Mechanical clock: how a hanging weight began to keep time

A sundial does not work under cloud and does not work at all at night. A water clock has to be refilled and looked after again and again. For centuries, people relied on these two to measure time. Then, around 1300 AD, a machine was built in Europe that ran day and night on nothing but a hanging weight. This was the mechanical clock, and every wall clock and wristwatch today descends from it.

The problem

Tie a weight to a rope and wind the rope round a drum, and the falling weight will turn the drum. But the weight will reach the bottom within moments. The real difficulty was how to hold back the power of the falling weight so that it was spent a little at a time, at equal intervals.

According to the University of Bristol project “Clocks and Clock-time in England” in the UK, the mechanical clock grew out of the medieval alarm, in which a slowly descending weight rang a bell. Craftsmen wanted a weight-driven device that would keep pace with the full cycle of day and night. Bristol says that in 1271 a writer named Robertus Anglicus wrote that makers had not yet succeeded in this.

Verge and foliot

The solution was a part called the escapement, which lets power “escape” step by step. In the first mechanical clocks, a system known as the verge and foliot did this work.

According to Bristol, the weight hung from a rope wound round a drum. Through gears, the power passed from the drum to a crown wheel. In front of this wheel stood an upright rod, the verge, carrying two small pallets. When the crown wheel pushed one pallet, the verge turned to one side and the other pallet caught the wheel. Then the same action followed in the opposite direction. In this way the wheel advanced one tooth at a time.

On top of the verge sat a horizontal bar, the foliot, with small weights at both ends. According to William B. Ashworth, a historian of science at the Linda Hall Library in the United States, the speed at which the foliot swung to and fro depended on these weights. According to Bristol, the clock could be made faster or slower by sliding the weights in or out along the bar.

When was this system devised? The sources differ. Bristol links it to the late 13th century, while Ashworth says it was invented in the first half of the 14th century. The name of the inventor is not known.

An earlier attempt

There had been earlier attempts to measure time by machine. In China in 1088, Su Song, an official and calendar expert, built a water-driven clock tower, whose large wheel carried 36 water buckets. According to “Asia for Educators”, an educational project of Columbia University, the tower was almost 40 feet high. Al-Jazari’s water-powered elephant clock was also covered in the 41st article of this series. What was new about the European mechanical clock was that it did not need water.

Tower clocks

The first mechanical clocks were very large and heavy, and were fitted in high towers. According to Bristol, one reason was that the weights needed a long space in which to fall, and another was that the sound of the bell should carry far. In England, most such clocks of the 16th century were wound once a day.

The old clock of Salisbury in England is a famous example from this period. According to Ashworth, records show that a mechanical clock was installed there in 1386, and most experts believe the clock that survives is that one, although some of its parts have been replaced. Its maker is unknown. It has no dial and no hands; it told the time only by striking bells, and for this it had a separate train of gears. The tower in which it was originally fitted no longer exists.

A second example is the clock at Wells. According to the records of the Science Museum in London, it dates from 1392, is made of iron and oak, and weighs about 2,000 kilograms. It too was driven by weights and was at first regulated by a foliot, but some time after 1657 it was converted to a pendulum. Its mechanism is now on display at the Science Museum.

The road to the pendulum

The verge and foliot clock was not very accurate, so it had to be corrected again and again against a sundial. According to Bristol, this system dominated from the late 13th century to the late 17th century. Then Christiaan Huygens completed the work of applying the pendulum to the clock in the late 1650s, and according to Bristol, within a few years pendulum clocks were being fitted in turret clocks as well. That story will come in the 53rd article of this series.

But the basic idea remained what it had been some 700 years earlier: a force to drive the clock, and a part to divide that force into equal portions. When we hear the chime of a city’s clock tower today, or the ticking of a wristwatch, we are hearing the echo of that old idea.