g gears or, more reasonably, a single large gear turned by a
trip lever. It did not matter much that the time-keeping properties were
poor in the long run; the model moved "by itself" and the great wonder
was that it agreed with the observed heavens "like the two halves of a
tally."

In the next, and essential, stage the turning of the water wheel was
regulated by an "escapement" mechanism consisting of a weighbridge and
trip levers so arranged that the wheel was held in check, scoop by
scoop, while each scoop was filled by the dripping water, then released
by the weighbridge and allowed to rotate until checked again by the
trip-lever arrangement. Its action was similar to that of the anchor
escapement, though its period of repose was much longer than its period
of motion and, of course, its time-keeping properties were controlled not
only by the mechanics of the device but also by the rate of flow of the
dripping water.

The Chinese escapement may justifiably be regarded as a missing link,
just halfway between the elementary clepsydra with its steady flow of
water and the mechanical escapement in which time is counted by chopping
its flow into cycles of action, repeated indefinitely and counted by a
cumulating device. With its characteristic of saving up energy for a
considerable period (about 15 minutes) before letting it go in one
powerful action, the Chinese escapement was particularly suited to the
driving of jackwork and other demonstration devices requiring much
energy but only intermittent activity.

In its final form, as built by Su Sung after many trials and
improvements, the Chinese "astronomical clock-tower" must have been a
most impressive object. It had the form of a tower about 30 feet high,
surmounted by an observation platform covered with a light roof (see
fig. 4). On the platform was an armillary sphere designed for observing
the heavens. It was turned by the clockwork so as to follow the diurnal
rotation and thus avoid the distressing computations caused by the
change of coordinates necessary when fixed alt-azimuth instruments were
used. Below the platform was an enclosed chamber containing the
automatically rotated celestial globe which so wonderfully agreed with
the heavens. Below this, on the front of the tower was a miniature
pagoda with five tiers; on each tier was a doorway through which, at due
moment, appeared jacks who rang bells, clanged gongs, beat drums, and
held tablets to announce th