which we shall
consider later.

During the century following this horological exuberance in Damascus,
the center of gravity of Islamic astronomy shifted from the East to the
Hispano-Moorish West. At the same time there comes more evidence that
the line of mathematical protoclocks had not been left unattended. This
is suggested by a description given by Trithemius of another royal gift
from East to West which seems to have been different from the automata
and hydraulic devices of the tradition from Procopius to
Ri[d.]w[=a]n:[19]

In the same year [1232] the Saladin of Egypt sent by his
ambassadors as a gift to the emperor Frederic a valuable
machine of wonderful construction worth more than five
thousand ducats. For it appeared to resemble internally a
celestial globe in which figures of the sun, moon, and
other planets formed with the greatest skill moved, being
impelled by weights and wheels, so that performing their
course in certain and fixed intervals they pointed out the
hour night and day with infallible certainty; also the
twelve signs of the zodiac with certain appropriate
characters, moved with the firmament, contained within
themselves the course of the planets.

[Illustration: Figure 10.--CALENDRICAL GEARING DESIGNED BY AL-BIRUNI,
_ca._ A.D. 1000. The gear train count is 40-10+7-59+19-59+24-48. The
gear of 48 therefore makes 19 (annual) rotations while that of 19-59
shows 118 double lunations of 29+30=59 days. The gear of 40 shows a
(lunar) rotation in exactly 28 days, and the center pinions 7+10 rotate
in exactly one week. After Wiedemann (see footnote 20).]

The phrase "resembled internally" is of especial interest in this
passage; it may perhaps arise as a mistranslation of the technical term
for stereographic projection of the sphere, and if so the device might
have been an anaphoric clock or some other astrolabic device.

This is made more probable by the existence of a specifically Islamic
concentration on the astrolabe, and on its planetary companion
instrument, the equatorium, as devices for mechanizing computation by
use of geometrical analogues. The ordinary planispheric astrolabe, of
course, was known in Islam from its first days until almost the present
time. From the time of al-Biruni (_ca._ 1000)--significantly, perhaps,
he is well known for his travel account of India--there is remarkable
innovation.

Most cogent to our purpose is a text, described for the fir