OLDEST ARRANGEMENT OF A CROWN-WHEEL ESCAPEMENT.
[Illustration: Fig. 148]
Plate 147 shows us the oldest known arrangement of a crown-wheel
escapement in a clock. _R_ is the crown wheel or balance wheel acting
upon the pallets _P_ and _P'_, which form part of the verge _V_. This
verge is suspended as lightly as possible upon a pliable cord _C_ and
carries at its upper end two arms, _B_ and _B_, called adjusters,
forming the balance. Two small weights _D D_, adapted to movement along
the rules or adjusters serve to regulate the duration of a vibration. In
Fig. 148 we have the arrangement adopted in small timepieces and
watches: _B_ represents the regulator in the form of a circular balance,
but not yet furnished with a spiral regulating spring; _c_ is the last
wheel of the train and called the _fourth wheel_, it being that number
distant from the great wheel. As will be seen, the verge provided with
its pallets is vertically placed, as in the preceding plate.
[Illustration: Fig.
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