Barometric Error
With a decrease in the pressure of the air, and consequent fall of the barometer, the pendulum increases its arc of vibration; with an increase in the pressure of the air, and consequent rise of the barometer, the pendulum diminishes its arc of vibration. In the Westminister clock the pendulum vibrates 2.75 deg. on each side of zero, and Sir Edmund Beckett pointed out that with this large arc the circular error just compensates for the barometric error. Where the escapement is suitable, this is doubtless the best way of neutralizing the barometric error; but it is not applicable to the dead-beat, for extra run on the dead faces of the pallets or larger angle of impulse than usual is found to be detrimental, as the oil thickens.