Turret clock
The first mechanical clocks didn't show the time; they rang it. They stood in cathedral towers, weighed hundreds of kilograms and drifted by a quarter of an hour a day. But for the first time, time was measured by a machine rather than by the Sun or by water.
- c. 1300the first mechanical clocks in Europe
- c. 1386Salisbury Cathedral clock, the oldest still working
- ± 15 minthe usual error in a day
Clocks that rang
The English "clock" and the German "Glocke" go back to the Latin clocca, a bell. Monasteries needed to wake the monks for the night office, and towns needed to open their gates and markets. Around 1300 a machine appeared in Europe that struck the hours by itself.
On the right is a reconstruction of a late 14th-century movement, like the Salisbury Cathedral clock. Such clocks often had no dial at all.
A cage of wrought iron
The movement is built in a frame of iron bars held by wedges and rivets, without a single screw. Smiths decorated the corner posts with Gothic finials, like the spires of the cathedral the clock stood in.
A stone in the tower shaft
The clock's motor is a heavy weight on a hemp rope. It sinks down the tower shaft and turns the barrel, and with it the great wheel. Each morning the keeper wound the weight up with a winch, and the clock ran for another day.
The 96-tooth great wheel drives an 8-leaf pinion, and the 60-tooth second wheel drives the 6-leaf pinion of the crown wheel.
A wheel with sideways teeth
Let the weight go without a brake and it would crash down in a second. The brake is the crown wheel, whose teeth stick out sideways like the points of a crown, and a vertical staff in front of it, the verge, with two pallets.
The upper pallet catches a tooth at the top of the wheel, the lower one at the bottom. The wheel pushes on a pallet and turns the verge until the tooth slips past. At once the other pallet catches a tooth on the opposite side, and everything repeats in the other direction.
A bar with two weights
On top of the verge hangs the foliot, an iron bar with small weights. Its inertia slows each swing: the further the weights from the axis, the slower the swing and the slower the clock.
Move the pointer or swipe sideways over the drawing to move the weights to other notches. The readout shows how many minutes the clock will gain or lose in a day.
A quarter of an hour a day
We have moved the weights out by a single notch, and in our model the clock already loses almost an hour a day. Unlike a pendulum, a foliot has no rhythm of its own. It swings only because the wheel pushes it, and any change in friction or in the weight's pull changes the rate.
So the keeper checked the clock against a sundial every day and moved the weights.
Then came the pendulum
In 1656 Huygens showed that a pendulum gives a clock a rhythm of its own. Over the following decades almost every turret clock in Europe was converted: the foliot came off and a pendulum went on the verge. Salisbury's clock was converted too, and in 1956 restorers gave it back its original verge and foliot.
What came before
The mechanical escapement didn't appear out of nowhere. In 1088 Su Song built an astronomical tower in China, some twelve metres high: water turned its wheel, and an escapement metered the water into buckets. But Europe's first mechanical clocks did without water, so frost could not stop them.
Nobody knows who invented the verge escapement. In treatises of the 1270s astronomers still complain that they can't build a wheel that keeps pace with the heavens. A couple of decades later such clocks were standing in the cathedrals of England and Italy.