Summary
Because the bob of a freely swinging pendulum traces out a circular arc, which is not isochronous, Huygens had to adjust its swing in such a manner that the pendulum could beat equal times irrespective of the amplitude of its swing. The problem was not new to him. In 1657, when he first applied the simple pendulum to a clock, Huygens was fully aware of its nonisochronous nature for swings of large amplitude; and although at that time he did not know the path that the bob should follow in order to achieve isochronism, he was able to compensate for the irregularity of the simple pendulum by confining its swing between two curved metal bands whose shape he determined empirically. As he explained later to Pierre Petit:
In a simple pendulum the swings that are elongated more from the perpendicular are slower than the others. And so in order to correct this defect (the opposite to that which you believed) at first I suspended the pendulum between two curved plates …, which by experiment I learned in what way and how to bend in order to equalize the larger and smaller swings. And I remember having so well adjusted two clocks in this manner, that in three days they never showed between them a difference of even seconds: although in the meantime I often changed their weights, rendering them heavier or lighter. However, later because I found that with these plates the slightest tilt of the clock altered the length of the pendulum, I got rid of them, at the same time trying to make the vibrations of the pendulum narrower by means of the gears.
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- Unrolling TimeChristiaan Huygens and the Mathematization of Nature, pp. 71 - 96Publisher: Cambridge University PressPrint publication year: 1989