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IIIThe Escapement

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Chapter III · Act I — Reading Time

The Escapement

late 13th century – 15th century

The decisive invention: a machine that divides time into equal beats.

In the late thirteenth century a machine appears in Europe that does not flow: it beats. A weight pulls a wheel round, and a mechanism stops it and lets it go at equal intervals. Water is no longer needed: time is counted in beats.

The verge and foliot

One beat every 4 s

Drag to turn it · arrow keys on a keyboard · double-click to face it

Verge and foliot escapement. The foliot has no rate of its own: it swings because the wheel drives it, and so it gains or loses with the pull of the weight. With its weights where they start, it beats every four seconds, like the restored foliot of Salisbury Cathedral.

  1. 1Crown wheel
  2. 2Verge and pallets
  3. 3Foliot
  4. 4Regulating weights

  1. A weight on a cord turns the clock’s wheels. The last of them is the crown wheel, its saw teeth standing on its rim like a crown.

  2. In front of it stands a staff with two pallets. One stops a tooth; the wheel pushes it and turns the staff.

  3. The foliot turns with the staff. As the upper pallet lets its tooth go, the lower one catches a tooth on the far side: the wheel escapes half a tooth at each beat.

  4. To regulate it, the foliot’s weights are moved: the further out they hang, the harder the bar is to turn, and the slower it beats.

A machine that always beats the same cannot stretch the summer hours by itself: it would have to be reset every morning and every evening. With the bells of the towers, in the fourteenth century, towns come to count equal hours.

12 + 12hours of day and of night, different every season

24equal hours, those of the town’s bells

The first clocks

  1. Richard of Wallingford

    The abbot of St Albans describes his abbey’s astronomical clock. Its escapement is not a verge but one of his own, the strob: two wheels on the same arbor. The clock is finished some twenty years after his death.

  2. c. 1336

    The hours of Milan

    In the bell tower of San Gottardo a clapper strikes the bell as many times as the hour: once at the first hour of the night, twice at the second, up to twenty-four. So, writes the chronicler Galvano Fiamma, one hour can be told from another.

  3. 1348–1364

    Dondi’s astrarium

    In Padua, Giovanni de’ Dondi builds a clock with seven dials for the sun, the moon and the five known planets. His treatise holds one of the earliest drawings of a verge escapement.

  4. c. 1386

    Salisbury Cathedral

    It probably had no dial: it was made to strike the hours on a bell. At the end of the seventeenth century it was given a pendulum, and in 1956 it went back to a foliot. It is said to be the oldest clock still working, though its date is disputed.

  5. The Prague Orloj

    The clockmaker Mikuláš of Kadaň and the astronomer Jan Šindel set up an astronomical clock on the Old Town Hall. Its dial still shows the unequal hours, as curved lines, beside the twenty-four of Old Czech time, counted from sunset.

The Line of Precision

The error a day of each chapter’s clocks, on a logarithmic scale. The line moves on with the story: each chapter draws its own stretch.

This chapter’s precision

Foliot tower clockc. 1386
≈ 15 min a day
  1. c. 1390 BCE: Clepsydra, ≈ half an hour a day
  2. c. 1386: Foliot tower clock, ≈ 15 min a day

This chapter’s sources