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