THE METROPOLITAN MAIN DRAINAGE WORKS. 57 
high-water mark to several feet below it, the task is one 
requiring the greatest skill and care ; and this difficulty is still 
farther increased by the existence of previous systems and net- 
works of drains , by subterranean lines of water-pipes and gas- 
pipes, lines of houses, the excavation of cellars, roads, streets, 
railways, bridges, and intersecting rivers ; to each of which 
belong rights of property, which must be respected. If we 
could cut an open trench at a given incline, the liquid sewage 
would flow along it just as a river in its channel, moving along 
by its momentum the solid objects in its course ; but the 
offensive nature of town-refuse requires that its course should 
be covered over, to prevent injury to the health of the inhabit- 
ants from its noxious emanations. Once inclosed, the sewer 
assumes the nature of a pipe, and becomes immediately subject 
to the laws of hydraulic pressure. Filled with the sudden 
influx of waters from storms, it would burst; stopped with 
solid accumulations, it would require cleansing. It must be 
made steep enough to induce motion in its contents, large 
enough to carry off storm-waters, and sufficiently ventilated 
for workmen to enter to clear out stoppages and obstructions. 
Other subsidiary points, it will be seen, come in. For example, 
if we observe a contour map of London, we shall see the 
irregularities and undulations of its surface are not altogether 
devoid of a certain method and order, and in the main resolve 
themselves into the general valley of the Thames, like the 
sloping sides of a double roof towards a central gutter. It is 
evident that, if we allowed the sewage to flow laterally down 
from the highest slopes into a central main drain, running 
through the entire length and median line of the metropolis 
like an under-ground river parallel with the Thames, we 
should require a culvert of enormous size, to provide not only 
for the (sewage, but the rain-fall of the whole district, and that 
the hydraulic pressure upon the masonry of the culverts would 
be of the highest tension, while the liability to accident would 
be of the most risky character. The more, then, we can divide a 
district, the smaller the streams of sewage the drains will have 
to carry, and the less the danger from sudden floods when there 
are several channels for the waters to run off by. Lienee the 
principle of intercepting drainage. If we wished to stop the 
run of water from our roofs into the central gully, two or 
three smaller gutters placed more or less diagonally along 
their sides would arrest at intervals the water flowing down 
their slopes, and convey that which fell on the different areas, 
thus isolated from each other, in separate streams to the head 
of the “ down-pipe ” into the streets. On the like principle 
the main drainage of the metropolis consists of intercepting 
sewers at various levels, so that instead of one great drain for 
