SEWERAGE OF COUNTRY TOWNS. 135 
successfully applied it in so many places. Its advantages are in- 
contestable. In the first place the quantity of sewage to be led 
is invariable. Having ascertained the water supply to the area 
to be sewered it is only necessary to calculate the size of the 
sewers from a datum which is the passage of the whole flow in 
eight hours, the pipes not to run more than half full; and upon 
that calculation it will be found that the size of the conduits, in- 
cluding the outfall sewers, is but quite small even for considerable 
towns. <A consequence is that the whole of the conduits can be 
made of glazed e. w. p., so that they are of the very best form for 
self-cleansing. Thus that condition is fulfilled which is comple- 
mentary to ventilation ; for although ventilation is always neces- 
sary it will be found incapable of keeping sewers sweet unless they 
are of such a form that their own flow keeps them free from 
deposit, and passes the sewage on to the outfall before it has time 
to begin to putrefy. Then again, if it be necessary to pump or 
raise the sewage from any part of the area, the quantity to be 
raised and the cost of raising it are kept as low as possible ; or if 
it be intended to treat the sewage at the outfall chemically and 
by filtration, its small quantity and constant composition are the 
important features of economical work of that kind ; or if it be 
designed to use the flow for cultivation (as should be designed 
wherever cultivation is practicable), its constant quantity and 
constant composition are precisely the conditions which render 
sewage farming possible on profitable terms. All these advan- 
tages turn upon the separation of the sewage from all other flows 
whatever: thence also the size of the conduits can be closely 
calculated without the slightest fear that they will ever be un- 
expectedly overtaxed ; thence also all those uncertain and trouble- 
some calculations of the margin which ought to be allowed for 
possible rainfalls are got rid of, and with them all the expense of 
the immense sewers of the partially-combined system, which are 
never required on more than a few days of the year, which on 
those few days egregiously fail to answer the purpose for which 
alone they are built, and which all the rest of the time are eating 
