60 
POPULAR SCIENCE REVIEW. 
hithe and Deptford, a length of ten miles to 18 feet 
below datum. From Deptford one general outfall sewer, 
11 feet 6 inches in diameter, conveys the drainage matter 
through Greenwich, by the Woolwich Lower Road, by a deep 
tunnel under the town of Woolwich, then along the Plum- 
stead Road, and afterwards north-easterly across the marshes 
to a point called Cross Ness at Half-way Reach on the Thames 
— a distance of seven miles and a half — where the whole sewage 
will again be lifted into a vast covered reservoir, six acres 
and a half in extent, and discharged after the turn of high 
water into the river. 
The objects sought to be obtained by these works are 
primarily to remove from the river Thames, in its course 
through London, the noxious contamination of the nearly 
sixty millions of gallons of sewage now daily poured into it, 
by intercepting that sewage in its progress towards the river, 
and diverting it in covered channels as far as possible, by 
gravitation, to points some fourteen miles below London 
Bridge, where it will be discharged into the bottom of the 
river during the first two hours of ebb-tide only, when the 
sewage, deodorized, will be further diluted by twenty times 
the volume of water which now dilutes it in the London 
area. In this way it is considered the ebb-tide will prac- 
tically convey the sewage to an ultimate distance of twenty- 
six miles from London Bridge, and surrender it there to the 
s.ea. 
When a plan is formed, it requires to be put in execution : 
having determined the lines of drainage, we have to construct 
the drains. It is evident, from the undulating nature of the 
surface, that there must be differences of inclination in various 
portions, and that at places the ground will sink beneath the 
plane of the sewer, unless it be excavated very deep indeed, 
to do which would be to diminish the power of gravitation, or 
to necessitate uplifting by engine-power — a perpetual cost it 
would, of course, whenever possible, be most desirable to 
avoid. 
The ground, therefore, is accurately surveyed, and all the 
irregularities of elevation referred to a given base-line — the 
ordnance datum, 12ft. Gin. below Trinity high-water mark. 
In London this operation is greatly facilitated by the re- 
gistered marks of the Ordnance Survey, who have accurately 
levelled through the metropolis, and affixed inscribed marks 
upon the houses and public buildings in every street and 
highway, so that the height of these marks being exactly 
known, they can be at once adopted, without recourse to a 
tedious process of levelling from the datum-line itself. 
Upon such a section, thus obtained and plotted down on 
