"Guy King" wrote in message
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I remember seeing the sort of thing Dave is describing as a child
living in London (Fulham - not a million miles from Dave). Definately
purpose designed rather than sawn off soil pipes. Look a little like
periscopes with a grill on the front.

I also assummed they were some sort of drain vent, but i've no idea how
they work/ed

I always thought they were cellar ventilation.

Drain vents for interceptor traps. Manhole near road has what is effectively
a U bend in it to isolate air movement to and from main sewer. Rodding eye
(plugged) above u bend usually. Vent allows free passage of waste without
air movement problems.

I have such a manhole and the vent was there but has been removed, requiring
a vent pipe further back.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)