We think of the tall-cased "grandfather's clocks" as antique; but this tower-clock of de Vick's outdoes them in antiquity by some four hundred years. And its most interesting feature is its curious likeness in mechanical principle to the clocks of modern times. Like most early clocks, it has only one hand—the hour-hand. Its ponderous movement is of iron, laboriously hand-wrought; the teeth of its wheels and pinions were cut out one by one. It was driven by a weight of five hundred pounds, the cord of which was wound round a drum, or barrel. This barrel carried, at one end, a pinion, meshing with the hour-wheel, which drove the hands; the flange at the other end of the barrel formed the great wheel, or first wheel of the train. This meshed with a pinion on the shaft of the second wheel, and this in turn with a lantern-pinion upon the shaft of the escape-wheel. All of this is, of course, essentially the modern train of gears, only with fewer wheels. 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_. [Illustration: 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. [Illustration: _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.