The escapement is the most important part of the whole mechanism, because it is the part which makes the clock keep time. It is an interrupter, checking the movement almost as soon as, under the urge of the mainspring, it starts forward. The frequency and duration of these interruptions determines the rate of running. Without this, the movement would run down swiftly; with it, the operation stretches over thirty hours, involving 432,000 interruptions.
A Time Piece of the Middle Ages
The huge and elaborate Clock of Strasbourg Cathedral, in Lorraine, was built in 1352 and is an example of the first clocks.
De Vick's escapement is shown in the illustration. The escape-wheel was bent into the shape of a shallow pan, so that its toothed edge was at a right angle to the flat part of the wheel. Near it was placed a verge, or rotating shaft, so called from a Latin word meaning "turning around." On this verge were fastened two flat projections called pallets, diverging from each other at about an angle of one hundred degrees. The width between the pallets, from center to center of each, was equal to the diameter of the wheel, so that one would mesh with the teeth at the top of the escape-wheel and the other with the teeth at the bottom.
de Vick's Clock
Now, if the upper pallet were between the teeth at the top of the wheel, the pressure of the wheel trying to turn would push it away until the teeth were set free. But, in so doing, it would cause the verge to turn and bring the lower pallet between the teeth at the bottom of the wheel. And since the bottom of the wheel was, of course, traveling in the opposite direction from the top, the action would be reversed, and the lower pallet would be pushed away, bringing the upper one back between the teeth of the wheel again; and so on, "tick-tock," the wheel moving a little way each time, and the pallets alternately catching and holding it from going too far.