These clocks were of invaluable assistance to the astronomers, affording as they did a means of keeping time "more accurate than the sun itself." When Picard had corrected the variation caused by heat and cold acting upon the pendulum rod by combining metals of different degrees of expansibility, a high degree of accuracy was possible.
But while the pendulum clock was an unequalled stationary time-piece, it was useless in such unstable situations as, for example, on shipboard.
But here again Huygens played a prominent part by first applying the coiled balance-spring for regulating watches and marine clocks.

The idea of applying a spring to the balance-wheel was not original with Huygens, however, as it had been first conceived by Robert Hooke; but Huygens's application made practical Hooke's idea.

In England the importance of securing accurate watches or marine clocks was so fully appreciated that a reward of L20,000 sterling was offered by Parliament as a stimulus to the inventor of such a time-piece.

The immediate incentive for this offer was the obvious fact that with such an instrument the determination of the longitude of places would be much simplified.
Encouraged by these offers, a certain carpenter named Harrison turned his attention to the subject of watch-making, and, after many years of labor, in 1758 produced a spring time-keeper which, during a sea-voyage occupying one hundred and sixty-one days, varied only one minute and five seconds.

This gained for Harrison a reward Of L5000 sterling at once, and a little later L10,000 more, from Parliament.
While inventors were busy with the problem of accurate chronometers, however, another instrument for taking longitude at sea had been invented.