56 
THE METROPOLITAN MAIN DRAINAGE WORKS. 
BY S. J. MACKIE, F.G.S. 
W HEN people see tlie roadways torn up, and the traffic 
blocked by fencings of planks round deep well-boles 
in our busy streets, they are puzzled often to conceive what 
connection these isolated diggings and workings can have 
with each other. Beyond the general idea that the new 
drainage works are to take the sewage away from our houses 
and deposit it in the river at Barking Reach, they have little 
notion of what the metropolitan drainage operations are ; and 
yet, in these gigantic undertakings, there is little in the 
general plan that is not readily intelligible and simple. To 
get rid of anything we must make use of natural or artificial 
force. The refuse from our houses could be carted away, or it 
might be allowed to run off. In the first case, labour and 
artificial locomotion would be required ; in the latter, the 
natural force of the earth's gravitation would do the work for 
us. The former would be costly and troublesome ; the latter 
is inexpensive and gratuitous. Eor liquids to run naturally 
away, we require an inclination of the surface, so that the con- 
stituent atoms may come under the influence of gravitation, in 
the same way as a handful of marbles will roll down a sloping 
board. But if the fluid be required to move a solid, the rate 
at which it flows must be increased, or, in other words, the 
inclined plane must be made the steeper, that the motion of 
the fluid may approach more nearly to that of a directly 
falling body. The steeper the incline, the greater the power 
of a liquid to move and carry along extraneous solid bodies. 
In drainage works, then, the primary object is to use gravita- 
tion as the removing power, because it is enduring and ever 
continuous, costs nothing, and wants no looking after ; while 
the secondary object incident on the application of this prin- 
ciple is to get as “ good a fall " — that is, as steep an inclina- 
tion — as possible. In a small town merely crowning the crest 
of a solitary hill this matter would be simple enough ; but 
where a large city like London spreads over many square, miles, 
and includes in its area undulating and irregular land of every 
degree of elevation and depression, from 400 feet above Trinity 
