In 1669 Newton became professor of mathematics at Cambridge. Three years later he joined the Royal Society. The Royal Society was a group of learned men from all branches of science. Before long Newton began again to study the problems of motion. He had already discovered the essential ideas, but it still remained for him to solve the difficult mathematical problems. At last he seemed to have solved the main difficulties. But he did not publish his findings at once.

Only in 1687 did he at last publish his new theory. Newton's great work, The Mathematical Principles of Natural Philosophy, marked the triumph of the Scientific Revolution. The very title is significant. Newton had found the mathematical principles, the scientific laws which governed the movements of earth and heavens. The book completed the working out of a new view of nature, a task begun by Copernicus.

The result was an exact mathematical world. Newton put forward three laws of motion in the book.

The first law stated that bodies will tend to move in a straight line with uniform motion unless acted upon by a force. Thus, a bullet shot from a gun moves straight ahead until it is stopped by a target or it slows and falls as a result of the friction¹ caused by moving through air.

The second law stated that the force applied² to a body is in proportion to the acceleration³ of the body. Thus, the harder you throw a ball the faster it will move.

The third law said that every action has an equal and opposite reaction. Thus, when you hit a punching bag, it bounces right back at you.

Newton also worked out a mathematical expression for gravity. It applied equally to the apple falling from the tree and the moon going around the earth. Newton was soon recognized as the leader of English science. In 1703 he became president of the Royal Society.

Science was never quite the same after Newton's discoveries. Little wonder that the eighteenth-century poet Pope, looking back at Newton's work, wrote: "Nature and Nature's laws lay hid in night; God said, Let Newton be! -- And all was light."

Newton, however, never rested on his fame. He continued to work and study. In his last years he once said to a friend, "I do not know what I may appear to the world, but to myself I seem to have been only like a boy, playing on the seashore, and now and then finding a smooth pebble⁴ of a pretty shell, while the great ocean of truth lay undiscovered before me."