Today, Su-Sung's wonderful clock. The University of
Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
created them.
When sixteenth-century Jesuit
missionaries went to China, they found timekeeping
in a deplorable state. Not even sundials were
reliable! And the clocks they brought as gifts were
seen only as playthings. Timekeeping was hardly on
China's radar screen.
Of course, the purpose of all ancient clocks was
not so much the simple telling of time as it was
display. Old clocks typically had bells and dials,
and they displayed planetary motions.
In the West, water clocks had evolved from remote
antiquity until mechanical clocks finally replaced
them seven hundred years ago. The Greek name for a
water clock was clepsydra. That means "a
stealer of water" because all water clocks depended
on a steady flow of water to meter time.
Greco-Egyptian engineers of the second century BC
had added feedback control to regulate the water
flow. That idea was carried forward by Arab
artisans until the Moors of medieval Spain were
building the finest clocks in the West.
The Chinese had also built water clocks for
millennia, but without feedback control. In Western
water clocks, a float on the surface of a steadily
draining tank drove the displays. But float
indicators exerted scant force for driving extra
machinery. The Chinese, on the other hand, created
a new kind of water-wheel-driven clock during the
eighth to eleventh centuries. A steady inflow
filled buckets around the rim, one at a time. As
each bucket became heavy enough to trip a
mechanism, it fell forward carrying the bucket
behind into place under the water spout. That water
wheel provided power to drive displays of lunar
cycles, the movements of the heavens, and time as
well.
Those clocks reached their apogee when the emperor
of the Sung dynasty charged an official, Su-Sung,
with creating the grandest clock that'd ever been
built. Su-Sung assembled a team and finished the
clock by 1092. It was
huge -- forty feet high.
The tick-tock motion of the falling buckets has
caused some historians to call it a mechanical
clock. But it had nothing resembling the
inertial escapement that began turning European
clocks into precision instruments by 1300. Neither
did it have the feedback control of Arab water
clocks.
Invading Tatars stole the clock when they ended the
Sung dynasty in 1126. They couldn't get it running
again, and the high art of Chinese clockmaking
disappeared. Even before the Tatar invasion,
Taoistic reformers had come into power and let the
great clock fall into disrepair. When Jesuits
eventually brought Su-Sung's book on clockmaking
back to Europe, it astonished the West -- even
though the escapement clock was then light-years
beyond it.
Su-Sung's clock seems to've been pretty accurate.
Whether it reached the fifteen-minute-a-day
accuracy of the best Western water clocks, we don't
know. But, for a time, the Chinese were ahead of
the West once again, with the grandest clock in the
world.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
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