Anchor escapement teeth, which made the escape wheel rotate once per minute so the second hand could be attached to its shaft. In a 30 tooth escape wheel the pallets span about 7½ teeth. The impulse angle of the pallets, which determined the swing of the pendulum, was 3°-4°. The anchor was the second widely used escapement in Europe, replacing the 400-year-old verge escapement in pendulum clocks. The pendulums in verge escapement clocks had very wide swings of 80° to 100°. In 1673, seventeen years after he invented the pendulum clock, Christiaan Huygens published his mathematical analysis of pendulums, "Horologium Oscillatorium". In
Anchor escapement is still a frictional rest escapement because the sliding of the escape tooth on the dead face adds friction to the pendulum's swing, but it has less friction than the recoil escapement because there is no recoil force. In contrast to the backward slant of the anchor escape wheel teeth, the deadbeat escape wheel teeth are radial or slant forward to ensure that the tooth makes contact with the 'dead' face of the pallet, preventing recoil. In 1826 British astronomer George Airy proved that a pendulum that is driven by a drive impulse that is symmetrical about its bottom equilibrium