the amount of rain which may be expected to fall; and,
secondly, the proportion of this rainfall which will reach the
sewers. The maximum rate at which the rain-water will reach the
outfall sewer will determine the size of the sewer and capacity
of the pumping plant, if any, while if the sewage is to be
stored during certain periods of the tide the capacity of the
reservoir will depend upon the total quantity of rain-water
entering it during such periods, irrespective of the rate of
flow.

Some very complete and valuable investigations of the flow of
rain-water in the Birmingham sewers were carried out between
1900 and 1904 by Mr. D. E. Lloyd-Davies, M.Inst.C. E., the
results of which are published in Vol. CLXIV., Min Proc.
Inst.C.E. He showed that the quantity reaching the sewer at any
point was proportional to the time of concentration at that
point and the percentage of impermeable area in the district.
The time of concentration was arrived at by calculating the
time which the rain-water would take to flow through the
longest line of sewers from the extreme boundaries of the
district to the point of observation, assuming the sewers to be
flowing half full; and adding to the time so obtained the
period required for the rain to get into the sewers, which
varied from one minute where the roofs were connected directly
with the sewers to three minutes where the rain had first to
flow along the road gutters. With an average velocity of 3 ft
per second the time of concentration will be thirty minutes for
each mile of sewer. The total volume of rain-water passing into
the sewers was found to bear the same relation to the total
volume of rain falling as the maximum flow in the sewers bore