and a most curious perpetual-motion device, the
mercury wheel, used as an escapement or regulator. The Alfonsine book on
clocks contains descriptions of five devices in all, four of them being
due to Isaac b. Sid (two sundials, an automaton water-clock and the
present mercury clock) and one to Samuel ha-Levi Adulafia (a candle
clock)--they were probably composed just before _ca._ 1276-77.

[Illustration: Figure 14.--ISLAMIC PERPETUAL MOTION WHEEL, after
manuscript cited by Schmeller (see footnote 26).]

The mercury clock of Isaac b. Sid consists of an astrolabe dial, rotated
as in the anaphoric clock, and fitted with 30 leaf-shaped gear teeth
(see fig. 13). These are driven by a pinion of 6 leaves mounted on a
horizontal axle (shown very diagrammatically in the illustration) and at
the other end of this axle is a wheel on which is mounted the special
mercury drum which is powered by a normal weight drive.

It is the mercury drum which forms the most novel feature of this
device; the fluid, constrained in 12 chambers so as to just fill 6 of
them, must slowly filter through small holes in the constraining walls.
In practice, of course, the top mercury surfaces will not be level, but
higher on the right so as to balance dynamically the moment of the
applied weight on its driven rope. This curious arrangement shows point
of resemblance to the Indian "mercury-holes," to the perpetual-motion
devices found in the medieval European tradition and also in the texts
associated with Ri[d.]w[=a]n, which we shall next examine.

[Illustration: Figure 15.--ANOTHER PERPETUAL MOTION WHEEL, after the
text cited in figure 14.]

It is of the greatest interest to our theme that the Islamic
contributions to horology and perpetual motion seem to form a closely
knit corpus. A most important series of horological texts, including
those of Ri[d.]w[=a]n and al-Jazar[=i], have been edited by Wiedemann
and Hauser.[23] Other Islamic texts give versions of the water clocks
and automata of Archimedes and of Hero and Philo of Alexandria.[24] In
at least three cases[25] these texts are found also associated with
texts describing perpetual-motion wheels and other hydraulic devices.
Three manuscripts of this type have been published in German translation
by Schmeller.[26] The devices include a many chambered wheel (see fig.
14) similar to the Alfonsine mercury "escapement," a wheel of slanting
tubes constructed like the noria (see fig. 15), wheels of