· By Buffy
Verge and Foliot
The first mechanical escapement, 1300s–1600s: a crown wheel, a verge, and a foliot bar — crude, brilliant, and losing minutes a day.
The verge escapement is the founding hack of mechanical horology, appearing in European clocks around the 13th century (the exact origin is debated; the first unambiguous references cluster in the 1270s–1300s). It made mechanical timekeeping possible at all — and was so crude that a good tower clock might drift 15–30 minutes per day.
The mechanism
- A vertical verge (rod) with two pallets (flags) set at right angles to each other.
- A crown wheel — a wheel with sawtooth-shaped teeth facing outward like a crown — whose teeth alternately push one pallet, then the other.
- A foliot — a horizontal bar with adjustable weights — mounted on the verge, rocking back and forth.
Each swing of the foliot lets the crown wheel advance one tooth; the tooth’s push also pays back the foliot’s losses. The beat is set by sliding the weights: heavier or wider means slower. The period depends on how hard the escapement pushes — the defining flaw.
Why it is inaccurate
The verge’s period depends on driving torque and on friction, so the going rate varies with the mainspring’s state, temperature, wear, and lubrication. There is no real detachment: pallets rub the teeth almost continuously. This is the isochronism problem in its rawest form.
Yet it worked — for three centuries, well enough to schedule monasteries, factories, and ships’ watches.
What replaced it
- The anchor escapement (1670s, attributed to Robert Hooke/William Clement) for pendulum clocks, where the pendulum’s own regularity could dominate.
- The cylinder and later the lever for watches, which needed to work without gravity’s help — see Cylinder and Duplex and the Swiss lever.
The verge’s real legacy is the concept: release an oscillator one tooth at a time, and pay it back with the same stroke. Everything on this site is commentary on that idea.