No other discovery ever made in science approaches in importance to the discovery of the principle of universal gravitation--the principle that every particle of matter is attracted by every other particle with a force proportioned inversely to the square of their distances. Vague ideas of some such principle had long been floating in the minds of some men; had probably been thus floating since ever men began to think seriously over the phenomena of inanimate nature. But the discovery of the principle was, however, as distinctly the achievement of Newton as "Paradise Lost" is the work of Milton. We find it hard now to form to ourselves any clear idea of a world to which Newton's principle was unknown. It would be almost as easy to realize the idea of a world without {273} light or atmosphere. Newton is called by Sir David Brewster the greatest philosopher of any age. Sir John Herschel assigns to the name of Newton "a place in our veneration which belongs to no other in the annals of science." In this book we have only to record the date at which the pure and simple life of this great man came to its end. The important events of his career belong to an earlier period; his teachings and his fame are for all time. The humblest of historians as well as the greatest may ask himself what is the principle of history which bids us to assign so much more space to the wars of kings and the controversies of statesmen than to the life and the deeds of a man like Newton. In the whole history of the world during Newton's lifetime, the one most important fact, the one fact of which the magnitude dwarfs all other facts, is the discovery of the principle of gravitation. Yet its meaning may be explained in fewer words than would be needed to describe the nature of the antagonism between Walpole and Pulteney, or the reason why Queen Anne was succeeded by King George.