A great master of classical architecture, in De Architectura Vitruvius also deals with astronomy, mechanics, hydraulics and mathematics. In fact, he believes that since all the arts are interconnected, in order to succeed in any one of them, it is necessary to have a knowledge, albeit theoretical, of the rest. Among the valuable information that accompanies his treatise, that concerning the invention of the water clock attributed to Caesibius of Alexandria, a Greek engineer who had worked mainly in the field of hydraulic and pneumatic machines, presumably in the 3rd century BC, is of singular interest. Regardless of Vitruvius’ account, however, it seems to be established that the real inventors of the water clock were the Egyptians. Of their instruments, one, currently preserved in a museum in Cairo and of particular importance, was found in pieces in 1904 in the temple of Ammon at Karnak and dates back to 1400 BC. It is an alabaster bowl with inscriptions on the outside and a hole in the bottom: the angle of the sides, about 70°, is calculated so as to allow equal flow of water at any level, taking into account both the viscosity of the water and the variation of pressure. The outlet hole could be made of precious metal or consist of a perforated gemstone, fixed to the alabaster bowl to prevent wear and tear. 
The Egyptians increasingly perfected the technique of these water clocks, another of which, found in the Temple of Horus at Edfu, is presumably about a millennium later than the one at Karnak. The Greeks used water clocks similar to the one at Karnak to calculate the duration of arguments in court, and one of these, discovered in the Agora in Athens, measured a six-minute cycle. After the destruction of the city in 200 B.C. first by Philip V of Macedon and later by Sulla during the Mithridatic War, the pax romana began and many Roman constructions, still fairly well preserved in the region north of the Acropolis, gave it new splendour. First among these was the Doric propylaeum, erected in honour of Athena Archegetis with the gifts made to the city by Caesar and Augustus, later incorporated into Hadrian’s Gymnasium. Just north of the loggia of Athena Archegetis one can admire, almost intact, the monumental clock built in the 1st century B.C. by Andronicus Cirrestes, located in the Tower of Winds (Varro, De Re Rustica, III, 5). It is an octagon of Pantelic marble, rising on three steps and covered by a circular pyramid roof. Two small vestibules and some fluted Corinthian columns stand in front of two doors built so that one could enter and exit by passing the hydraulic clock, located inside and regulated by water flowing from a cylindrical deposit located in a round tower at the back of the south façade of the Tower of Winds. According to Cicero, Pompey brought back from a campaign in the East one of these devices which, because of its precision, was used, similarly to the Greeks, to set a limit to the speeches of orators. Julius Caesar, too, is thought to have made use of a water clock during his expedition to Britain in 55 B.C., which enabled him to observe that the summer nights in those regions were shorter ‘quam in continente’ (De Bello Gallico, V, 13).
Let us now return to Vitruvius. In Chapter 8 of Book IX, the writer examines water clocks, dwelling first on that of Caesibius and recounting how Caesibius himself came to discover the principles he later applied in the construction of hydraulic machines. Vitruvius explains the construction of water clocks as follows: ‘A hole is made in a piece of gold or in a pierced gemstone (because these materials do not wear out with the passage of water) through which the water, always coming out in the same way, raises a cork or tympanum on which is fixed a line formed with teeth equal to those of a versatile wheel, whose teeth, pushing against each other, make the wheel move and turn slowly.
The spaces of the hours are described above a column or pillar and are pointed out in diem totum with a stick by a statuette rising from below. The shortness or length of these in each day and month is regulated by wedges, which go further out or go further in. These wedges, also called mete, are truncated cones, one solid and the other hollow, worked in such a way that one can enter into the other: by loosening or tightening them with the same regulation, more or less water falls into the basin’. The description is completed by the rules needed to construct winter water clocks, called anaphoric clocks, and to administer the water so that it is properly regulated. In hourglasses, different in structure but the same in mechanism, the hours were measured by the time taken by a certain amount of water to pass from one place to another. The water clock was, however, inaccurate for two reasons: firstly, because water is subject to evaporation, it gradually decreases in volume and in the long run becomes muddy and cannot be distilled; and secondly, because a hydrodynamic law, according to which as the surface of the water descends, the pressure exerted by the liquid on the bottom of the vessel decreases, preventing the inflow from taking place at a constant speed. The masterpiece of Alexandrian engineers was considered to be the monumental clock of Gaza, located in the market square on a semi-cylindrical wall, which concealed the hydraulic mechanism. At the top appeared a gorgon with rolling eyes; at the bottom a figure of Helios passed through twelve illuminated doors at night, marking the twelve hours. At the stroke of these, out of one of the doors came Heracles, wearing the attribute of one of the twelve labours, to be crowned by an eagle; other statues, also depicting Heracles, stood in three aedicules and the hero, in the middle one, marked the hours by beating his club on a bronze lion skin. Procopius, head of the school of Gaza in the early 6th century, dedicates the sixth ekphrasis to ancient art and mechanics, describing the clock in question in detail and evocatively. […]
In 1673, Claude Perrault, an architect, physicist, naturalist and member of the Académie Royale des Sciences in Paris, translated Vitruvius’ De Architectura, highlighting the importance of water, which is normally used to impart movement to machines. In his work Oeuvres diverses de Physique et de Mechanique Perrault discusses the new invention of the pendulum clock that runs on water. The description of this mechanism is very precise and detailed: once again, it is shown that the continuous and regular flow of water produces an effect identical to that of springs and counterweights on other pendulums.
