Al-Jazari’s Castle Water Clock: Analysis of its Components and Functioning

The first machine described by al-Jazari in his famous treatise of mechanics Al-Jami‘ bayn al-‘ilm wa 'l-‘amal al-nafi‘ fi sina‘at al-hiyal (A Compendium on the Theory and Useful Practice of the Mechanical Arts) is a monumental water clock known as the castle clock.

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By Professor Salim T. S. Al-Hassani

Table of contents

1. Introduction
2. Clock's Appearance
3. How it Works: Mechanism of the Castle Clock
4. Servicing and Maintenance
5. Notes on Construction
6. Appendix: Different Line Drawings and Views of the Computer Assisted Reconstruction of the Castle Clock
7. Acknowledgement
8. Bibliography and References

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1. Introduction

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Figure 1: Manuscript view of the castle clock. Source: Museum of Fine Arts, Boston, Egyptian manuscript, Mamluk period, Accession number: 14.533 (ink, opaque watercolor and gold on paper, 39.37 x 27.62 cm). (Source).  

The castle water clock is one of the grandest clocks mentioned in al-Jazari's book. Details of its construction and operation have been described quite explicitly at the beginning of Al-Jami ‘ bayn al-‘ilm wa ‘l-‘amal al-nafi ‘ fi sina ‘at al-hiyal (A Compendium on the Theory and Useful Practice of the Mechanical Arts). The first chapter of Category I of the treatise devotes to this detailed description ten sections [1]. We follow in our study of al-Jazari's device his own narrative, but our description given below is not concerned with exact details of its construction but concerned with how components are linked with each other and with the purpose of the clock and its functioning. The analysis thus provided is conceived to accompany computer animations; it is also an interpretation of the clock's appearance to viewers and a study of its internal workings. Further, basic notes on the clock's operating system have been provided to aid understanding of components and some are referenced to technical drawings found at the end.

2. Clock's Appearance

2.1. What an Observer Would Have Seen During the Day

The clock has many motions throughout the day, and would have been very pleasing to watch and listen to. From the point of view of an observer, he would have seen the sun's disc on the eastern horizon about to rise, the moon will not be seen at all and six zodiac signs are visible, while the first point of the constellation Libra is about to set.

The crescent moon would steadily be travelling from left to right on the frieze, and when in between two doors the upper door opens to reveal a figure of a man, the lower door flips round to reveal a different colour. This will occur as each solar hour of sunlight has passed. Soon after this happens, the two falcons will tilt forward and spread their wings, and a ball will drop out of their beaks and into the vase. The observer will hear a cymbal like sound, and both falcons will lean back to their original position and close their wings.

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Figure 2: View of the computer assisted reconstruction of the castle clock.  

At the point when the sixth door is about to open, the sun is at its highest altitude, and three zodiac signs have risen from the east, and three have set on the west. The crescent moon would be between the sixth and seventh doors as the sixth door opens. It is now that the musicians will begin to play their instruments. The observer would hear and see the percussionists beat their drums very lively, and a brief moment later the observer would hear the sound of the trumpets. The two falcons would still perform their duty on this hour, and it can be concluded that the clash of cymbals would have occurred together as the percussionists started to beat their drums. This would be a fair assumption judging from the distance travelled by the lead ball, and the flow of water over the water wheel.

The musicians would play at the ninth and twelfth hour. On the twelfth hour, the sun would be about to set on the western horizon, and all the zodiac signs at the start of the day would have set too. In this configuration, the zodiac disc has rotated about 180o. At this point it would have been the start of night, as the sun has set. The servant must now very quickly prepare the clock for the night ahead.

2.2. What an Observer Would Have Seen During the Night

At the beginning of night the observer would see the moon on the east horizon of the clock. Light will begin to show through the first glass roundel and the crescent moon will again be moving from the left to the right of the observer. Due to the disc with 28 holes and the lamps, the moon roundel will be illuminated such that it looks similar to the phase of the actual moon for that night.

When it is midnight, the crescent moon will be between the sixth and seventh door, and the sixth door figure will have fallen and opened the door. This will trigger the mechanism for the musicians to play, and also rotates the plate to deposit water in the second trough. It must be noted again that the bronze balls are not placed back in their slots for the night, as this could have been quite disturbing to hear clashing cymbals during the night. The musicians only play twice during the night, at midnight, and at daybreak, which coincides with the opening of the twelfth door.

