beautiful marble bell tower which leans
over as if it were just about to fall to the ground. Yet it has stood
in this position for hundreds of years and has never given a sign of
toppling. The foundations on which it rested sank down into the ground
on one side while the tower was being built (it took over 200 years
to build it), and this made it tip. But the men who were building
it evidently felt sure that it would not fall over in spite of its
tipping. They knew the law of stability.

[Illustration: FIG. 13. The Leaning Tower of Pisa.]

All architects and engineers and builders have to take this law into
consideration or the structures they put up would topple over. And
your body learned the law when you were a little over a year old, or
you never could have walked. It is worth while for your brain to know
it, too, because it is a very practical law that you can use in your
everyday life.

If you wish to understand why the Leaning Tower of Pisa does not fall
over, why it is hard to walk on stilts, why a boat tips when a person
stands up in it, why blocks fall when you build too high with them,
and how to keep things from tipping over, do the following experiment
and read the explanation that follows it:

EXPERIMENT 12.[2] Unscrew the bell from a doorbell or a
telephone. You will not harm it at all, and you can put it
back after the experiment. Cut a sheet of heavy wrapping paper
or light-weight cardboard about 5 x 9 inches. Roll this so as
to make a cylinder about 5 inches high and as big around as
the bell. Hold it in shape by pasting it or putting a couple
of rubber bands around it. Cut two strips of paper about an
inch wide and 8 inches long; lay these crosswise; lay the
bell, round side down, on the center of the cross. Push a
paper fastener through the hole in the bell (the kind shown in
Figure 14) and through the crossed pieces of paper, spreading
the fastener out so as to fasten the paper cross to the
rounded side of the bell. Bend the arms of the cross up around
the bell and paste them to the sides of the paper cylinder so
that the bell makes a curved bottom to the cylinder, as shown
in Figure 15.

[Illustration: FIG. 14.]

[Footnote 2: TO THE TEACHER. If you have a laboratory, it is well to
have this cylinder already made for the use of all classes.]

[Illustration: FIG. 15. In this cylinder the center of weight is so
high that it is n