This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
The whole duty of a good clock is to drive the hour hand at a regular rate of travel around the dial, twice in twenty-four hours. So the barrel turns entirely over twice, letting down two coils of cord. In an eight-day clock the cord is wound around the barrel sixteen times. On the rear end of the barrel is fixed the hour wheel. Both turn together. The minute wheel and the second wheel are fitted there, too. All of these turn on other wheels and pinions. The axles of the pinions are extended and pushed through the dial to carry the hour, minute and second hands. So these hands just have to turn when the wheels turn.
Sometimes a clock goes too fast or too slow, and must be regulated. A grandfather's, or other pendulum clock, is regulated by pushing the "bob" of the pendulum up or down on the rod. The time it takes a pendulum to swing, depends upon the length of the rod. In a watch, or a spring clock, there is a key-stone shaped indicator plate right over the coiled spring. One side is marked S (slow) and the other F (fast). A movable pin over this plate regulates the swing of a fussy little wheel that rocks back and forth like a cradle.
Of course you have guessed that this rocking wheel in the watch is really the pendulum wheel. There is no room in a watch for a long pendulum rod and "bob" to swing. And there is no room for a cord and weight and big barrel to furnish power. The "engine" of a watch is a blue-steel coiled spring. The escapement, or little saw-toothed wheel and catch, is in the watch, just as it is in the clock. Ask your papa to open his watch at the back. Over these parts the plate is cut away, so you can find all of them.
Ask your papa to keep the back door of his watch open while he winds it up by the stem. Then you can see the loose, open coils of the main spring come closer together, so the spring fills less space. A clock is wound with a key that slips through a key-hole, over the axle of the barrel, in the dial face. The key winds the spring in a spring clock, or winds the cord around the barrel in a pendulum clock. The other key-hole in the clock face is to wind up the wheel that controls the striker. At the end of every hour, the hammer connected with this wheel is lifted to strike a bell, from one to twelve times. To put a wheel, a spring and hammer and a bell in so a clock could do its own striking, was a wonderful improvement. Once a town or a church had to keep a man to pull a bell rope, to make the big clock in the tower strike the hours.
A watch is the most wonderful little machine in the world. Packed away in mama's watch, that is no bigger around than a silver quarter and less than half an inch thick, are one hundred and fifty or more separate parts. There are wheels of many sizes and shapes, pinions, plates, screws, rivets, pins, springs and "jewels." Many of these parts are so small that watch-makers pick them up with tweezers or magnets, and find the places for them by screwing a magnifying glass in one eye. Wouldn't you like to go through a watch factory and see all these parts made and put together?