[Illustration: Figure 13.--ASTROLABE CLOCK, REGULATED BY A MERCURY DRUM, from the Alfonsine _Libros del saber_ (see footnote 22).]
In addition to this geared development of the astrolabe, the same period in Islam brought forth a new device, the equatorium, a mechanical model designed to simulate the geometrical constructions used for finding the positions of the planets in Ptolemaic astronomy. The method may have originated already in classical times, a simple device being described by Proclus Diadochus (_ca._ 450), but the first general, though crude, planetary equatorium seems to have been described by Abulcacim Abnacahm (_ca._ 1025) in Granada; it has been handed down to us in the archaic Castilian of the Alfonsine _Libros del saber_.[22] The sections of this book, dealing with the _Laminas de las VII Planetas_, describe not only this instrument but also the improved modification introduced by Azarchiel (born _ca._ 1029, died _ca._ 1087).
No Islamic examples of the equatorium have survived, but from this period onward, there appears to have been a long and active tradition of them, and ultimately they were transmitted to the West, along with the rest of the Alfonsine corpus. More important for our argument is that they were the basis for the mechanized astronomical models of Richard of Wallingford (_ca._ 1320) and probably others, and for the already mentioned great astronomical clock of de Dondi. In fact, the complicated gearwork and dials of de Dondi's clock constitute a series of equatoria, mechanized in just the same way as the calendrical device described by Biruni.
It is evident that we are coming nearer now to the beginning of the true mechanical clock, and our last step, also from the Alfonsine corpus of western Islam, provides us with an important link between the anaphoric clock, the weight drive, 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.