If now the cage is lifted and the weight thus removed from the jaw _a_,
the spring _g_ pushes the plunger _b_ outwards and allows the oil to
run from _e_ back into _c_.
=27. Pneumatic Fans.=--A pneumatic fan, shown in section in Fig. 26,
is one in which the shock of the landing is partially relieved by a
cushion of compressed air. The fan is keyed at the bottom to the
shaft _a_ that rotates it, as in Fig. 24. The cylinder _b_ contains
the plunger _c_, which is kept at the top limit of its motion by the
spring _d_. When the cage lands in the jaw _e_, the plunger descends,
compressing the air in the cylinder _b_. The air escapes slowly through
the ¹/₁₆-inch hole _f_, thus allowing the cage to settle into place
with very little shock. These fans should be made of wrought-iron or
cast-steel so as not to be easily broken.
[Illustration: FIG. 26]
=28. Cage Chairs.=--In the case of a cage required to stop at a large
number of levels, it is expensive to provide fans at each level, and to
obviate this a strong steel bar or dog may be used under each corner
of the cage, all four bars being connected to a lever on the cage, by
means of which they can be thrown out at will so as to rest on supports
provided at each level. Fig. 27 shows Gray’s patent cage chair, which
operates on this principle. The sliding bars _a_ are connected by the
cross-bars _b_, which are pivoted at the center and operated by the bar
_c_ through the links _d_. By moving the lever _e_ into the position
shown, the bars _a_ are thrown out so as to rest in notches or on wall
plates in the shaft. The springs _f_, through the cross-bars _b_, force
the sliding bars _a_ back under the cage when the lever _e_ is released.