366 On Drainage and Sewerage of Towns. 
area of the town, but algo that of its suburbs; and the size 
of this sewer is very often dependent more upon the neces- 
sity of a rapid discharge of the water poured into it during 
heavy rains, than upon the actual drainage from the town. 
In any calculations, therefore, which are made for deter- 
mining the size of these main sewers, attention must be paid 
to the area of the district for which it is the outlet; to the 
maximum quantity of rain known to fall within a given 
period; to the usual discharge from the town. And it must 
also be remembered, that the effect ofa perfectsystem of drain- 
age is to carry off the water which falls on the surface much 
more rapidly than would happen were it allowed, as is now 
the case, to find its way gradually to the outlet or main 
sewer by such imperfect channels as it can make for itself ; 
and, therefore, that the more perfect the system of drainage, 
the larger must be the main outlet. 
The branch sewers, which convey the drainage of different 
portions of the town to the main sewer, must be constructed 
with reference to the maximum amount of water which will 
pass through them in the heaviest rains. 
It would be false economy to make the sewers too small 
to convey the maximum charge, and equally so to make them 
larger than necessary. The size will be determined by a 
joint consideration of the quantity of water to be passed 
through them in a given time, and the velocity of discharge, 
which depends upon the fall or the slope which can be given 
to the bottom. 
It is desirable to maintain, as far as possible, an uniform 
fall in the sewers, for if the upper portion be steep and the 
lower portion more nearly horizontal, the water flowing less 
rapidly through this latter would not be carried off as fast 
as it would be brought down from aboye, and there would 
