37).]

Having discussed the Villard drawings which are already cited in
horological literature, we must draw attention to the fact that this
medieval architect also gives an illustration of a perpetual motion
wheel. In this case (fig. 21) it is of the type having weights at the
end of swinging arms, a type that occurs very frequently at later dates
in Europe and is also given in the Islamic texts. We cannot, in this
case, suggest that drawings of clocks and of perpetual motion devices
occur together by more than a coincidence, for Villard seems to have
been interested in most sorts of mechanical device. But even this type
of coincidence becomes somewhat striking when repeated often enough. It
seems that each early mention of "self-moving wheels" occurs in
connection with some sort of clock or mechanized astronomical device.

Having now completed a survey of the traditions of astronomical models,
we have seen that many types of device embodying features later found in
mechanical clocks evolved through various cultures and flowed into
Europe, coming together in a burst of multifarious activity during the
second half of the 13th century, notably in the region of France. We
must now attempt to fill the residual gap, and in so doing examine the
importance of perpetual motion devices, mechanical and magnetic, in the
crucial transition from protoclock to mechanical-escapement clock.

Perpetual Motion and the Clock before de Dondi

We have already noted, more or less briefly, several instances of the
use of wheels "moving by themselves" or the use of a fluid for purposes
other than as a motive power. Chronologically arranged, these are the
Indian devices of _ca._ 1150 or a little earlier, as those of Ri[d.]w[=a]n
_ca._ 1200, that of the Alfonsine mercury clock, _ca._ 1272, and the
French Bible illumination of _ca._ 1285. This strongly suggests a steady
transmission from East to West, and on the basis of it, we now
tentatively propose an additional step, a transmission from China to
India and perhaps further West, _ca._ 1100, and possibly reinforced by
further transmissions at later dates.

One need only assume the existence of vague traveler's tales about the
existence of the 11th-century Chinese clocks with their astronomical
models and jackwork and with their great wheel, apparently moving by
itself but using water having no external inlet or outlet. Such a
stimulus, acting as it did on a later occasion when Galileo