All of these efforts were preparatory for the greatest of all Chinese
medieval clocks, the 'Cosmic Engine' of Su Sung, built in the year 1092.
Su Sung's clock is known in considerable detail due to the miraculous
preservation, over nine centuries, of his book Nezv Design for a Mechanized
Armillary Sphere and Celestial Globe. This work describes the design and
construction of the great clock in full detail. Some drawings in the book
have recently been discovered to be those of Chang Ssu-Hsun in 976, to whose
earlier clock Su Sung's must have borne a closer resemblance than had been
thought until now. Modern working models of the clock have been constructed,
and in Plate 80 may be seen one at the Science Museum in London. These models
are, of course, based on the full descriptions and drawings in Su Sung's
Su Sung's clock was actually an astronomical clock tower more than
thirty feet high, like the previous one of Chang. But on top of Su Sung's
tower was additionally a huge bronze power-driven astronomical instrument
called an armillary sphere (see page 36), with which one could observe the
positions of the stars. A celestial globe inside the tower turned in
synchronization with this sphere above, so that the two could constantly
be compared. We are told that the observations made on the demonstrational
globe inside and by the observa- tional sphere above 'agreed like the two
halves of a tally'.
On the front of the tower was a pagoda structure of five storeys,
each having a door through which mannikins and jacks appeared ringing bells
and gongs and holding tablets to indicate the hours and other special times
of the day and night. All of these time-indicators were operated by the same
giant clock machinery which simultaneously turned the sphere and the globe.
This machinery consisted, as usual, of a huge vertical water-wheel
with scoops at the end of each blade, into which water dripped from a water
clock. Every time the wheel turned one notch upon the filling of a scoop,
there was a ratchet-pin which came down to prevent the wheel recoiling backwards.
As for the forward motion of the wheel, it went forward one scoop every quarter
of an hour. Needham describes the machine as follows:
The wheel was checked by an escapement consisting of a sort of weigh-bridge
which prevented the fall of a scoop until full, and a trip-lever and parallel
linkage system which arrested the forward motion of the wheel at a further
point and allowed it to settle back and bring the next scoop into position
on the weigh-bridge. One must imagine this giant structure going off at full-
every quarter of an hour with a great sound of creaking and splashing, clanging
and ringing; it must have been very impressive, and we know that it was actually
built and made to work for many years before being carried away into exile.