"Time", says Jacques in As You Like It, "travels at divers paces with divers persons." Shakespeare, writing in 1599, was referring to the individual's perception of the passing hours, but in those days Jacques's remark might equally have applied to the varying rates at which the few clocks available kept the time. Sixteenth century time-pieces were notoriously approximate in their indications. It was not until 40 years after Shakespeare's death, when pendulums came into use, that accurate time-keeping first became possible.
In Elizabethan times, the "foliot balance", a weighted horizontal arm that swung erratically from side to side, was the best known regulator for the primitive clocks. It slowed down the rate at which the clock unwound, but its effectiveness as a control device left much to be desired, and even the best of time-pieces would gain or lose up to 15 minutes every day.
They say it was Galileo, as a young man of 17, who first discovered the delightfully elegant secret of the simple pendulum. Gazing at the massive sanctuary lamp suspended from the dome in the cathedral at Pisa, he noticed, by timing its rhythmic movement with his pulse, that that no matter how narrow or wide might be its arc of swing, the time per oscillation was the same. Many years later, in 1656, Christopher Huygens used this feature of the pendulum to regulate a clock, and was able to build time-pieces that would gain or lose no more than 10 seconds every day.
But despite this great improvement there was still a problem. The accuracy of a pendulum clock depended crucially on the pendulum's length, and that in turn depended on the temperature. Pendulums were usually made of iron or steel with a lead bob fastened to the end: in winter the rod contracted and the clock went fast, and in summer the rod expanded, and with a longer pendulum the clock lost time.
This problem was solved around 1725 when John Harrison, destined centuries later to become the hero of Dava Sobel's Longitude, developed the so-called "grid-iron" pendulum. Brass and steel, he knew, expand for a given increase in temperature in a ratio of 3 to 2 so he built a pendulum with nine alternating steel and brass rods, pinned together in such a way that the expansion and contraction of the unlike rods counteracted each other, thus eliminating any change in length with variations in the temperature.
Following further improvements, by the early 1900s the pendulum clock had evolved to the stage where it was capable of keeping accurate time to within one hundredth of a second in a day.