In Italy, to speak again of water clocks (highly appreciated since antiquity and for which the Romans themselves had coined the term horologium, a word that soon spread throughout all the regions of Europe that were part of the Roman Empire), we need to go back to the second half of the 19th century. In fact, the invention of the hydrochronometer dates back to 1867. It was invented by the Dominican Father Giovanni Battista Embriaco, who devoted himself to his studies of mechanics applied to watchmaking in the solitude of the Minerva monastery. This instrument (which was judged worthy of a prize at the Universal Exhibition in Paris in 1866, even though it was then without a striking mechanism) was then, once perfected and fitted with a large striking mechanism, presented at the National Exhibition in Milan in 1881. The clock, made in the mechanical watchmaking workshop by the Oranaglia brothers, is located on the central islet of an artificial lake on the Pincian Hill, fed by the Marcia water, which flows in several rivulets and cascades from a rock: it is one of the ornaments with which the Roman City Hall wanted to embellish the famous promenade. In Father Embriaco’s hydrochronometer, not to be confused with hourglasses, water performs the office of the true motor of a clockwork machine, simple but exact, as well as an anchor. The latter is placed between two springs parallel to the clock’s regulating pendulum, which are alternately lifted by the anchor at every second minute. The fall of the springs is always equal, whatever the amount of water moving the boat, and this causes a perfect isochronism in the oscillations of the pendulum, which is not set in motion by the water, but by the two springs. 
The water discharges with each swing of the pendulum from each compartment of the boat, falls alternately on two large bronze lilies placed below, which form a kind of balance, and makes them swing with a motion synchronous to that of the pendulum. From one end of the axle of these lilies runs a rod, which transmits the movement to the hour machine and causes the minutes, hours and quarters to be marked on four large displays or dials. The water, which is indispensable for setting the clock in motion, pours into a container underneath, worked in the shape of a basket, and fills it every quarter of an hour. The basket, suspended by two brass chains from the axis of a wheel, has twelve teeth on one side and three on the other: teeth that are used to raise the hammers that strike the hours and quarters. Every quarter of an hour, the basket fills with water, descends with its own weight and sets the very simply designed striking wheel in motion. Once at the bottom, the same basket empties itself by means of a siphon and rises back up to fill itself again. The hydrochronometer now described is housed in a kind of cast iron tower, built to the design of the municipal architect Herzog, who wanted to depict a rustic tangle of trunks and tendrils arranged in such a way as to allow the play of water and the movement of each individual part of the device to be seen behind the four large glass plates.
On the hydrochronometer of the Pincian Hill, everyone can read Father Embriaco’s name, engraved by order of the Roman City Hall in recognition of a versatile genius who, in the shadow of the cloister and in the silence of a cell, promoted and cultivated the study of physical and mechanical sciences. 
But Rome is a city full of charm, with a thousand unknown corners, which one discovers little by little: when one turns into a small square, when one walks through one of the many streets of the historic center, when one goes to browse among the doorways. And right on Via del Gesù, at number 62, on the back wall of the courtyard of the Berardi building, now condominium property, you can admire a fountain with a new water clock, again designed by Father G.B. Embriaco and second, in Rome, after the better-known one at the Pincio. The clock is located inside a shell-shaped niche, rests on the bottom of a two-invaded fountain, with a small seal immersed in the water, and works on one side only. Around the niche, a frame with four caryatids can be seen, on which two marble busts rest on either side of the clock. A third water clock, similar to the previous two and also made by Father Embriaco, was located in a courtyard of the former Ministry of Finance, now the Treasury.
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Through the kind disposition of some officials, it was learned that, as early as 1965, this garden had been excavated for the construction of the thermal power plant and that during the work, carried out by the Civil Engineers, the clock had been removed and since then (unfortunately!) there has been no news of it. Father Embriaco’s hydrochronometers can undoubtedly be considered, for their simplicity and exactness, among the most beautiful inventions, possessing in fact the singular prerogative of never needing to be wound: the regular and constant flow of water in fact provides the necessary task.
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The images illustrating the editorial are taken from: Giovanni Battista Piranesi, Diverse maniere di adornare i camini ed ogni altra parte degli edifizi… In Rome. Generoso Salomoni, 1769 [20 A I 67]