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GALILEO AND THE LAMPS
 IN Italy about three hundred years ago there lived a
young man whose name was Galileo. Like Archimedes he
was always thinking and always asking the reasons for
things. He invented the thermometer and simple forms of
the telescope and the microscope. He made many
important discoveries in science.
 One evening when he was only eighteen years old he was
in the cathedral at Pisa at about the time the lamps
were lighted. The lamps—which burned only oil in
those days—were hung by long rods from the ceiling.
When the lamplighter knocked against them, or the wind
blew through the cathedral, they would swing back and
forth like pendulums. Galileo noticed this. Then he
began to study them more closely.
He saw that those which were hung on rods of the same
length swung back and forth, or vibrated, in the same
length of time. Those that were on the shorter rods
vibrated much faster than those on the longer rods. As
Galileo watched them swinging to and fro he became much
interested. Millions of people had seen lamps moving in
this same way, but not one had ever thought of
discovering any useful fact connected with the
When Galileo went to his room he began to experiment.
He took a number of cords of different lengths and
hung them from the ceiling. To the free end of each
cord he fastened a weight. Then he set all to swinging
back and forth, like the lamps in the cathedral. Each
cord was a pendulum, just as each rod had been.
He found after long study that when a cord was 39 1/10
inches long, it vibrated just sixty times in a
 minute. A cord one fourth as long vibrated just twice
as fast, or once every half second. To vibrate three
times as fast, or once in every third part of a second,
the cord had to be only one ninth of 39 1/10 inches in
length. By experimenting in various ways Galileo at
last discovered how to attach pendulums to timepieces
as we have them now.
Thus, to the swinging lamps in the cathedral, and to
Galileo's habit of thinking and inquiring, the world
owes one of the commonest and most useful of
inventions,—the pendulum clock.
You can make a pendulum for yourself with a cord and a
weight of any kind. You can experiment with it if you
wish; and perhaps you can find out how long a pendulum
must be to vibrate once in two seconds.