When the musicians first play at midnight, the observer would see six glass roundels illuminated with light, and by daybreak all twelve roundels would have illuminated. The servant must quickly repeat the process as it did at the beginning of the day, and this is repeated throughout the days of the year.

3. How it Works: Mechanism of the Castle Clock

3.1. The Main Reservoir

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Figure 3  

All the power is from the main reservoir, which is an adaptation to the outflow clepsydra (Fig. 3). The float inside uses two semi-spheres of copper soldered together to form a ‘turnip-like' shape and the reservoir uses four cylinders of copper welded together and a tap placed at its base. The two semi-spheres are sealed with wax to enable it to float, but sufficiently heavy to power the mechanics of the clock by pouring sand into it through a hole at the top. Water constantly flows out of the reservoir and the float will move down with the surface of the water. The force due to the floats' weight is transmitted throughout the clock via ropes and pulleys.

The reservoir is filled with enough water for that day, as the hours are unequal for each day. As the float moves downwards, its linear motion is converted to angular motion and provides the means to trigger various mechanisms around the clock for users to observe the time. The flow chamber and its regulator can adjust for the unequal hours.

3.2. The Float Chamber and the Flow Regulator

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Figure 4  

The float chamber is placed directly below the main reservoir's tap and will regulate the outflow from the reservoir (Fig. 4). Its purpose is to maintain a constant outflow from the main reservoir, by overcoming the always-changing head of pressure. This is done by the use of a float, which also acts as a bung for the reservoir's tap. Due to its plug like shape, if the float is at the top of the chamber, i.e. when the chamber is full, it would stop the outflow of water from the reservoir temporarily. This chamber is an adaptation of an inflow clepsydra and also an example of a feedback system. This chamber maintains a constant head of pressure and outflow rate from the reservoir.

A flow regulator is connected to the base of the chamber, and this device allows users to change the outflow rate but still maintaining the constant head of pressure in the float chamber. Allowing the user to change the outflow rate from the main reservoir, the hours of daylight can be adjusted accordingly throughout the year. The regulator will have twelve unequal divisions (or markings) on its face to represent each sign of the zodiac. It was further sub-divided into even more divisions, and these divisions act as a guide to the positioning of the regulator for a particular day of the year. Through a process of trial and error, al-Jazari perfected his design for the castle clock.

To adjust the clock, the user would simply have to rotate the regulator so that the onyx would be in the position of the required division. The position of the onyx determines the hydraulic gradient of the flow out of the chamber. Water flows out of the onyx, and it is collected on to a ‘plate'.

3.3. Plate and Valve Trough

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Figure 5  

This plate is round in shape with a lip around its perimeter and is mounted on a pole, which gives it freedom of rotation about its centre. Extending from the plate is a spout approximately 0.25m in length. Water from the plate will flow out through this and is directed into a particular division of the valve trough.

The plate is rotated by the motion of the sixth, ninth, and twelfth doors. Rope is attached between these doors and a plug in the valve trough.

The trough consists of an upper and lower deck, and the upper deck is further divided into three separate compartments (Fig. 5). The upper deck is to contain the water obtained from the plate, and the three compartments correspond to the sixth, ninth, and twelfth hour doors respectively.

A plug prevents flow into the common lower deck, which earlier mentioned was connected to the figures at the back of the doors. So in the first trough, the plug would be attached to the sixth door figure, the second to the ninth, and the third to the twelfth. When these doors are released or opened, they drop the figure, and due to the lead weight attached to these figures, will pull the plug away and release the water into the lower compartment, which is common to all the other troughs.

When the plug is released, it only has restricted movement, so this will make the rope taut, and will redirect the plate to deposit its water in the next trough, i.e. the ninth hour trough and so on.

When the water is allowed to flow into the lower compartment, it will flow out of a spout and on to a small water-scoop-wheel causing it to turn.

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Figure 6  

3.4. The Waterwheel and The Five Musicians

The waterwheel is axially connected to a series of cams and provides motion to the musicians' arms (Fig. 6). It is now clear that the musicians only play on the sixth, ninth, and twelfth hours of daylight. The cams are configured in a way so that the musicians play in unison a melodic tune. The cams only provide motion to the percussionists' arms, as the two trumpeters do not move. The use of real drums has been employed to give a more authentic sound.

