Nevertheless this form of vent-pipe
produces a gurgling noise, and would be better for a trumpet-shaped
mouth. A special feature of the pipe in B is that unless it is placed
symmetrically about the centre of the bell its weight tends to throw the
bell out of the vertical, and it may have to be supported at its upper
part; conversely, if the pipe is arranged concentrically in the bell, it
may be employed as part of the guiding arrangement of the bell itself.
Manifestly, as the pipe must be long enough to extend through the roof of
the generator-house, its weight materially increases the weight of the
bell, and consequently the gas pressure in the service; this fact is not
objectionable provided due allowance is made for it. So tall a vent-pipe,
however, seriously raises the centre of gravity of the bell and may make
it top-heavy. To work well the centre of gravity of a holder bell should
be as low as possible, any necessary weighting being provided
symmetrically about its circumference and close to its bottom edge. The
whole length of an ascending vent-pipe need not be carried by the rising
bell, because the lower portion, which must be supported by the bell, can
be arranged to slide inside a wider length of pipe which is fixed to the
roof of the generator-house at the point where it passes into the open
air.
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