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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 phenomenon.
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.
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