These three methods are represented in the annexed diagram
(Fig. 1). In Al the water-levels in the tank and bell remain always at
_l_, being higher in the tank than in the bell by a distance
corresponding with the pressure produced by the bell itself. As the bell
falls a pin _X_ moves the lever attached to the cock on the water-
pipe, and starts, or shuts off, a current passing from a store-tank or
reservoir to a decomposing vessel full of carbide. It is also possible to
make _X_ work some releasing gear which permits carbide to fall into
water--details of this arrangement are given later on. In A^1 the water
in the tank serves as a holder seal only, a separate quantity being
employed for the purposes of the chemical reaction. This arrangement has
the advantage that the holder water lasts indefinitely, except for
evaporation in hot weather, and therefore it may be prevented from
freezing by dissolving in it some suitable saline body, or by mixing with
it some suitable liquid which lowers its point of solidification. It will
be observed, too, that in A^1 the pin _X_, which derives its motive
power from the surplus weight of the falling bell, has always precisely
the same amount of work to do, viz.
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