In A^3 the interior of the bell is fitted with a circular
plate which cuts off its upper corners and leaves a circumferential space
_S_ triangular in vertical section. This space is always full of
air, or air and water, and has to be deducted from the available storage
capacity of the bell. Supposing the bell transparent, and viewing it from
above, its effective clear or internal diameter will be observed to be
smaller towards the top than near the bottom; or since the space _S_
is closed both against the water and against the gas, the walls of the
bell may be said to be thicker near its top. Thus it happens that as the
bell descends into the water past the lower angle of _S_, it begins
to require more space for itself in the tank, and so it displaces the
water until the levels rise. When high, as shown in the sketch marked
A^3(a), the water-level is at _l_, below the mouth of a pipe
_P_; but when low, as in A^3(b), the water is raised to the point
_l'_, which is above _P_. Water therefore flows into _P_,
whence it reaches the carbide in an attached decomposing chamber. Here
also the water in the tank is used for decomposition as well as for
sealing purposes, and its normal level must be maintained exactly at
_l_, lest the mouth of _P_ should not be covered whenever the
bell falls.
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