Editing Pendulum (section)

=== The Airy condition ===
Clockmakers had known for centuries that the disturbing effect of the escapement's drive force on the period of a pendulum is smallest if given as a short impulse as the pendulum passes through its bottom [[Equilibrium point|equilibrium position]].<ref name="Marrison" />  If the impulse occurs before the pendulum reaches bottom, during the downward swing, it will have the effect of shortening the pendulum's natural period, so an increase in drive force will decrease the period.  If the impulse occurs after the pendulum reaches bottom, during the upswing, it will lengthen the period, so an increase in drive force will increase the pendulum's period.  In 1826 British astronomer [[George Airy]] proved this; specifically, he proved that if a pendulum is driven by an impulse that is [[Symmetry (mathematics)|symmetrical]] about its bottom equilibrium position, the pendulum's period will be unaffected by changes in the drive force.<ref>{{cite journal
|last=Airy
|first=George Biddle
|date=November 26, 1826
|title=On the Disturbances of Pendulums and Balances and on the Theory of Escapements
|journal=Transactions of the Cambridge Philosophical Society
|page=105
|url=https://books.google.com/books?id=xQEBAAAAYAAJ&pg=PA105
|access-date=2008-04-25
|volume=3 (Part 1)}}</ref>  The most accurate escapements, such as the [[Deadbeat escapement|deadbeat]], approximately satisfy this condition.<ref>[https://books.google.com/books?id=OvQ3AAAAMAAJ&pg=PA75 Beckett 1874], p.75-79</ref>