just about to impinge upon the advancing pallet _P'_. This it does, and
will at first, through the impulse received from the tooth _d_ be forced
back by the momentum of the pallet, that is, suffer a recoil; but on the
return journey of the pallet _P'_, the tooth _e_ will then add its
impulse to the receding pallet. The tooth _e_ having thus accomplished
its mission, will now slip by and the tooth _c_ will come in lock with
the pallet _P_ and, after the manner just described for _e_, continue
the escapement. Usually these escape wheels are provided with teeth to
the number of 11, 13 or 15, and always uneven. A great advantage
possessed by this form of escapement is that it does not require any
oil, and it may be made to work even under very inferior construction.

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. 149]

Here it will quickly be seen that regarded from the standpoint of
regularity of motion, this arrangement can be productive of but meager
results. Subjected as it is to the influence of the slightest variation
in the motive power and of the least jar or shaking, a balance wheel
escapement improvided with a regulator containing within itself a
regulating force, could not possibly give forth anything else than an
unsteady movement. However, mechanical clocks fitted with this
escapement offer indisputable advantages over the ancient clepsydra; in
spite of their imperfections they rendered important services,
especially after the striking movement had been added. For more than