y been at
Glastonbury abbey.

[Illustration: FIG. 1.--Verge Escapement.]

These old clocks had what is called a verge escapement, and a balance.
The train of wheels ended with a crown wheel, that is, a wheel serrated
with teeth like those of a saw, placed parallel with its axis (fig. 1).
These teeth, D, engaged with pallets CB, CA, mounted on a verge or staff
placed parallel to the face of the crown wheel. As the crown wheel was
turned round the teeth pushed the pallets alternately until one or the
other slid past a tooth, and thus let the crown wheel rotate. When one
pallet had slipped over a tooth, the other pallet caught a corresponding
tooth on the opposite side of the wheel. The verge was terminated by a
balance rod placed at right angles to it with a ball at each end. It is
evident that when the force of any tooth on the crown wheel began to act
on a pallet, it communicated motion to the balance and thus caused it to
rotate. This motion would of course be accelerated, not uniformly, but
according to some law dependent on the shape of the teeth and pallets.
When the motion had reached its maximum, the tooth slipped past the
pallet. The other pallet now engaged another tooth on the opposite side
of the wheel. The motion of the balls, however, went on and they
continued to swing round, but this time they were opposed by the
pressure of the tooth. For a time they overcame that pressure, and drove
the tooth back, causing a recoil. As, however, every motion if subjected
to an adverse acceleration (i.e. a retardation) must come to rest, the
balls stopped, and then the tooth, which had been forced to recoil,
advanced in its turn, and the swing was repeated. The arrangement was
thus very like a huge watch balance wheel in which the driving weight
acted in a very irregular manner, not only as a driving force, but also
as a regulating spring. The going of such clocks was influenced greatly
by friction and by the oil on the parts, and never could be
satisfactory, for the time varied with every variation in the swing of
the balls, and this again with every variation of the effective driving
force.

[Illustration: FIG. 2.--Galileo's Escapement.]

The first great step in the improvement of the balance clock was a very
simple one. In the 17th century Galileo had discovered the isochronism
of the pendulum, but he made no practical use of it, except by the
invention of a little instrument for enabling doctors to count their
pa