You can make a pendulum with an apple and a string hung from a gas jet. It took another hundred years to fit wheels and weights to pendulums so they could be kept swinging for a whole day and night.
To understand what makes the wheels, go ’round in papa’s watch, a little boy or girl would better begin with the big grand- father’s clock in the hall. What a broad, pleasant, honest face it has. Twelve numbers it has, evenly spaced on a circle, like the old stone sundials, and two hands moving around to point the hours and minutes. On the lower side is a small dial with a tiny hand racing around it and counting the seconds. The hour and minute hand are fastened to axles that are pushed through a hole in the middle of the dial. Axles are the centers of wheels, you know, so there are wheels behind the dial. In the face of the clock are two more holes. These are key holes for winding up the weight and the hammer that strikes the hours. A watch has no key holes. It is wound up by the stem.
That is all you can see until you open the tall, narrow door below the face. There, in a sort of closet, the long pendulum swings back and forth, its bright brass "bob" winking in the light. And, at one side, a heavy iron weight hangs on a stout cord. "Tick-tock" is all a clock says to most grown-up people. But to children and poets it says all sorts of things. One thing it says, if you are small enough to squeeze in behind the pendulum, is: "Tick—tock, fennel-and-dock, jump-in-quick and climb-up-the-clock!" If you were a Brownie you could scramble right up the pendulum rod or the weight cord and find the "tick."
But eyes can climb where little boys can't. Look up into that Chinese puzzle of wheels behind the clock dial. The pendulum rod goes up to a sort of beam near the roof. There it is hung by a thin slip of steel that bends easily and makes a spring. It allows the pendulum to swing just so far, and then gives it a little push back. Now look up the weight cord. The cord is ever so long. The upper end of it is wound around a drum or barrel, very much as the rope of an old oaken bucket is wound on a windlass. The weight pulls on the cord and barrel all the time, but it cannot move them until the pendulum is set swinging. Start the pendulum and see what happens. The whole clock wakes up, like the palace in Sleeping Beauty.
The pendulum lifts one leg of an anchor-shaped piece of metal that is locked in the saw-teeth of a wheel. When this wheel is
