THE METROPOLITAN MAIN DRAINAGE WORKS. 
61 
paper to scale, tlie future works are designed and estimated 
exactly in every possible particular, so that the builders' 
contracts are taken upon these designs. Everything is 
preconceived and studied — the amount of rainfall upon each 
area ; the quantities of sewage now flowing, or to be provided 
for ; the incomings along the course of the sewer, so that its 
size may be determined ; and by starting with the smallest 
requisite size, and increasing its dimensions as it proceeds, 
economy of labour and material is acquired without, in the 
1 2 3 4 5 6 
Fig. 1. A, Commencement of sewer, say 4 feet, such dimensions gradually 
increase to 10 feet 6 inches at the termination B. 1 to 6 lateral incoming 
j sewers along its course. 
smallest degree, impairing its efficiency. The thickness of 
brickwork, where iron culverts are required, their size, form, 
and length ; how they are to be supported, how joined to the 
masonry of the sewer ; how the drains shall be carried over 
valleys, rivers, or low-lying marshes, whether by pipes, 
embankments, or viaducts of arches ; where there shall be 
pen-stocks, and flushing, and pumping-stations; the extent 
and height of reservoirs, the power and dimensions of the 
pumping- engines ; what kind of brick, or stone, or iron shall 
be used for each special purpose ; — in short, everything, down 
to the very mouldings and ornamentation of any sightly por- 
tions, is all contemplated, drawn on the plans, and entered in 
the specifications. Modifications, from unforeseen circum- 
stances, or for various reasons, often take place during the 
practical performance of such undertakings ; but in the present 
case it is but justice to notice the extreme care and accuracy 
with which all the details, even to the minutest, have been got 
out and strictly adhered to. When, then, the contractor goes 
to work, he has before him the exact measurements, eleva- 
tions, and descriptions of everything he has to do. Suppose, 
for example, he has a thousand yards of sewer to construct, 
starting with an elevation of 90 feet and falling one foot in a 
hundred yards, or to 80 feet elevation at its termination. If 
the surface were a true level, the operation of laying the drain 
would be very simple : he would merely have, at every 
hundred yards apart, to sink down a foot lower in each case, 
and give a general incline in laying the floor of the drain from 
one spot to another. But no surface exists so level and true 
that this can be done ; the exact height, therefore, of the 
exact spot where the sinking is to be made to “ drive the 
headings/' or the excavations of the earth-tunnel in which 
