THE ESCAPEMENT.
The spring or weight transmits its power through a train of cogs to the escapement, or device for regulating the rate at which the wheels are to revolve. In clocks a pendulum is generally used as the controlling agent. Galileo, when a student at Pisa, noticed that certain hanging lamps in the cathedral there swung on their cords at an equal rate; and on investigation he discovered the principle that the shorter a pendulum is the more quickly will it swing to and fro. As has already been observed, Huygens first applied the principle to the governing of clocks. In Fig. 202 we have a simple representation of the "dead-beat" escapement commonly used in clocks. The escape-wheel is mounted on the shaft of the last cog of the driving train, the pallet on a spindle from which depends a split arm embracing the rod and the pendulum. We must be careful to note that the pendulum controls motion only; it does not cause movement.
The escape-wheel revolves in a clockwise direction. The two pallets a and b are so designed that only one can rest on the teeth at one time. In the sketch the sloping end of b has just been forced upwards by the pressure of a tooth. This swings the pallet and the pendulum. The momentum of the latter causes a to descend, and at the instant when b clears its tooth a catches and holds another. The left-hand side of a, called the locking-face, is part of a circle, so that the escape-wheel is held motionless as long as it touches a: hence the term, "dead beat"—that is, brought to a dead stop. As the pendulum swings back, to the left, under the influence of gravity, a is raised and frees the tooth. The wheel jerks round, and another tooth is caught by the locking-face of b. Again the pendulum swings to the right, and the sloping end of b is pushed up once more, giving the pendulum fresh impetus. This process repeats itself as long as the driving power lasts—for weeks, months, or years, as the case may be, and the mechanism continues to be in good working order.
COMPENSATING PENDULUMS.
Metal expands when heated; therefore a steel pendulum which is of the exact length to govern a clock correctly at a temperature of 60° would become too long at 80°, and slow the clock, and too short at 40°, and cause it to gain. In common clocks the pendulum rod is often made of wood, which maintains an almost constant length at all ordinary