president's address — 8ECTI0N H. 157 



" 2. It must Iiave a continuous fall from the head to the 

 outlet in order that its contents may he kept 

 moving, so that there shall be no halting- lo 

 putrefy on the way, and no depositing of silt 

 that would endanger the channel. 

 " 3. It must be perfectly ventilated, so that the poisonous 

 gases that necessarily arise — even when decom- 

 posing matters are being carried along in water — 

 shall be diluted with fresh air, and shall have 

 such means of escaj^e as will prevent tliem from 

 forcing their way into houses through the traps in 

 the house drains. 

 "4. It must be provided with means for inspection and 



flushing. 



"5. The branches by which it receives its supply should 



be so I'ejiulated as to admit nothing; that will be 



liable to choke up the channel. 



" G. There must be some device to prevent the gases of 



the sewer from rising through the house drains 



or through the street basins. 



" 7. Its size should be so adjusted to its work that it will 



remove all water that ordinary rains bring to it, 



and so that the usual dry weather flow shall 



keep it free from silt and organic deposit." 



Mr. Waring considers that any sewer deficient in any one 



of the first six requirements is a fair subject for indictment 



as a dangerous nuisance. In designing a system of sewerage 



for a g;iven town, one of the first thinos that has to be con- 



sidered is the greatest quantity of fluid which the sewers 



will be required to discharge. Now, it is clear that the 



sewers must be capable of discharging all the liquid wastes 



which will be delivered to them, which is usually estimated 



by assuming that they are equal to the water supply which 



is most conveniently estimated in terms of the population 



using the sewers. The volume of sewage to be jirovided 



for depends upon the water supply, and allowance must also 



be nuide for half the daily quantity flowing oft' in six 



hours. 



Now, there are various systems which have been proposed 

 for the sewerage of towns, which may be described as 

 follows : — 



a. The combined system, in which both surface and sub- 

 soil waters are admitted to the sewers in addition 

 to the sewage. 



