SEWERAGE OF A SEASIDE TOWN. 771 



and length of the outfjill sewer, for the same eonsiderations 

 often rendei- it desira])le to lay large colleetino- sewers at a 

 o-reat de])th — through the ridges, for instance, that divide 

 different drainage hasins — so as to bring' all the sewage to 

 one place to he ])inn])ed. The effect of them upon the 

 hygienic aspect of the system will be shown by the fact that 

 large sewers mean lai'gely increased capability and prqba- 

 bility of producing sewer gas, and largely increased capacity 

 for holding it, at the same time as lai-gely increased diffi- 

 culty of effecting any remedial ventilation or any remedial 

 cleansing by ffushing. The capacity of a sewer has to be 

 calculated upon the maximum cpiantity of sewage it. will 

 reasonably have to provide for. The usual practice of 

 engineers is to limit this pi'ovision to four times the maximum 

 quantity of house sewage properly so called, and thus alloM 

 for the rainfall ujion houses and yards that cannot be 

 conveniently and economically separated from the house 

 slops. Thus, in London, Sir J. Bazalgette provided for 

 106 million gallons of sewage and 324 million gallons of 

 rain-water a day ; and at Melbourne, in his report to the 

 Royal Commission, Mr. Thwaites jiroposed to provide, in 

 his sewerage scheme for that city, for 42 million gallons 

 of sewage and 120 million gallons of rain-water. The 

 consequence of making this j^rovision for rain-water, and it 

 should be made, is this : — The minimum quantity of sewage 

 passes through the sewers in hot, dry weather, that is, when 

 there is no rain ; and consequently at that time, even at the 

 hour of the day when most sewage jiasses, the sewei's ai-e 

 only quarter full, all the rest of the space in them being 

 occupied Avith sewage-tainted air, all the more tainted as the 

 slowly-flowing sewage is the longer subject to the heated 

 atmosphere. For this hot, dry weather is just the time 

 when the sewage naturally gives off" most of this tainted air, 

 so that the larger the sewer the larger the volume of sewer 

 gas, and the longer the time it has in which to develop 

 itself ; and furthermore, as the larger a sewer is the greater 

 is the difficulty of ventilating it, this larger volume of sewer 

 gas is all the more ditHcult to dilute, and therefore when it 

 is forced into houses, or into the open air of streets and 

 courts, it is all the more poisonous. 



To illustrate what I mean, let us take a 10-feet barrel 

 sewer laid with a fall of 1 in 3000, as being necessary to 

 carry the sewage of a town to a sewage farm — (one of' the 

 proposed outfall sewers at Melbourne is 11 feet 3 inches in 



