We have seen that a political climate favorable to intellectual activity evolved under the Abbassides — and this climate coincided with important needs for hydraulic development. Scientists and often engineers made use of all the fruits of Greek and Hellenistic science in parallel with major construction projects. They produce precise mechanisms, water clocks (clepsydres) and other marvelous machines following the tradition of the ancient scholars of Alexandria, Philon, Ctesibios, and Heron.
Three brothers — Muhammad, Ahmad, and al-Hasan Banu Musa, known to the caliph al-Mamun around 820, wrote numerous treatises on these mechanisms. Of particular note is the Kital al-Hiyal, Book of Ingenious Mechanisms, written in Baghdad about 850. This book contains descriptions of many devices that reveal a perfect mastery of hydrostatics.
The fact is that Archimedes, as well as other authors such as Aristotle, Euclid, and Heron, had been translated into Arabic by that time. Al-Khazini, a scientist of the 12th century, put together a synthesis of the hydrostatics of Archimedes and the premises of dynamics of Aristotle and his commentators. In his book, called Kitab mizan al-hikma (book of the balance of wisdom), he differentiates between the two types of action that can be exerted on mobile bodies in water — hydrostatic forces, and hydrodynamic forces arising from the movement of the body. These latter forces “differ because the shapes of the bodies differ”. He also extends the theories of Archimedes to hollow bodies, as well as to hollow bodies carrying a load. Here the theory of ship buoyancy makes its appearance.
A characteristically Aristotlean trait appears in the vision of groundwater developed in the 11th century by the mathematician al-Karagi, a vision that we cited earlier in the context of his treatise on qanats:
“God — may he be blessed and exalted — created a compact universe without empty space, and attributed to each element — the celestial sphere, stars, fire, air, water, and earth — its own place, a place to which it tries to return if separated from it. Dense bodies like earth and water (…) seek the center of the universe, and the most dense arrive there first; from which it follows that the earth is at the center of the water that surrounds it.”[358]
And from this comes the explanation of groundwater movement:
“It is in the nature of water to seek, by its movement, the center of the earth, and not to rise.”
Springs are then explained by the slope of the impermeable layers over which groundwater flows:
“When the groundwater has its bed upon a hard surface, and this hardness, lying next to a fissure where the water flows, extends to the summit of a mountain, the water emerges and can be tapped at this summit, if it is nearer the center of the earth than the place from which the fissure is supplied.”[359]
Other authors also reveal a great deal of refinement in Arab research into these mechanisms. One is al-Muradi, who in the 11th century describes automatic controls that are powered by waterwheels. Another is al-Jazari who, at the very beginning of the 13th century, brought the art of the clepsydre (water clock) to its pinnacle.
We have been emphasizing the influence of the Greek and Hellenistic scientists on Arab thought. But we must not ignore the Chinese influence insofar as technology is concerned. After the battle of Talas, Chinese prisoners introduced the Chinese technology of the hydraulic pestle to the nascent paper industry at Samarcand; we will come back to this in Chapter 8.[360] This technology subsequently spreads to all the Arab world, then into the Occident. A century later, in 850, the historian al-Jahiz assembles an inventory of “products” imported from China to Iraq. In this inventory he naturally mentions silk, but also includes a curious list that contains, in order, “female slaves” and “hydraulic engineers.”[361]
