Since, therefore,
acetylene is in practice always forced through mains and service-pipes in
virtue of the pressure imparted to it by the gasholder and since, for
reasons already given, only the rising-bell type of gasholder can be
regarded as satisfactory, it becomes important to know the relations
which subsist between the dimensions and weight of a gasholder bell and
the pressure which it "throws" or imparts to the contained gas.
The bell must obviously be a vessel of considerable weight if it is to
withstand reasonable wear and tear, and this weight will give a certain
hydrostatic pressure to the contained gas. If the weight of the bell is
known, the pressure which it will give can be calculated according to the
general law of hydrostatics, that the weight of the water displaced must
be equal to the weight of the floating body. Supposing for the moment
that there are no other elements which will have to enter into the
calculation, then if _d_ is the diameter in inches of the
(cylindrical) bell, the surface of the water displaced will have an area
of _d^2_ x 0.7854. If the level of the water is depressed _p_
inches, then the water displaced amounts to _p_(_d^2_ x 0.
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