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This castle clock in al-Jazari’s 1206 book was invented by a Greek who wrote under the pseudonym of Archimedes. One existed in the Greek Christian city of Gaza prior to the year 529. Many centuries later, al-Jazari made a small improvement in the mechanism for adjusting the speed to make the days longer in the summer.

A float valve kept Tank#2 exactly full from water in tank #1. Tank#2 delivered water at a constant rate to tank#3. Once an hour, when tank#3 tipped over, one or 2 balls at the top of the clock would be released to power opening of display doors, and the released water struck musical instruments and blew a flute or horn.

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This castle clock in al-Jazari’s 1206 book was a traditional Pseudo-Archimedes water clock invented by a Greek who wrote under the pseudonym of Archimedes.

All the water clocks of the Islamic Golden Age were nearly identical to this clock except for notes a,b,c, and d below:
a) artistic differences in the statues
b) The seasonal time-of-sunrise calibration lines were spaced at incorrect intervals in the early pre-Islamic Pseudo-Archimedes clocks. Al-Jazari used trial and error to determine the proper spacing, which was also affected by seasonal differences in thermal expansion and in viscosity of the water.
c) Al-Jazari’s elephant clock was shaped like a rotating rider on an elephant, a clock-shape used in the water clocks of ancient India. It and some other of his clocks used a sinking bowl attached to the wall of the tank on a hinge.
d) Al-Muradi’s clocks in Muslim Spain used a less accurate and less convenient method of making seasonal adjustments.

The earliest clock of this “Pseudo-Archimedes” design to be described, using ropes and pulleys to move figures once every daylight hour, was a Greek public clock in the (Byzantine Christian) city of Gaza in the year 529 (before the birth of the prophet Mohammed).
It had an upper row of 12 doors and a lower row of 12 doors, and looked and behaved like the later Islamic Golden Age water clocks, and had the identical internal mechanism.

Like all similar water clocks of the Islamic Golden Age which would follow, the clock in Gaza displayed temporal hours with the day beginning at sunrise. This required that one June daytime hour be longer than one June nighttime hour.

“The Christian rhetorician Procopius described in detail prior to 529 a complex public striking clock in his home town Gaza which featured an hourly gong and figures moving mechanically day and night.”

The Gaza clock “struck the number of the hours, counting two sequences from one to six; the striking was widely audible. The clock, erected in a large square, set in motion an elaborate mechanism of mechanical figures. The eyes in the head of the Medusa on the tympanum of the facade moved every hour. Below the Medusa head was a series of twelve doors, in front of which the figure of the sun god was moving about. Every hour a door opened, and out came Hercules with the attributes of one of his twelve mythical labors. Among the musical and figural mechanisms, Procopius singles out an eagle with beating wings that wreathed the clock. At night a light moved behind the thresholds of the doors.”
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The 2nd oldest Pseudo-Archimedes water clock to be described was the one given to Charlemagne by Sultan Harun-al-Rashid in 807. It was very similar to the Gaza clock, but with the head of Medusa removed and Hercules replaced with a horseman.  

The Pseudo-Archimedes clock used a very complex mechanism to calculate temporal hours. Tank#1 (the top tank) was the reservoir tank. Each morning someone turned on the faucet to add enough water to last the daylight hours and then turned off the faucet. [At sunset enough water would be added to last the nighttime hours.] Tank#2 was a small tank designed to stay full and automatically replenish itself from tank#1 as water drained from tank#2 into tank#3 at a steady rate. A cone-shaped plug, at the top of tank#2, floated to block off water input from tank#1 when tank#2 was full, thus maintaining a nearly constant pressure at the bottom of tank#2. A device which raised or lowered the exit point from tank#2 to tank#3 compensated for 80% to 90% of the seasonal variation in the time of sunrise and sunset. It had to be manually adjusted at sunrise and again at sunset.

A pulley rope attached to the falling float in tank#1 moved a sun god which opened one pair of swinging doors each daylight hour. (In the Muslim modification, the sun god was replaced with a crescent moon).

At sunrise, a man would need to load 12 balls into the top of the clock. At sunset, a man needed to light the torch.

Tank#4 contained a waterwheel that had cogs for striking musical instruments. Flow of water into tank#5 (an air tank) blew an air horn or flute.

A pulley rope connected to a float and wrapped around the ball-release mechanism rotated it. Once every daylight hour a ball would pass over a hole and escaped.

The ball would roll down a ramp, activating levers that would cause statues to move and make noises once an hour. Some noises would be made less often.

At night, a lamp attached to the ball-holder moved to indicate the hour, and holes would be uncovered to aid in counting the hour.

The Muslim Golden Age clockmaker craftsmen made minor variations in the design of the ball-release mechanism, such as whether it was mounted vertically or horizontally, and whether it contained 12 balls, 12 pairs of balls, or 13 balls.

The sign of the zodiac display and the phase of the moon display were adjusted manually.

Even as early as the 1st century in Greek-ruled Alexandria, Hero of Alexandria explained how to use ropes, pulleys, camshafts and segmental gears to move automata in complicated paths, make them bend over or move their arms, and play musical instruments.  For more examples of the automata, search YouTube for “Hero of Alexandria” or “Heron of Alexandria”.  

The city of Damascus had at the Jayrun gate a Pseudo-Archimedes clock as early as the 900’s that was rebuilt in the late 1100’s and described by Al-Jazari in 1206.
The top doors revealed statues. The bottom doors revealed colors.
[ and Al-Jazari’s “The Book of Knowledge of Ingenious Mechanical Devices”, translated with commentary into English by Donald Hill. ]

In Cordoba in 11th century Muslim Spain, Ibn Khalaf al-Muradi built Pseudo-Archimedes clocks that “were neither as accurate nor as convenient as using a float-chamber with a properly calibrated flow regulator [A History of Engineering in Classical and Medieval Times, Donald Hill, p. 233].
Some used an epicyclic gear that was an invention of 2nd century Alexandria.

The castle clock in al-Jazari’s 1206 book was a traditional Pseudo-Archimedes clock.  Al-Jazari made critical valves of agate. He experimented with different ways to release water from tank#3 to tank#4, such as siphons or using the falling ball to tip the tank over.

Image by al-Jazari, via Wikimedia Commons.

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