Time before clocks
For most of history, people told time by the sun. A shadow moving across the ground or a marked dial gave a reasonable idea of the hour in daylight, as covered in the history of the sundial design. At night or on cloudy days, other devices were used: water clocks, which measured time by the flow of water, candles marked with divisions, and sand glasses. The earliest water clocks appear in Egypt and Mesopotamia, and by classical times they were widely used and sometimes elaborate.
Each of these methods had problems. Sundials cannot work in the dark. Water flow varies with temperature and the pressure of the supply. Candles burn unevenly. A device that kept going by itself, powered by a falling weight and regulated by a mechanism, would solve many of these issues, and in medieval Europe that device gradually appeared.
Where the mechanical clock begins
The mechanical clock, in the sense of a gear-driven device regulated by an escapement, appeared in Europe in the late thirteenth century. Written references to clocks in monasteries and towns start around the 1280s and 1300s. Exact dates and builders are mostly unknown, and the earliest machines are usually known from records of payment or repair, not from surviving hardware.
Historians have also pointed to earlier mechanisms elsewhere. A large astronomical clock tower built in China by Su Song in the late eleventh century used a water-driven wheel with a form of escapement. Whether it influenced European clockmakers is uncertain, and there is no clear evidence of a direct link. It is safest to say that several traditions of complex timekeeping devices existed, and the European mechanical clock developed in its own context.
How an early clock worked
The core design was simple in principle. It had four main parts.
- A weight. A heavy weight hung on a cord wound around a drum. As it descended, gravity supplied the power.
- A gear train. A series of toothed wheels carried the motion from the drum to the other parts, slowing it down.
- An escapement. This is the key invention. Without one, the weight would simply drop and the gears would spin freely. An escapement releases the gear train one tooth at a time, in a regular series of small steps.
- A regulator. In early clocks this was a horizontal bar called a foliot, with small weights at the ends, that swung back and forth and controlled the pace of the escapement.
The earliest escapement is usually called the verge escapement, in which a rod with two small flags, called pallets, alternately catches and releases a toothed wheel. The name and details are discussed in the article on the verge escapement. The result is the characteristic ticking.
Early clocks had no dial in many cases and no hands. Their main purpose was to strike a bell at the hours, to call monks to prayer or to mark the working day in a town. The word “clock” itself is thought to come from words for a bell.
Accuracy
Early mechanical clocks were not especially accurate. They are often said to have gained or lost a quarter of an hour or more per day, though there is no precise figure that applies to all of them. They also had to be checked against a sundial and reset regularly. Friction, temperature, the quality of the metal and the condition of the oil all mattered.
Their advantage was reliability in a different sense: they ran day and night, in any weather, and sounded the hour without a person watching them. That made them useful for towns even when they were less exact than a skilled observer with a good sundial.
Towers, towns and public time
By the fourteenth century, large clocks were being built in the towers of churches, cathedrals and town halls across Europe. A well-known example is the clock at Salisbury Cathedral, dated to 1386, which survives in a substantially original form. Several other surviving or documented clocks of the same era, including those at Wells and Dover Castle, are discussed by historians, though the claim for the oldest surviving working clock is disputed.
Astronomical clocks went further. In the 1360s, Giovanni de Dondi in Padua built a complex astronomical instrument, known from his own written description, that showed positions of the Sun, Moon and planets. It shows that by the mid-fourteenth century, makers could build very sophisticated gear trains.
Public clocks changed how people organized their day. Hours of equal length, struck by a bell, were easier to coordinate than the unequal hours of daylight used earlier. How much this changed daily life is debated, since many people still lived by the sun and the church bell.
From weights to springs
Weight-driven clocks need a tall case or tower for the falling weight. Around the fifteenth century, clockmakers developed the coiled spring as a power source, making smaller clocks and eventually watches possible. Springs deliver uneven power as they unwind, which led to devices such as the fusee to even out the force.
The great leap in accuracy came later, with the pendulum clock in the seventeenth century, followed by improved escapements and, in the eighteenth century, the marine chronometer built for navigation. Those developments grew out of the framework established by the first medieval clocks.
Why the medieval clock matters
The mechanical clock was one of the first complex machines to be built in large numbers, and its gears, weights and escapement gave a model for later mechanisms. It also set the pattern of dividing the day into equal units that is still used. Many of the people who made it are unknown, and its early history is full of gaps. What can be said with confidence is that by about 1300 the idea was in use in Europe, and by about 1400 it was well established.