and upon Primrose-hill, &c.