In that great city dwelt a man called Archimedes. He was a rich and wise man, a friend and relative of the king, and had he chosen, he could have spent all his days in the pursuit of pleasure, going to the races and matches and theatres, and dining every day at the king’s table. But his mind was given to the pursuit of science and of truth. His great delight lay in studying the laws of Nature, and finding out the secrets of her working. He believed that everything happens according to some law or principle, and that if once he could discover the law he could rule the world. Archimedes was born 300 years before Christ. When he was young, war was raging in Sicily between the Greeks and Romans and the people of the city of Carthage in Africa, each wishing to possess the beautiful and fertile island. The king of Syracuse joined the Romans, and as they proved the strongest, he was able to reign in peace for fifty years. During that time the trade and commerce of the city flourished. The king built many ships, some of them larger than any seen before, and these ships sailed to Egypt and Greece, Africa and Spain, and along the coasts of France and Italy, exchanging grain and fruit for purple and gems, Arabian horses, and amber from the shores of the Baltic. Archimedes spent much of his time in the harbour and in the shipbuilding yards, watching the sailors and shipwrights at their work, and helping them with his new inventions. He saw them lift heavy weights, and taught them how to do it without great exertion. They already knew how to use a lever, or crowbar, for lifting heavy stones. They pushed the end of the lever under the stone, and rested the lever or bar on another stone close to it. [Illustration: Fig. 1.--A Lever to move the Earth.] In that way they could exert a great force. Archimedes measured the force they could exert in this way, and found it was increased when the bar was made longer and the distance between the stone and the support was made less. When the distance between the support and the hand was five times the distance between the support and the stone, Archimedes found that the force of the hand was multiplied by five, and one man could lift a stone which it would take five men to lift without a lever. “Now,” he said, “if you make the lever long enough there is no weight too heavy for us to lift.” He told the king about this discovery, and said to him, “If you gave me some place on which to stand, I could move the whole earth.” [Illustration: Fig. 2.--The Endless Screw.]