The Castle Clock

Introduction

Al-Jazari opened  “The Book Of Knowledge Of  Ingenious Mechanical Devices”  with a monumental clock, perhaps the most complex of all ten water clocks and candle clocks explained in the book: The Castle Clock.

Sometimes you know you read a wonderful book the second you read the first paragraph:

“Call me Ishmael. Some years ago – never mind how long precisely – having little or no money in my purse, and nothing particular to interest me on shore, I thought I would sail about a little and see the watery part of the world. It is a way I have of driving off the spleen and regulating the circulation.”

Moby Dick by Herman Melville

In the right hands, the beginning of a novel can make you feel like you were abducted from reality, and you are drifting down a river that will take you to other worlds. Not only engineers who open al-Jazari’s book are captured immediately by the magic of the machines he designed eight hundred years ago. We will never know whether al-Jazari intended a powerful opening to demonstrate his ability at its best, or whether he positioned the machines in random order and was surprised by the very question. This post aims to explain the Castel Clock and discuss what we can learn about al-Jazari from the text.

How does it work?

The Castle Clock had a complicated movement throughout the day, and it is on the boundary between a clock and an automaton(a machine that performs a function according to a predetermined set of instructions). There is something theatrical in many automata. Sometimes it is by design, like the automata in Greek theater used for “Deus ex machina”, literally “god from the machine”. Sometimes there are other objectives, such as the lion automaton built by Leonardo da Vinci for François I, king of France. When the King tapped the lion with his sword, its body opened, revealing lilies, a symbol associated with French royalty. The clock by al-Jazari is also very theatrical.

At the beginning of the day, all twenty-four doors, in two rows, are closed, and the Golden Crescent, which is a little hard to see in the picture, is positioned to the left. During the day, the half-moon is moving right, and every hour, three things are happening:

• The upper doors open, and a figure comes out and stands as if he had suddenly emerged.
• The lower door is rotating on its axis, and the text “Allah al-Malik” meaning ” God is the King or Owner of Dominion”
• The two falcons, with outstretched wings, lean forward and cast a bronze ball into a vase; inside the vase, a cymbal hangs, producing a sound audible from afar.

The picture of the falcon is taken from a dream or myth. Horus is one of the most significant ancient Egyptian deities. He was most often depicted as a falcon. Horus fought many battles with Seth, the god of the desert, during which he lost his left eye; then a new eye was created for him, called “the eye of the Moon” or “the diamond,” which symbolizes an endless vision. I have no reason to assume that al-Jazari was familiar with Egyptian mythology, but who knows?

Above the upper row of doors, we can see the Zodiac sphere. At the beginning of the day, the sun will be on the eastern horizon, about to rise. The sun climbs until noon, then descends until nightfall, and the six signs that have been visible will disappear, and the six that have been hidden will appear. At noon, the drummers drum, the trumpeters blow, and the cymbalist plays his cymbals for a while.

Al-Jazari does not explain why multiple mechanisms are used to display the time. The crescent actually functions as a modern analog clock hand, and the rest are just “decoration” and maybe a resonance box. In the world of modern engineering, it could be considered excessive and even wasteful, but there is magic that has passed through the centuries of the Falcons, even without additional information.

Erich Kästner, the wonderful author of Pünktchen und Anton(Dot and Anton in English), was concerned: “By the children who would prefer to eat porridge for three days than deal with such complex issues as his reflections” [my translation from Hebrew]. He came up with a different font: “so if you see something like that you can skip it altogether…” It seems to me that this is even more needed for technical explanations of engineers who will be in blue.

The Castle Clock is a sophisticated version of the classical water clock, or clepsydra, in which time is measured by the regulated flow of water out of a vessel.  The difficulty is that the water flow rate is not uniform and depends on the water’s pressure (altitude) in the vessel. To overcome this problem, al-Jazari used a conical plug and a float chamber.

The main reservoir feeds the float chamber through a conical plug; thus, whenever the water level drops, the valve (a float that is a conical plug) goes down with the water level, allowing the chamber to be refilled. Every time the chamber is filled with water, the conical plug seals it, isolating it from the main reservoir. In this way, the float chamber is always full of water, so the water flow is constant and does not depend on the water level in the main reservoir.

At sunrise, a servant makes sure all doors are closed and that the time cart is on the right side (looking from the back). During the day, water will flow at a rate determined by the flow regulator, and the main float will drop as the water level in the main reservoir falls. The main float is made of copper, and it is quite heavy.  When it drops, it pulls the rope, which, through the pulley, turns the main disk and pulls the time cart attached to the golden crescent, moving to the left at a constant velocity, indicating the time elapsed since sunrise. Every hour, the cart will progress one door, and a smart mechanism will open the doors while dropping down two bronze balls. The balls would roll down and reach an opening above the heads of the Falcons. The curving claws of the Falcons are welded to a copper tube that can rotate on its axis. The falcon stands upright because of a balancing weight. When the bronze ball drops down, it changes the balance, and the falcon would lean forward, and the falcon’s wings, attached to the body on a hinge, will spread open, and the ball will fall on the cymbal hidden in the vase. Now that the falcon’s head is light again, the balancing weight will bring it to its original position. The clock is packed with similar inventions and  “patents”.

