The fountain that changes its shape and the controversy with the Banu Musa

“Allah has promised the believing men and believing women gardens beneath which rivers flow, wherein they abide eternally.”

Qur’an 9:72

The fountain with two tipping buckets, Topkapi manuscript, 1206


The Muslim paradise is called Jannah ( جنّة), literally “garden.”  Every time heaven is mentioned in the holy book of Qur’an, there is a description of flowing water and fruit-bearing trees. This is not surprising because Islam came from the desert, hot and arid lands. I would like to ignore the other attributes of al-Jannah such as houris; splendid companions of equal age, lovely eyed and virgins, who will accompany the faithful. My only focus is on the scenery. Like Garden of Eden in the Bible, there are four rivers in Jannah, the Euphrates flows according to both books, but the other rivers are different, and they have a common source named Salsabil (سلسبيل).

Gardens were significant to Islam from its inception. The garden landscaping has a spiritual meaning which exceeds the human need for shade and water. They are perceived as a place of rest and contemplation, an earthly equivalent to life in heaven. This metaphor reached its peak in Chahar-bagh, (چهارباغ), in which the garden was divided into four parts by water channels; the four water channels being the four rivers of paradise with a fountain in the center of a pool, representing Salsabil. I think that al-Jazari’s deep interest in fountains is related to the importance of gardens in Islam. 

A miniature of paradise from the 16th century

How does the fountain work?

The fountain of al-Jazari is installed in a pool. For an hour the fountain shoots up from the main orifice and then for one hour it emits six curving jets from six nozzles, and the process repeats itself. Today it is a trivial task for any engineer, but in the 12th century, with no electronics or electric valves, it was almost a miracle. The technical explanation, as always, will be colored in blue, so anyone who is not interested in early control systems can skip those bits.

At some distance from the pool, al-Jazari built a high house into which the water flowed. This section does not appear in the drawing. The water from the high house flew into the copper bowl welded to a pipe with four openings. This is the same drawing as above, but with my captions.

At the bottom of the titling pipe, al-Jazari welded a ring which is seated on an axle so that the pipe is like a kids seesaw. The right side is slightly heavier, and therefore it tilts to the right, and the water comes from both openings on the right. The main opening fills the tank and the narrow pipe which shoots the water up in the air. The secondary opening is much smaller, and it slowly fills the tipping bucket (in red). In the drawing, the tipping bucket is almost full. After one hour, the weight of the water at the front-end is heavy enough to make the tipping bucket swing, and the black rod will push the tilting pipe upward so the seesaw would tilt to the left and water fill the other tank, the wide pipe around the narrow pipe and comes out in six jets. The process repeats itself.

Banū Mūsā

Al-Jazari opens the fourth Category “On fountains which change their shapes at known intervals and on perpetual flutes” with a brief statement:

“I did not follow the system of the Banū Mūsā, may God have mercy upon them, who in earlier times distinguished themselves in the matters covered by these subjects.”

The Banū Mūsā brothers are the predecessors of al-Jazari and are important to understanding his work. Banū Mūsā, the sons of Moses, is the name shared by three scholars, brothers from the ninth-century, sons of Mūsā ibn Shākir, a Persian astronomer. At a young age, they join the famous House of Wisdom, a library and a translation center in Baghdad. It is known that the brothers wrote together more than 20 books, but most have been lost over the years. Their most famous book and only two copies survived is The Book of Ingenious Devices (كتاب الحيل Kitab al-Hiyal( which al-Jazari is referring. The book was commissioned by the Abbasid Caliph of Baghdad, Abu Jafar al-Ma’mun ibn Harun (786–833), who instructed the Banu Musa to acquire all of the Hellenistic texts that had been preserved during the decline and fall of Roman civilization. This rescue operation has cultural importance, which exceeds by far the current post. Some of the devices described in their Book were inspired by the works of Hero of Alexandria and Philo of Byzantium, as well as ancient Persian, Chinese, and Indian engineering. However, many of the other devices described in the book were original inventions by the Banu Musa brothers. Donald Hill, who translated this book, as well as al-Jazari’s book, wrote:

“The Banu Musa went “well beyond anything achieved by Hero or Philo.” Their preoccupation with automatic controls distinguishes them from their Greek predecessors, including the “use of self-operating valves, timing devices, delay systems, and other concepts of great ingenuity.”

The book describes the construction of 100 devices, including seven automatic fountains.

What is the Controversy?

Al-Jazari did not specify which fountain he is referring to, but he did write:

“They made the alternation [fountain water shapes] with vanes turned by wind or by water so that the fountains were changed at every rotation, but this is too short an interval for the change to appear [to the full effect]. Then in two designs they used a pipe like an almost horizontal balance arm. The water flowed through it …”

It is clear that this is the fountain he is referring to:

A fountain that alternates water shapes by use of a balance from
The Self Changing Fountain of Banu Musa bin Shakir by Prof. Salim T S Al-Hassani

He concludes: ” I do not know whence this confusion [came], from the original or from the copy.”

For those who really want to dive into the details, you can see here the fountains the Banu Musa. There could be no argument that al-Jazari borrowed key concepts from the Banu Musa, including the placement of a narrow pipe within a wide pipe, the concept of two water tanks and variable feeding with time. His main disagreement is over the control method. In his opinion, the intervals were too short, and the result could be erratic. He’s probably right. Al-Jazari explains what’s wrong with the design, but the details are of little importance. The technology changed so dramatically that the historical techniques to control the timing are only an odd puzzle of how we can control timing before we had, electronics and electric valves. However, curiosity and skepticism are the best guides for every engineer today, just like eight centuries ago.

Curiosity and Doubts

Anybody who taught high school or Bachelor’s degree in science or technology knows that academic success is no guarantee for curiosity, healthy doubts, or critical thinking in general. Excellent students can answer the questions in the exam but find it difficult to ask questions about a scientific paper or engineering presentation, to test if the assumptions are robust and can stand rigorous evaluation, if there is an alternative explanation or if there can be another model. Many excellent and feel uncomfortable with the new requirements so different from their previous experience in school. In parenthesis, as an educator, I have to say that this is not a decree of fate and schools can do a lot, but that’s another discussion. My encounter with al–Jazari is limited to his book, but beyond is high of engineering capabilities, it is clear he was curious and had a healthy measure of inquisitiveness and skepticism. He checked the water regulator attributed to Archimedes and found it insufficient, he read the Banu Musa and had his doubts regarding the control method. Beyond the benefit of the healthy engineering skepticism, as he adds question marks, I like him more.


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