weights
swinging on arms as described by Villard of Honnecourt, and a remarkable
device which seems to be the earliest known example of a weight drive.
This latter machine is a pump, in which a chain of buckets is used to
raise water by passing over a pulley which is geared to a drum powered
by a falling weight (see fig. 16); perhaps for balance, the whole
arrangement is made in duplicate with common axles for the corresponding
parts.

[Illustration: Figure 16.--ISLAMIC PUMP POWERED BY A WEIGHT DRIVE,
after the text cited in figure 14.]

The Islamic tradition of water clocks did not involve the use of gears,
though very occasionally a pair is used to turn power through an angle
when this is dictated by the use of a water wheel in the automata. In
the main, everything is worked by floats and strings or by hydraulic or
pneumatic forces, as in Heros devices. The automata are very elaborate,
with figures of men, monkeys, peacocks, etc., symbolizing the passage of
hours.

MEDIEVAL EUROPE

Echoes from nearly all the developments already noted from other parts
of the world are found to occur in medieval Europe, often coming
through channels of communication more precisely determinable than
those hitherto mentioned. Before the influx of Islamic learning at the
time of transmission of the Toledo Tables (12th century) and the
Alfonsine Tables (which reached Paris _ca._ 1292), there are occasional
references to the most primitive mechanized "visual aids" in astronomy.

The most famous of these occurs in an historical account by Richer of
Rheims about his teacher Gerbert (born 946, later Pope Sylvester II,
990-1003). Several instruments made by Gerbert are described in detail;
he includes a fine celestial globe made of wood covered with horsehide
and having the stars and lines painted in color, and an armillary sphere
having sighting tubes similar to those always found on Chinese
instruments but never on the Ptolemaic variety. Lastly, he cites "the
construction of a sphere, most suitable for recognizing the planets,"
but unfortunately it is not clear from the description whether or not
the model planets were actually to be animated mechanically. The text
runs:[27]

Within this oblique circle (the zodiac on the ecliptic of
the globe) he hung the circles of the wandering stars (the
planets) with marvellous ingenuity, whose orbits, heights
and even the distance from each other he demonstrated to
his pupils most ef