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Al-Jazari borrowed from another Muslim the idea of attaching a thin rope between a large candle and a counterweight. As the candle burned away, the counterweight caused a small statue of a man to rise, causing his sword to point to the hour. Al-Jazari invented a push-and-twist “bayonet fitting“ to hold the candle.
Later Alexandria Greek designs of water clocks wasted less water, using float valves to keep the upper tank full. Greeks and Muslims knew that temperature greatly affected the speed of water clocks.
Al-Jazari built a mercury clock in which a pulley rope was wrapped around a small barrel half-filled with mercury. The barrel contained compartments connected to each other by small holes.
The ancient Hindu water clocks used a bowl with a hole design that took 4 hours to sink, causing a gong to be hit, Later Hindu designs automatically reset the bowl.
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CANDLE’S WEIGHT CLOCK
Al-Jazari (1136 – 1206) borrowed from another Muslim the idea of attaching a thin rope between a large candle and a counterweight. As the candle burned away, the counterweight caused a small statue of a man to rise, causing the sword in his hand to point to a horizontal line labeled with the hour.
Pre-Islamic candle clocks instead had markings on the candle to show how much candle had burned away. The idea of using a statue with a weapon tip pointing to the hour had been used earlier by Ctesibius in one of his water clocks.
Al-Jazari invented a push-and-twist “bayonet fitting“ to hold the candle. This fitting is used today to hold lenses in cameras and as an alternative to screw-in lightbulbs.
Al-Jazari’s book described a mercury clock in which a pulley rope was wrapped around a small barrel half-filled with mercury. The barrel contained compartments connected to each other by small holes. The rate at which the pulley rope fell was determined by how fast mercury could flow through holes between compartments.
Greeks and Muslims knew that temperature affected the speed of water clocks, but did not understand why.
The viscosity of water is much less at higher temperatures. If, in a theoretical worst case scenario, the water were passing through a long thin tube, the clock would run 2 hours a day faster if the temperature were increased by only 4 degrees centigrade (7.2 degrees Fahrenheit).
Actual water clocks have somewhat less of a viscosity problem because they do not use a long tube but instead send the water through a small hole of a sinking bowl or through a small float-controlled valve.
Attempts can be made to lessen the viscosity effect by using a larger diameter hole to increase the rate of flow, or maintaining a constant temperature. A large container of water from a constant-temperature underground spring can be used, or the clock can be housed in an insulating wooden case and placed in a basement where the temperature does not change much. Under ideal conditions a water clock might have an accuracy of plus or minus 5 minutes, but plus or minus a half hour is more likely.
HINDU WATER CLOCKS
The ancient Hindus used a bowl with a hole design that took 4 hours to sink, causing a gong to be hit, reminding someone to reset the bowl. Later Hindu designs automatically reset the bowl.
Hindus prior to al-Jazari built water clocks in many shapes, including clocks shaped like a scribe with a pointing pen sitting on an elephant.
CHINESE WATER CLOCKS
Chinese water clocks used a very different design, involving an escapement-like lever shutting off the water flow while the bucket fell, until the weight of the other end of the lever brought the empty bucket back to its starting position. This pendulum-like delay lessened the effect of viscosity on the speed of the water clock.
MUSIC TEMPO DEVICE
Ibn-Firnas (810 – 887) invented a music tempo device that only “kept the beat” but was unable to display elapsed time or time of day..
Ibn Yunus could not have built a pendulum clock because pendulum clocks require the Verge escapement mechanism (derived from a European Christian invention for ringing a bell).
The source of this false claim about ibn Yunus was a translation error made in 1684 by the English scholar Edward Bernard.
MOON PHASE DISPLAY
Abū Ishāq Ibrāhīm al-Zarqālī (Arzachel) (1029-1087) in Muslim Spain built a device that “by magic” displays the correct phase of the moon.
The height of the water in the device controlled the movement of the moon plase display.
I assume that (in another room) someone sets the height of a water spillway, which (by means of a hidden siphon) controls the water level in the moon phase display device.
GEBER’S BOOK ON CLOCKS
The alchemist Geber wrote a 4 volume work on building water clocks, probably based on him having read about the water clocks of Greeks such as Pseudo-Archimedes, Hero of Alexandria and Ctesibius.
CLOCK IN AN OBSERVATORY
Taqi al-Din completed in 1577 an observatory near Istanbul that contained a “mechanical clock”. I assume it was a Pseudo-Archimedes water clock for measuring the time interval between the sightings of two stars. If the sightings were less than 8 minutes apart from each other, and assuming the water clock had an accuracy of plus or minus one percent, the water clock would have had an error of only plus or minus a twelfth of a minute in measuring the time interval.
Image by al-Jazari, via Wikimedia Commons.
image credit https://en.wikipedia.org/wiki/File:Al-Jazari_-_A_Candle_Clock.jpg