2. How it Works: Mechanism of the Castle Clock
2.1. The Main Reservoir 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. 2.2. The Float Chamber and the Flow Regulator 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'. 2.3. Plate and Valve Trough 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. 2.4. 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. 2.5. 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. 2.6. The Main Pulley and the Crescent Disc 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. 2.7. The Zodiac, Sun and Moon Spheres 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. 2.8. The Clock Door and Cart A cart is made and a track for it to travel on is fixed near the twelve doors of solar sun hours. The cart is directly connected to the crescent-disc by a strong in-elastic string, and the crescent-disc will pull the cart as it rotates around. To ensure that the string is taut, the other end of the cart has a string that has a lead weight attached. On this cart is an arm that extends from the top of the cart to the nails on the back of the figures.  | | Large image | | Figure 9b&9c: The left and right figures show the upper door figure and lower door graphic, respectively. The Arabic sentence reads: "Dominions is God's". |
The upper sets of doors have figures with nails attached that help prompt up the doors on hooks attached to the clock wall. As the cart moves, it will travel along its track that is parallel to the twelve doors (Fig. 9a). The figure is connected to the upper doors, and as long as the figure is propped up, the door will remain closed. So the cart moves along, and will ‘knock' the figure off its hook. The upper doors will open and the figure drops on to a catch releasing the lower doors. This happens very quickly and indicates to observers that a solar hour of sunlight has passed. Both sets of doors are configured in such a way that they favour gravity. The upper doors are tilted slightly, and will open if there is no tension in the string, and the lower doors are pivoted in the middle across its width, but is tapered larger at the top. The pictures used on the upper and lower doors can be seen in the figures 9b and 9c. The falling of the figure triggers various other mechanisms around the clock. Each figure dropping triggers the motion of the falcons, and the musicians are triggered by the opening of the sixth, ninth and twelfth figures dropping. These are the figures that have lead weights fixed to the back to give a greater force to trigger the above mechanisms. The cart also gives motion to the crescent-moon. The crescent-moon projects out from the cart through a slit in the front wall of the clock, and moves along the width of the clock. It is a visual guide for people to see when the next hour is coming because it moves with the cart, and thus the doors open as the crescent-moon passes each door (Fig. 9d). 2.9. The Falcons and the Vase On each hour of sunlight that passes, the falcons will tilt forward, spread their wings, and release a bronze ball out of their mouth. The ball will drop onto a cymbal for all to hear. The cymbal is concealed in a vase, and also acts as a catchment for the balls (Fig. 10a). Above the propped up figures is a piece of wood as wide as the sets of doors. Two rows of twelve slots are drilled, and a bronze ball is placed into each. There is a thin blade that prevents the ball from falling out, and the blade is connected to the figure via a slack string (Fig. 10b). When the figure falls on the hour, this string will become taut and rotate the blade such as to release two balls simultaneously. The ball drops into another channel that is common to all, and from here one ball will travel to one falcon, and the other ball to the other falcon. This is done by directing the balls from the slots to the falcons' head via a thin copper tube. The falcons are made of copper and pivoted at their feet. They are kept at a rest position so that the head is tilted back towards the clock (Fig. 10c). The wings and tail are separate and are pivoted similarly to a real falcon, and the string is attached at the ends of the wing and tail to the wall behind. When a bronze ball is in its head, the whole falcon will rotate about its perch, as if it is tilting forward. The bird will seem like it is spreading its wings and curling its tail as the strings get taut. When far enough forward, the ball drops out of the beak into the vase, creating a clashing sound. The falcon is again at an imbalance, and will return to its original position due to a counterweight attached to the perch like a cantilever with a weight at the end. Its wings and tail also return to their initial position. 2.10. The Triggering of Motion to the Musicians When the sixth, ninth, and twelfth figures are released, they drop with much greater force due to a lead weight attached to its back. This has been mentioned repeatedly within the description but it must be strongly emphasized to fully understand the motions occurring. The weight must be sufficiently heavy to pull the plug from the valve trough, and also be sufficiently taut, so that the taut rope will push against the long spout of the plate, causing it to rotate and redirect its flow to the next trough. This prepares the musicians for the ninth hour, and so on. The water will then flow over the waterwheel causing it to turn, and give life to the arms of the percussionists. The flow of water after the waterwheel was described previously and details of the function of the clock shall end here. End Notes [1] Al-Jazari, The Book of Knowledge of Ingenious Mechanical Devices, op. cit., pp. 35-39. Table of contents Introduction
1. Clock's Appearance
2. How it Works: Mechanism of the Castle Clock
3. Servicing and Maintenance
4. Notes on Construction
5. Appendix: Different Line Drawings and Views of the Computer Assisted Reconstruction of the Castle Clock
by: Prof. Salim T. S. Al-Hassani, Thu 13 March, 2008
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