THE METROPOLITAN MAIN DRAINAGE WORKS. 63 
therefore, must be made for these well-known alterations in 
iron materials. 
For the illustration of engineering operations, no better 
portion of the metropolitan drainage works can possibly be 
taken than that between Old Ford and the Stratford Road, , at 
Bow. At Old Ford the High Level and Middle Level Sewers 
make a junction or rather meet at a pen-stock, by which, for 
repairs, or during storms, communication can be made between 
them, or with a storm overflow into the River Lea ; for the 
sewers themselves run on side by side, touching, but not 
actually joining. 
Fig. 3. Plan of Old Ford Pen-stock. H. L. S. (High Level Sewer), a gate. 
M. L. S. ^Middle Level Sewer), b Gate. / Connecting gate, e e' Storm 
weir, c Storm outlet running into the river Lea at x. 
The accompanying diagram, representing in a general way 
the arrangement at Old Ford, will convey an idea of the nature 
and use of a “ pen-stock.” The two main lines of sewers are 
seen approaching each other, the arrows indicating the direc- 
tion in which the sew r age is flowing. At f is a gate usually 
closed, each current of sewage then passing along its own 
special sewer. If an impediment take place in the High 
Level Sewer in the portion at c, or further on its course 
towards the outfall, the gate a being closed stops the sewage 
from passing along it, and the intermediate gate f being 
opened allows it to flow into the Middle Level main. Vice 
versa, the gate b bars the passage of the Middle Level sewage 
when the opening of the communicating gate / gives vent 
from it into the High Level main. The same in the case of 
storm-waters when the flow is excessive, for it is necessary to 
bear in mind that ordinarily the drains are not even half full 
or anything like it. But during storms the volume of fluid 
passing is enormously augmented, and unless provisions were 
made for such storm-waters, their hydraulic pressure in 
extreme cases would burst the barrels of the drains. In 
order, therefore, to provide against such emergencies, 
additional communications are also made for the surplus 
storm-waters to run off by “ storm-outlets ” into the rivers 
or old drains. A provision of this sort is made at Old Ford, 
