pecial
purposes, such as timing sermons and boiling eggs, they have not been of
any practical value.

The clepsydra naturally suggested to the mechanical mind the idea of
driving a mechanism for registering time by the force of gravity acting
on some body other than water. The invention of the _weight-driven
clock_ is attributed, like a good many other things, to Archimedes, the
famous Sicilian mathematician of the third century B.C.; but no record
exists of any actual clock composed of wheels operated by a weight prior
to 1120 A.D. So we may take that year as opening the era of the clock as
we know it.

About 1500 Peter Hele of Nuremberg invented the _mainspring_ as a
substitute for the weight, and the _watch_ appeared soon afterwards
(1525 A.D.). The pendulum was first adopted for controlling the motion
of the wheels by Christian Huygens, a distinguished Dutch mechanician,
in 1659.

To Thomas Tompion, "the father of English watchmaking," is ascribed the
honour of first fitting a _hairspring_ to the escapement of a watch, in
or about the year 1660. He also introduced the _cylinder escapement_ now
so commonly used in cheap watches. Though many improvements have been
made since his time, Tompion manufactured clocks and watches which were
excellent timekeepers, and as a reward for the benefits conferred on his
fellows during his lifetime, he was, after death, granted the
exceptional honour of a resting-place in Westminster Abbey.

THE CONSTRUCTION OF TIMEPIECES.

A clock or watch contains three main elements:--(1) The source of power,
which may be a weight or a spring; (2) the train of wheels operated by
the driving force; (3) the agent for controlling the movements of the
train--this in large clocks is usually a pendulum, in small clocks and
watches a hairspring balance. To these may be added, in the case of
clocks, the apparatus for striking the hour.

THE DRIVING POWER.

_Weights_ are used only in large clocks, such as one finds in halls,
towers, and observatories. The great advantage of employing weights is
that a constant driving power is exerted. _Springs_ occupy much less
room than weights, and are indispensable for portable timepieces. The
employment of them caused trouble to early experimenters on account of
the decrease in power which necessarily accompanies the uncoiling of a
wound-up spring. Jacob Zech of Prague overcame the difficulty in 1525 by
the invention of the _fusee_, a kind of conical pul