(68)
The number of openings
of air would not extend right through the system of sewers. you have in the streets must be arranged with due regard to level of such streets.
789.-You would have some surface outlets?
A. Yes, but their action depends on the pressure of atmosphere and also on the amount of heated liquid being turned into the sewers in the lower parts of the city. In some recent experiments at Hampstead I found that with sections of three feet by two, and gradients of about one in forty or fifty, there was a strong current of air down the sewer. Air currents in sewers are very irregular.
790.—A small current would be sufficient to carry the air down?
A.-Well, it depends on circumstances, but every current of water carries air
with it.
791.-Do you mean to say the pressure in a sewer is very much greater than the air outside?
A.-No; it never can exceed the seal of the trap, which is usually three inches that is, 146th part of an atmosphere or about a tenth of a pound to the inch. A great many traps are considerably less than that, and as soon as the pressure rises the air
escapes.
792. Do you think any harm would arise from openings it the street?
A-I think in the present condition of house drains very serious harm would arise from openings in the street. The house drains are in an extremely bad condition. Matter in a state of decomposition entering into the main sewers immediately causes very noxious gases to arise out of the ventilators into the street, and any one in the neighbourhood would certainly smell a very strong smell if nothing worse. I think it would be a very bad plan to open ventilators in the streets whilst the present condition of the house drains exists.
793.-There are some already?
A.-There are some, but I would not increase the number.
794.-Then would confine the sewer gas?
you
A.-No; if you keep the house drains in bad condition they would have it in the houses. If the house drains are in bad condition you cannot take the gas into the streets and pour it out there. It is bound to find its way into the houses.
The warm air in the houses always tends to draw the gas in.
795.-But if we could get the ventilationi mproved?
A. You could not prevent the passage of sewer gas into the houses through the defective house drains.
796.-You don't think you could draw the gas into the main sewers?
A.-No.
797.-You are content with the present system of drainage in Hongkong then?
A. In the present state of the house drains, I don't think we can do much more.
798. So we cannot look for any improvement until the house drains are improved?