"But those who are more curious in these matters reckon eight winds; among such was Andronicus Cyrrhestes, who, to exemplify the theory, built at Athens an octagonal marble tower, on each side of which was sculptured a figure representing the wind blowing from the quarter opposite thereto. On the top of the roof of this tower a brazen Triton with a rod in its right hand moved on a pivot, and pointed to the figure of the quarter in which the wind lay."
Vitruvius, On Architecture (I.6.4)
Situated at the foot of the Acropolis in the Roman agora (marketplace), the Tower of the Winds is one of the best preserved buildings of classical antiquity. It takes its name from the sculptured frieze that surrounds it, each wall of the octagonal tower presenting a personification of the eight winds, with a sundial beneath each one, the lines in the marble scarcely visible and their bronze gnomons long since lost. On the apex of the roof, as Vitruvius relates, there was a bronze weathervane in the form of the sea-god Triton who, rotating in the wind, indicated its direction by pointing to that side of the building which from the wind was blowing. Writing in the third or fourth century AD, Marcus Cetius Faventinus made a revised abridgement of Vitruvius for builders of private houses. In it, he averred that "most people assert that there are twelve winds, just as in the city of Rome there is a bronze triton with twelve busts of the winds, created in imitation of Andronikos of Kyrrhos' model. Holding his rod above the head of a wind, he shows that same wind to be blowing" (Liber Artis Architectonicae, II).
Vitruvius dedicated his treatise on architecture to Augustus, who became emperor in 27 BC, when it may have been written. The only other classical author to mention the Tower is the Roman scholar Varro, who compares it with the tholos in the aviary of his sumptuous villa at Casinum.
"Inside, under the dome of the rotunda, the morning-star by day and the evening-star at night circle around near the lower part of the hemisphere, and move in such a manner as to show what the hour is. In the middle of the same hemisphere, running around the axis, is a compass of the eight winds, as in the horologium at Athens, which was built by the Cyrrestrian; and there a pointer, projecting from the axis, runs about the compass in such a way that it touches the wind which is blowing, so that you can tell on the inside which it is" (On Agriculture, III.5.17).
Varro wrote a decade earlier, in 37 BC, which provides the latest date that Cyrrhestes' Tower could have been constructed (its terminus ante quem), or it even may have been another decade before that, when Antony plundered the villa. An estimate for the construction of the Tower of Andronicus of Cyrrhus, therefore, may be about 50 BC. Not only were there sundials on the exterior of the building but inside it housed a horologium ("hour indicator"), an instrument to measure time (and so the name of the building itself).
The device was a clepsydra ("water thief") or water clock. First used to measure the time persons were allotted to speak in the Athenian law courts, (Pseudo) Aristotle describes it as a vessel with a stopper at its neck that controlled the stealthy flow of water from several small holes at the bottom (Problems, XVI.8; a hundred years before, Aristophanes has a character in Wasps. ll. 827ff suggest a chamber pot might do just as well). It takes its name from a spring that supplied water to a cistern on the side of the Tower. An overflow outlet assured that the water level was constant (and so the water pressure). Water then flowed from this reservoir to a tank inside the Horologium, where its rising level was measured by a float or lines on a scale to mark the passing hours. With each new day, it was emptied and filled again. There still are grooves in the marble to channel the water, which drained through a hole in the middle of the floor.
Situated so that it faces the points of the compass, the northeast and northwest sides of the building have porticoes flanked by Corinthian columns (distyle) that give access to the interior. On the southern wall opposite, a circular structure (top and right) held the cistern that supplied water to the clepsydra inside. (In 1772, a decade after The Antiquities of Athens had been published, Oxford University began construction on the Radcliffe Observatory, which was based on the design of the Tower.)
References: "Vitruvius and Attic Monuments" (1997) by Antonio Corso, The Annual of the British School at Athens, 92, 373-400; "The Water Clock in the Tower of the Winds" (1968) by Joseph V. Noble and Derek J. de Solla Price, American Journal of Archaeology, 72(4), 345-355; "The Tower of the Winds" (1967, April) by Derek J. de Solla Price, National Geographic, 131(4), 587-596 (the National Geographic Society had funded the work of Noble and Price and a popularized account of their investigation first appeared in its publication); "The Tower of the Winds and the Roman Market-Place" (1943) by Henry S. Robinson, American Journal of Archaeology, 47(3), 291-305; "Twisting in the Wind: Monumental Weathervanes in Classical Antiquity" (2016) by Dunstan Lowe, Cambridge Classical Journal, 62, 147-169; Vitruvius and Later Roman Building Manuals (1973) by Hugh Plommer.
