62 
POPULAR SCIENCE REVIEW. 
the sewer is to be built, must be ascertained. If there be an 
Ordnance-mark near at hand; say indicating 102 feet; the 
surveyors level from this by the spirit-level to the spot for the 
sinking ; they rule lines; as it were; with the telescope of their 
instrument parallel with the datum-line. 
Fig. 2. Method of levelling for determining the depth of the work. 
Suppose two levellings are taken; the fine horizontal wire 
across the lens of the telescope accurately adjusted to a perfect 
level cutting the mark of ten feet on the staff the assistant 
holds a convenient distance away on each occasion; it is 
evident we have added 20 feet in elevation to the original 
height at the spot c. One hundred and two feet plus twenty 
are 122 feet — the elevation; then; of that spot; and one 
hundred yards being the distance between u and c — the fall 
of the sewer adds one foot additional for the depth to be sunk 
at <3; making a total depth of 21 feet from the surface to the 
floor or invert of the drain. When; therefore; this depth is 
roughly attained by the well-sinker; the surveyor makes an 
accurate measurement with the tape, and drives pegs at the 
exact spots; the excavators make their tunnel; and the brick- 
layers work to these pegs just as they would work to given 
lines in their ordinary operation upon the surface. The form of 
the sewers often varies. Sometimes it is round; sometimes 
oval; rarely square ; sometimes compounded of various seg- 
ments of arches struck from different centres. These modi- 
fications depend on the quantity of sewage; the nature of the 
ground; and the character of particular obstructions ; the flat 
or square portions being; for example; adopted where the 
quantity of sewage is great; but the drain lying near the 
surface; or too close beneath some obstructive object to permit 
of its being built of its full height. Where open gaps occur; 
as at the underground railway- cutting in Victoria Street 
(Farringdon Street); iron culverts are used to bridge over the 
void beneath. But iron expands and contracts with variations 
of temperature; and such expansion or contraction is almost; 
one may say, irresistible in its force. In expanding; the iron 
must either contort itself, or push down or out the brickwork 
on which it presses ; in contracting; it would leave an open 
space between,; and the sewage would leak out. provision, 
