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 Engineering Clocks

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

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5. Notes on Construction

5.1. General Arrangement

The castle clock is quite a complex device, with lots of parts interconnected in some way to each other, and construction details are not given here, but can be found in al-Jazari's treatise, The Book of Knowledge of Ingenious Mechanical Devices [1]. The clock described by al-Jazari suggests it should stand about 3.5m high, and 3m wide. It is not typically necessary to build a clock of such size, so there are a few factors to consider when scaling it down.

The most important part of the clock is the main reservoir and how far the main float travels within it. The amount of water held within the reservoir is not as important, because the outflow rate can be carefully adjusted using the flow regulator, but it is very important that there is sufficient water to momentarily give ‘life' to the musicians.

As a reminder, the main float descends to its full length of travel in 12 hours, and all the connected pulleys would have rotated 180o, and the cart would have travelled the whole length of its road.

For example, if the float travels a distance x, then the cart travels a distance x too. So the circumference of the crescent disc is equal to 2x, because only half the disc is used to move the cart its full distance (see Fig. 12).

All the pulleys are of the same radius. There should be some allowance for the stretching of the ropes used, but ideally there should be none to ensure accurate time keeping.

The twelve sets of doors can now be equally spaced along the distance x, and knowing the radius of all the pulleys, the rest of the clock can be constructed relative to major components.

Drawings of various other components have also been included, and should be used as a guide (Fig. 13a to 13c). The dimensions given in the drawings are taken from the English translation of al-Jazari's book. They should not be followed too accurately, as parts should be built-to-fit, and not made-to-measure.

5.2. Calibration of Time

It is not easily described, but it can be said that even al-Jazari, a great engineering mind of his time, used trial and error methods to calibrate his huge castle clock. One must painstakingly mark the flow regulator according to flow through its onyx, and the onyx is carefully filed to an appropriate size for the correct range of flows for the whole year.

End Notes

[1] Al-Jazari, The Book of Knowledge of Ingenious Mechanical Devices, op. cit., category I, chapter 1, sections 1-10, pp. 17-41.

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by: Prof. Salim T. S. Al-Hassani, Thu 13 March, 2008