he mechanical escapement, has presented
one of the most tantalizing of problems. Without doubt, the crown and
foliot type of escapement appears to be the first complicated mechanical
invention known to the European Middle Ages; it heralds our whole age of
machine-making. Yet no trace has been found either of a steady evolution
of such escapements or of their invention in Europe, though the
astronomical clock powered by a water wheel and governed by an
escapement-like device had been elaborated in China for several
centuries before the first appearance of our clocks. We must now
rehearse a revised story of the origin of the clock as it has been
suggested by recent researches on the history of gearing and on Chinese
and other astronomical machines. After this we shall for the first time
present evidence to show that this story is curiously related to that of
the _Perpetuum Mobile_, one of the great chimeras of science, that came
from its medieval origin to play an important part in more recent
developments of energetics and the foundations of thermodynamics.[2] It
is a curious mixture, all the more so because, tangled inextricably in
it, we shall find the most important and earliest references to the use
of the magnetic compass in the West. It seems that in revising the
histories of clockwork and the magnetic compass, these considerations
of perpetual motion devices may provide some much needed evidence.

[Illustration: Figure 1.--FRAMEWORK STRUCTURE OF THE ASTRONOMICAL CLOCK
of Giovanni de Dondi of Padua, A.D. 1364.]

Power and Motion Gearing

It may be readily accepted that the use of toothed wheels to transmit
power or turn it through an angle was widespread in all cultures several
centuries before the beginning of our era. Certainly, in classical times
they were already familiar to Archimedes (born 287 B.C.),[3] and in
China actual examples of wheels and moulds for wheels dating from the
4th century B.C. have been preserved.[4] It might be remarked that
these "machine" gear wheels are characterized by having a "round number"
of teeth (examples with 16, 24 and 40 teeth are known) and a shank with
a square hole which fits without turning on a squared shaft. Another
remarkable feature in these early gears is the use of ratchet-shaped
teeth, sometimes even twisted helically so that the gears resemble worms
intermeshing on parallel axles.[5] The existence of windmills and
watermills testifies to the general familia