inute hand. By a skillful arrangement of cogs
it also moves the hour hand around the dial once in twelve hours. The
center wheel moves the third wheel. The chief business of the third
wheel is to make the fourth turn in the same direction as the center
wheel. The fourth wheel revolves once a minute, and with it turns
the tiny second hand.

Suppose that a watch has been made with only the main spring, the four
wheels, and the three hands, what would happen when it was wound? You
can tell very easily by winding up a mechanical mouse or a train of
cars or any other toy that goes by a spring. It will go fast at first,
then more and more slowly, then it will stop. This sort of motion
might do for a mouse, but it would not answer for a watch. A watch
must move with steadiness and regularity. To bring this about, there
is a fifth wheel. Its fifteen teeth are shaped like hooks, and it has
seven accompaniments, the balance wheel, the hair spring, and five
others. This wheel, together with its accompaniments, is able to stop
the motion of the watch five times a second and start it again so
quickly that we do not realize its having been stopped at all. A tiny
arm holds the wheel firmly, and then lets it escape. Therefore, the
fifth wheel and its accompaniments are called the "escapement." This
catching and letting go is what makes the ticking.

A watch made in this way would run very well until a hot day or a cold
day came; then there would be trouble. Heat makes metals expand and
makes springs less elastic. Therefore in a hot day the watch would go
more slowly and so lose time; while in a cold day it would go too fast
and would gain time. This fault is corrected by the balance, a wheel
whose rim is not one circle, but two half-circles, and so cunningly
made that the hotter this rim grows, the smaller its diameter becomes.
In the rim of the wheel are tiny holes into which screws may be
screwed. By adding screws or taking some away, or changing the
position of some of them, the movement of the watch can be made to
go faster or slower.

All this would be difficult enough to manage if a watch was as large
as a cart wheel, with wheels a foot in diameter; but it does seem a
marvel how so many kinds of wheels and screws and springs, one hundred
and fifty in all, can be put into a case sometimes not more than an
inch in diameter, and can find room to work; and it is quite as much
of a marvel how they can be manufactured and handled.