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F. Berthoud Escapement

From Horology magazine, December, 1937

F. Berthoud Escapement

DURING the year 1580, Galileo observed a swinging chandelier in the Cathedral of Pisa. By counting his pulse beats and watching the motion of the chandelier, he observed that whether the chandelier swung in a short or long arc, the duration of time was the same. From this he got the conception of the pendulum, being a mathematician, he discovered the laws of the pendulum. Up to this time, the verge escapement, or foliott was used and the arrangement of the escapement was such that it was not suitable for delivering motion to the pendulum.  Among Galileo's earliest statements he said no doubt some genius would invent an escapement that could deliver motion to the pendulum. 

In the year 1657, Huygens, a Dutch astronomer and mathematician cleverly converted the verge escapement in such a way that it would drive a suspended pendulum, which was our first pendulum clock, however, before Huygens had perfected his clock, Galileo had a conception and made drawings of a single impulse escapement that would maintain pendulum motion. He did not live, however, to construct the clock but in later years, his son took up the work and built a clock from the original drawings. A duplicate of that model is now housed in a British museum. When Galileo designed his single impulse escapement, he little dreamed that the basic principles would hold superior through centuries to come.

In 1749, Thomas Earnshaw of England, is credited with inventing a chronometer escapement. Some years later the British government offered a prize of £20,000 for a timepiece which would run so accurate that it would be possible to determine the correct longitude at sea. John Harrison of England was awarded this prize. His timepiece was controlled by a single impulse or chronometer escapement and today our pocket chronometers and marine chronometers are also constructed along the same lines.

In 1750, one of our foremost horologists, F. Berthoud of France, designed and built an escapement for clocks, the escapement being placed at the lower end of a pivoted pendulum. Attached to the extreme lower end of the pendulum rod was a plate having a small recess into which the escape wheel teeth dropped, giving motion to the pendulum.

The diagram shows the pendulum traveling to the left. The freely fitted pawl attached to the plates will pass over the unlocking lever. On the return journey the pawl will drop back in place and engage the unlocking lever, which is pi voted to an arbor and on this arbor a locking lever is attached which engages the escape wheel tooth when the wheel is released, giving impulse to the pendulum.

Attached to the left side of the locking lever is a small weight which causes the locking lever to drop back into its proper position, engaging the next tooth of the escape wheel. The depth of the lock is controlled by a banking screw.

Comparing this escapement with our chronometer escapements of today, we find the impulse roller and the balance staff takes place of the impulse plate at the lower end of the pendulum.

The unlocking jewel in the chronometer is in the form of a lever and the locking jewel in our modern chronometer is in the form of a lever also. Both these clocks of similar construction were made. We can take it for granted however, that a heavy pendulum swinging on pivots would create considerable friction, consequently requiring a great deal of power to maintain motion causing early wear and making the clock short lived.

Comparing these escapements with Galileo's conception, we find the same basic principles employed. A diagram of his work and a short biography will be found in the American Horologist, April, 1936, issue.

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