Timekeeping  /  A float and a gear
c. 270 BC

A float and a gear

Ctesibius lets water lift a float; a rack turns a toothed wheel and a figure points to the hour.

A float and a gear
A float water clock on Ctesibius’ principle, as an 18th-century encyclopaedia drew it. Vitruvius describes a figure pointing at hour lines on a column; this later version turns the same float and rack into a hand on a dial. The mechanism is the same: steady inflow, rising float, toothed rod, wheel.Photograph: Engraving of 1753 (Chambers’ Cyclopaedia tradition), public domain, via Wikimedia Commons
Water flows in at a steady rate and lifts a float. A toothed rod on the float turns a wheel, and the wheel moves a pointer along the hour marks.Drawing: Watch Historian
The need
A draining bowl only sinks; nothing on it points to the hour. Vitruvius tells us that Ctesibius, the son of a barber in Alexandria, worked out the principles of the water clock.
How it worked
Water enters through a hole drilled in gold or a gem, so that it never wears or clogs. The steady inflow lifts an inverted bowl, the float. A toothed rod on the float turns a toothed drum, and a small figure rising with it points a rod at the hours marked on a column. Vitruvius mentions no dial; the clock face with a hand, as in the engraving, is a later way of showing the same idea.
What it changed
For the first time flowing water is turned into rotary motion that can drive a pointer, and also figures, falling pebbles and trumpets. Every geared clock display descends from this idea.
Where it is now
No ancient example survives. The description is in Vitruvius, On Architecture, book 9; the date rests on a Greek epigram that places a device by Ctesibius in the shrine of Arsinoe, about 270 BC.

SourcesVitruvius, De architectura 9.8 (Morgan translation) · Athenaeus 11.497 (Hedylus’ epigram) · Kenny, “The Date of Ctesibius”, Classical Quarterly 26 (1932)

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