774 
PKOCEEDINGS OF SECTION H. 
"being that the sewers become charged with sewer gas generated by 
the decomposition of the slimy sewage adhering to the drains, which 
the best system of ventilation would not overcome, and the natural 
sequence would be the spread of disease and the prevailing nuisance 
from escaping sewer gas. It is therefore of the utmost importance 
that sewers of this description should be systematically Hushed for 
cleansing and purifying purposes to as great an extent as possible 
without resorting to artificial arrangements. With this object in 
view, it is advisable to admit the rainfall which, by increasing the 
volume of sewage, and consequently the increased velocity of discharge, 
would have the desired effect of cleansing the drains. 
The additional cost of admitting the rainfall to the sewers in 
tropical and semi-tropical countries is a question of considerable 
importance, seeing that the excessive quantity of storm-water to be 
provided for would mean the construction of sewers of very large 
capacity, and so much out of proportion to the ordinary work of 
the sewers during periods of dry weather that a sufficient velocity of 
current would not be maintained to keep them clean, and consequently 
they would become immense cesspools of almost stagnant sewage, 
which by decomposition and generating sewage gas would prove 
highly injurious to the health of the citizens. 
A rainfall 2 inches in depth over one acre is equal in volume to 
7,260 cubic feet, and it is not unusual for the rate of fall to exceed 
T -J-p-ineh per minute. If this data be adopted, then the rate of flow to the 
outfall would be = 36-3 cubic feet per minute from each acre. 
Taking the population at an estimate of 200 per acre, and the 
consumption of water at 6 cubic feet per day, the volume of sewage 
would then he 1,200 cubic feet, 8 per cent, of which would be dis- 
charged per hour during the period of maximum flow, or at the rate of 
1*6 gallons per minute. 
As a 2 -inch rainfall, at the rate of fall of T £^-mch per minute, 
gives a volume of 72*60 cubic feet per minute, or forty-five times the 
volume of the sewage proper, it may safely be inferred that the 
additional cost attendant on the construction of sewers to provide 
for the discharge of rainfall would bo exorbitant and could not be 
contemplated. 
The additional cost of sewers is, however, not the only extra cost ; 
as, where the sewage has to be lifted by artificial means, the additional 
cost of machinery for pumping would be very heavy. The difficulty 
attending proper treatment of the liquid at the sewage disposal works 
would he considerably augmented, seeing that a much larger volume 
of water would require to be provided for, and the sudden influx of 
such largely increased volumes of liquid would have a tendency to 
throw* entirely out of gear all the machinery in operation for the 
treatment of the sewage. 
By the adoption of the £C separate system” of sewerage, or 
separate drains for the sewage proper and the rainfall, the drainage of 
a district can be carried out on the best and most economical basis. 
The drains for the sewage proper could be reduced to a minimum 
consistent to efficient working, provision being made for future 
increase of population. The drains being small compared with those 
