"While fancy, like the finger of a clock, runs the great circuit, and is still at home."
William Cowper,"The Winter Evening"(1785)
Around 1602 Galileo Galilei noticed that the swing period of a pendulum was nearly independent of the amplitude of the oscillation, and this became the most important discovery in the history of horology. In 1656 Dutch mathematician and astronomer Christiaan Huygens (1629-1695) was the first to use a pendulum as a regulating oscillator in a clock.
The swing period of a pendulum is only a function of its length and the local gravitational field, unlike the verge and balance (foliot) oscillator, which it replaced, which had an oscillation period that depended on the force exerted by the driving spring.
Within years of Huygens's discovery weight-driven pendulum clocks were appearing all over Europe. To provide a sufficient distance for the weights to fall, and to accommodate a reasonably long pendulum—a two-second tick-lock requires a pendulum 3 feet (1 m) long—these clocks were put in long floor-standing cases. These "grandfather" clocks were reliable to an impressive (in those days) twenty seconds a day. Around 1670 the invention of the anchor escapement led to improvements in timekeeping by enabling the amplitude of the pendulum oscillation to be reduced.
In 1676 the more fragile dead-beat escapement was introduced to high-accuracy regulating clocks. This escapement gave the pendulum a "push" only when it was near its vertical position. Coupled with a pendulum made of bars of different metals (usually brass and steel), it ensured that the length did not change as the temperature changed. The accuracy improved to about one second per day, an important aid in the work of astronomical observatories.