An over-shoot waterwheel is over a trough, and so the water is again contained. The water in the trough will flow into an air vessel that will eventually fill up. This air vessel has a siphon and also at the top is an airline that is connected to a flute. So when the vessel is filled with water, the water will start to siphon out of the vessel into a cistern. The siphoning of water occurs faster than the incoming of water, and so causes a change in pressure within the vessel. This will draw in air from the outside via a flute, and this mimics the sound of the trumpeters. This flute is placed on the wall of the clock between the two trumpeters, again to give a more authentic sound.

The cistern is the end of the power transferred by the water, and so the transmission of power from the float is now described to complete the internal motions of the clock.

3.5. The Float in the Main Reservoir

As a reminder, the float is made of two semi-spheres welded together, sealed and placed in the main reservoir. It is of sufficient weight to power the clock as it moves down with the water level of the reservoir.

A rope connects the float to the main pulley that provides motion to the rest of the clock that was not previously mentioned. Between the float and the main pulley is a small pulley that is directly above the reservoir. This is to keep the rope taut as the float moves down as the pulleys turn.

3.6. The Main Pulley and the Crescent Disc

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Figure 7  

There are in fact two pulleys separated by an axle of about 1 meter long (Fig. 7). The pulley that is connected to the float is at the rear of the clock, and the rope is tied to a ring in the pulley and looped around once before being tied to the float. This allows the pulley to turn relatively and proportionately to the float as it moves down.

There are two grooves in the main pulley, and the second groove provides motion to the sun, moon, and zodiac spheres that are at the top of the clock, but this will be described in more detail later.

On the other end of the axle to this pulley there is a crescent shaped disc for the revealing of light through the roundels. The disc is situated just behind the front wall of the clock behind the glass roundels.

The crescent disc has a crescent shape cut out of half its perimeter, and then an iron bar is bent to regain the shape of the disc. The purpose of the iron ring is to have the centre of gyration at the centre, and also the void will reveal a candle's light through the roundels. A candle is placed behind the disc in such a way that during the night, the light will illuminate the roundels for people to see that one hour of solar night has passed. During the hours of sunlight, the roundels are of little use but will still be operative, because the crescent-disc provides the motion to open the doors of sunlight hours.

3.7. The Zodiac, Sun and Moon Spheres

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Figure 8  

The motion of the spheres is provided by a pulley connected to the rear pulley mentioned in the previous section (Fig. 8). The spheres are only simple models that mimic the sky's motion and serve no real use, except to look very appealing and attractive.

Its operation is quite simple, but its construction is rather complicated, and has been exhaustively described by al-Jazari in section 9 of his description of the castle clock [1]. To best describe it, we mention that it has the form of a large dial with several rotating bezels to position the sun, moon and zodiac relative to each other. The rotating dial is the Zodiac, and it rotates with the pulley because it is fixed to it with bent iron bars. The rear pulley has 360 holes drilled around its perimeter, and this is for the purpose of aligning the zodiac for the day. A rope attached to a hoop in the second groove of the main pulley is connected to a nail. The rope is then wound around the groove of the spheres' pulley once, and the nail is slotted firmly into the hole. This nail can be slotted into any of the holes in the zodiac pulley quite firmly. It provides motion to the zodiac, the sun and the moon bezels.

The middle bezel represents the sun and this can be rotated about its centre. A hole is bored into the disc, and a glass roundel is placed into this hole with a thin layer of gold on its back. During the day, the sunlight reflects on the gold layer, and it is made bright just like the sun itself.

The inner bezel represents the moon, and a hole similar to the sun but slightly smaller is bored. A clear glass roundel is placed into this hole, but the moon has an additional feature to the others. Behind the moon bezel is another bezel with twenty-eight cut-outs spaced evenly around its front face. The holes are cut in such a way that when light passes through the holes, it projects light similar to the phases of the moon. This would be seen at night, as a candle within the clock house is lit, and on-lookers will see the illuminated roundel with a phase similar to the moon of that night.

3.8. The Clock Door and Cart