If two
of the tunnels are left open to this branch, and the third one shut off
from it by closing the doors, the vitiated air will be drawn from the
two working tunnels, through the connecting branch, while fresh air will
be partly sucked down the vertical shafts through their open ends and
partly at the center tunnel, which is supplied by forcing air down the
vertical shaft in communication with it, a stop or door being placed
just outside of the bottom of the shaft so as to compel the air to flow
to the center of the tunnel. It will be observed that no trains are
running in this air tunnel so long as it is so used; there are similar
doors for the working tunnel, but they are kept open, unless either of
them is required to be made into an air tunnel, so that the passing
trains run no risk of running into the doors. By means of the doors
above mentioned, any one of the three tunnels can be used as a fresh-air
tunnel, in which the men doing the repairs to the road would be clear of
the traffic, while the other two are used for the traffic, as well as
outlets for the mixed impure gas and air.
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