March 31, 1870] 
IN GEA NOSIS. 
559 
them is hidden beneath the surface of the ground, nothing 
but a personal inspection during the process of construc- 
tion can give any adequate idea of the vastness and intri- 
cacy of our drainage system. The following facts, collected 
from the engineer’s papers on the subject and reports to 
the Metropolitan Board of Works, may present to the mind 
some notion of these great works. There are about 1,400 
miles of sewers in London, 82 miles of which are intercept- 
ing sewers of “ The Main Drainage.” The area drained by 
the intercepting sewers is about 100 square miles. The 
total amount of sewage and rainfall which they will carry off 
is 63 million cubic feet per day, or equal to a lake as large 
as Hyde Park four feet in depth. There have been con- 
sumed in their construction about 340 million bricks, and 
upwards of 900,000 cubic yards of concrete. The total 
engine-power employed is 2,380 nominal horse-power, but 
this will soon be increased by about 400 horse-power on the 
construction of the Pimlico pumping station. The cost of 
these works when completed will be alittle over 4,000,000/, 
The accompanying map shows the position of the main 
intercepting sewers, pumping stations, outfalls, &c., and 
it will be seen that they run east and west, the reason for 
this being that the main sewers in existence before the 
construction of the main drainage, emptied themselves 
into the Thames, running more or less at right angles to 
it, and consequently, by constructing the intercepting 
sewers parallel to it, they would cross all the main outlets 
and cut off their sewage. In carrying out this scheme 
the great object to be kept in view was to discharge as 
much sewage as possible by gravitation, in order to avoid 
the great expense of pumping. To this end three lines of 
sewers are constructed on each side of the Thames, called 
respectively the High, Middle, and Low Level sewers, 
together with their branches. On the north side the 
High and Middle Level discharge their sewage by gravi- 
tation; that of the Low Level, which joins them at Abbey 
Mills, being pumped up a height of 36 feet into the 
Upper Level, when all three flow through the Northern 
Outfall Sewer to the reservoirs at Barking, and discharge 
their contents into the river by gravitation. On the south 
side the Low Level sewer meets the High and Middle 
Level at Deptford, where its contents are pumped a 
height of 18 feet into the Outfall Sewer, which carries the 
three streams to Crossness, where they can discharge by 
gravitation ; but as this can only be done at low water, 
they are generally pumped a height of 20 feet into 
the reservoirs. Such is the general outline of the Main 
Drainage scheme, as being carried out by Mr. Bazalgette, 
the able engineer of the Metropolitan Board of Works. 
Before commencing these works a large number of ex- 
periments and observations had to be made in order to 
decide the numerous knotty points that suggested them- 
selves: such as the determination of the position of the out- 
falls, the shape and inclination of the sewers, the amount 
of sewage and rainfall to be provided for, the amount to 
be pumped, &c., &c., each requiring a great amount of 
labour and study. The results arrived at, and which were 
acted upon, may be summed up as follows :—That it was 
necessary to take the sewage down: the river as far as 
Barking Creek; that its lowest mean velocity should 
be 14 miles per hour;.the quantity of sewage to be 
intercepted was, in populous districts, 750,000 gallons per 
square mile per day ; the amount of rainfall to be pro- 
vided for was a quarter of an inch per day; the form of 
the sewers was to be circular, and the sewage was to be 
discharged at or about the time of high water. The 
above are a few of the principles upon which the Main 
Drainage of London is based. 
THEWORKS.—The High Lever Sewer North commences 
near Hampstead Hill, where it intercepts the Fleet Sewer. 
It varies in size from 4ft. in diameter to oft. 6in. by r2ft., is 
about seven miles long, and drains an area of about ten 
square miles in its course from Hampstead to its junction 
withthe Middle Level at Old Ford. It passes under three 
railways, the New River, and Sir George Duckett’s Canal, 
in the last case the top of the sewer is within 2 ft. of the 
waters in the canal. 
The Middle Level Sewer commences near Kensal 
Green and flows to its junction with the High Level at Old 
Ford. The total length of the main line, together with 
the Piccadilly branch, is about 11} miles; the area drained 
is 17} square miles; in form it varies from 4ft. 6in. by 
3ft. to oft. 6in. by 12ft. Six miles of this sewer were con- 
structed in tunnel. In its course it passes under two 
canals and one railway, and over the Metropolitan line, 
across which it was carried without interfering with the 
traffic, the bottom of the sewer being only a few inches 
above the chimneys of the engines. 
At the junction of the High and Middle Level sewers 
there is a large “penstock” and “weir” chamber, for 
the purpose (should occasion demand) of letting storm 
waters or all the sewage flow into the River Lea. It is 
15oft. in length, 4oft. wide, and 36ft. high, and fitted 
with five large penstocks or valves, which are raised or 
lowered by machinery. 
The Low Level North will commence by a junction 
with the Western Drainage near Cremorne,and flow along 
the intended Chelsea Embankment to Pimlico, where its 
contents will be pumped up a height of 173 feet into 
the remaining length, which will flow vz@ the Thames Em- 
bankment to Abbey Mills. Its length, including branches, 
is about 124 miles, and it’varies in size from 6ft. gin. 
to 1oft. 3in. diameter. The area drained by it, including 
the Western Drainage, the sewage of which it receives, 
is 244 square miles. This sewer is by far the most 
interesting on the north side, in consequence of the large 
works in connection with it. In the first place, it receives 
the drainage of the Isle of Dogs, in consequence of 
which, what was formerly little better than a marsh is 
now perfectly habitable ; in the second place, it will pass 
along the two new embankments, viz., Chelsea and the 
Northern Thames ; and in the third, its contents will be 
twice lifted by steam power, the lift at Pimlico being over 
17ft., and that at Abbey Mills 36ft. At Blackfriars 
Bridge, where it intercepts the Fleet, there will be a 
large penstock-chamber fitted with four penstocks and 
as many tide flaps, to prevent the return of the tide at 
high water. A large portion of this sewer, namely, that 
extending from Blackfriars to Abbey Mills, will be exe- 
cuted for the most part in tunnel. 
The Northern Outfall Sewer, which receives the whole 
of the sewage on the north side of the Thames, is totally 
unlike any other portion of the Main Drainage system, 
being entirely above the ordinary level of the ground. 
The first length, from B to C (see plan), consists of a 
double line of sewers, each being oft. by oft. of the section 
shown in Fig. 1 running parallel 
to each other; at C it receives re 
the contents of the Low Level, 
and from this point to the re- 
servoirs at Barking it is of the 
section shown in Fig. 2, which 
is three parallel culverts oft. by 
oft. built upon a concrete founda- 
tion and covered by an earthen 
embankment, the top of which 
would serve as a road or rail- 
way. In its course to Barking 
it crosses the River Lea and 
six streams, four railways, and ten roads. Fig. 3 gives 
a general idea of the method adopted for carrying the 
sewers over the streams, railways, &c. It consists of three 
cast-iron culverts carried between four wrought-iron girders, 
the top being covered with cast-iron plates, which support 
the ballasting, roadway, &c. ; the parapets are ornamented 
with cast-iron mouldings. 
The contents of this sewer are received by the reservoirs 
at Barking. These are about gf acres in extent, and 16#ft. 
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