The book contains almost 50 pages explaining the various mechanisms with detailed construction instructions. Readers interested in the details can find them here and see the simulation here.

What did I learn about Al-Jazari?

We have no information about al-Jazari except what is in the text itself. We can “pick” the book to learn about al-Jazari and his world. Consider the adjustable flow regulator designed to ensure the clock movement adjusts to the changing length of the day. This controller is a small engineering marvel by itself, but I am interested in it because of the triple encounter it offers with al-Jazari and his world:

• First, al-Jazari is familiar with the literature of his time. The opening lines of the Castle Clock chapter are: “I followed the method of the excellent Archimedes in distributing the twelve signs of the Zodiac. Al-Jazari is probably referring “On the construction of water clock” – كتاب أرشميدس في عمل البنكامات. This book was attributed to Archimedes, but its source is unclear. This reinforces al-Jazari’s statement in the introduction:

“I have studied the books of the earlier [scholars] and the works of the later [craftsmen] –masters of ingenious devices with movements like pneumatic [movements], and water machines … I considered the treatment of this craft for a period of time and I progressed, by practicing it, from the stage of book learning to that of witnessing, and I have taken the view on this matter of some of the ancients and those more recent [scholars]. “

The question of openness or seclusion to the world for people of faith is a relevant question even today for Jews or Muslims. Maimonides, Rabbi Moshe Ben Maimon, the most important rabbinical arbiter in Jewish history and a polymath, scientist, and physician, lived in the same time frame in Cordoba, far from Diyarbakir in Anatolia, yet he was part of the same Muslim world. During his medical studies, he was introduced to Aristotle’s writings on natural science and felt no threat to his faith. He even wrote:

” Consequently he who wishes to attain to human perfection, must therefore first study Logic, next the various branches of Mathematics in their proper order, then Physics, and lastly Metaphysics.” Guide for the Perplexed

It’s amazing to read that today, when Orthodox Jewish children are forbidden to learn mathematics or sciences. Al-Jazari is more of an engineer than a philosopher; he does not directly address matters of faith, but his faith is embedded in the text. This doesn’t bother him at all to read and learn from pagan scholars.

• Secondly, in Diyarbakir, eastern Turkey, there are approximately 14.5 hours of daylight in the summer and 9 hours in the winter. Al-Jazari made considerable engineering efforts to ensure that there would be twelve hours between sunrise and sunset in summer and winter. This is the purpose of the flow regulator: to adjust the short hours in winter compared to the longer hours in summer. Time is neither an illusion nor a purely man-made concept. The Earth orbited the sun before there were humans around, and the sunrise and the sunset, as well as summer and winter, were here before we gave them their names. But the perception of time and its measurement are human inventions. If I had met al-Jazari and told him that a second, which was impossible to measure in his time, is the basic unit of time, and that its scientific definition is approximately 9 billion (for those who want precision, 9,192,631,770) cycles of the cesium atom between two energy levels. Not only would he not understand a word, but he would also think me really weird. He did not need such precision that did not fit his daily experience. But I use Waze, a navigation application, and we need accurate atomic clocks at this level of precision to bring me to my destination on time. In today’s world, the concept of time, which varies with the seasons, seems far-fetched, but in al-Jazari’s world, where sundials and water clocks were known, it made perfect sense.
• Thirdly, al-Jazari made detailed measurements of the water regulator attributed to Archimedes and found it insufficient. Then he explains in detail how he tried to solve the problem without success. It’s ridiculous to compare a modern engineer to al-Jazari, but it is delightful to read the report of a very talented engineer more than eight hundred years ago. It turns out his concerns are not very different from those of a current engineer. From the text, it turns out he did a “literature review” and theoretical calculations (in this case, unsuccessful), and planned and performed the experiments. He was also a skilled man who knew copper, bronze, and wood and their processing. When al-Jazari explains, for example, how to prepare the main water reservoir, he’s not satisfied with a drawing and the selection of material (copper); he also explains how to make a perfect cylinder using a precise wooden disk and how to ensure the cylinder has the same diameter throughout. For the technical reader, it is easy to sympathize with the difficulties and solutions. There is something appealing about this combination of a man of the books, an engineer, a craft master, and an artist, whom we can meet through the pages and the hundreds of years that have passed.