of motion, and that the celestial bodies were perfect, unchangeable, and of different composition from the earth.

It was about this time that the dispute arose whether observation and reasoning or the authority of the Church and the Bible, as interpreted by the Church, ought to have more weight in scientific matters, and the enemies of Galileo, who was known to have slight respect for tradition and authority, tried to draw him into a dispute with the Church. In 1615 the Inquisition declared it heresy to hold either of the doctrines that the sun was the centre of the world and immovable, or that the earth was not the centre of the world and in motion. As a result of this adverse report upon the Copernican theory the Pope ordered Galileo to be summoned and censured. In 1632 Galileo published his great treatise, the Dialogue on the Two Chief Systems of the World, the Ptolemaic and the Copernican. The dialogue form enabled him to present the Copernican case as strongly as he could through the mouth of one of the speakers without identifying himself with the opinions expressed, and he hoped that this would save him from appearing to defy the Inquisition. But as the advocate of the Copernican system always confutes the upholder of the Ptolemaic and promptly convinces the third speaker, who is neutral, it was easy to see on which side Galileo's sympathies lay. In fact the book was an unanswerable argument for Copernicanism. It aroused great opposition, and unfortunately the Pope believed that he was himself the original of the supporter of Ptolemy's views in the Dialogue, this speaker being continually held up to ridicule. Galileo was brought before the Inquisition three times, examined under threat of torture, compelled to abjure his errors, and condemned to imprisonment

and penance. The imprisonment was of a very mild description, however, and at the end of a year Galileo was allowed to retire to his country house near Florence, where he continued his work, though he was hampered in it and spied on by the agents of the Inquisition. The most important work of his last years was a book setting forth his discoveries in mechanics, a science which he may be said to have created. He died, a blind old man, in 1642.

Galileo and his contemporary the Würtemberger Kepler, who published an Epitome of the Copernican Astronomy, placed, like the Dialogue, on the Roman Index of Prohibited Books, had made it impossible, long before the half-century had elapsed that divided Galileo's death from the next astronomical discovery of prime importance, for any serious astronomer to be anything but a Copernican. During this period the telescope was much improved, and the invention of the pendulum clock, patented in 1657, revolutionized the art of exact astronomical observation.

In 1643, the year after Galileo's death, and the opening year of the Civil War in England, Isaac Newton was born. He was a man of the greatest eminence in many branches of science-astronomy, dynamics, optics, and pure mathematics-but the world knows him best as the discoverer of the law, vaguely guessed at in the speculations of Kepler and some other astronomers, of universal gravitation-viz. every particle of matter attracts every other particle with a force proportional to the mass of each, and inversely proportional to the square of the distance between them. This law and the laws of motion, investigated and defined by Galileo, Kepler, and himself, afford, as Newton showed in his Principia, published in 1687, a consistent explanation

of the observed motions of the bodies of the solar

system.

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Of Newton's work in mathematics it was said by a personal enemy : Taking mathematics from the beginning of the world to the time when Newton lived, what he had done was much the better half." The great French astronomer Lagrange declared: "Newton was the greatest genius that ever existed, and the most fortunate, for we cannot find more than once a system of the world to establish." His discoveries in astronomy were nevertheless the crown of a structure built by the thought of many men, and this he recognized, saying: "If I have seen farther than other men, it is because I have stood upon the shoulders of the giants."

It would be impossible in a short space to describe the developments in astronomical knowledge since the death of Newton in 1727. Of attempts to explain the origin of the visible universe the nebular hypothesis of the French astronomer Laplace is the most striking. This is that the solar system has been formed by some process of condensation out of some such mass as one of the nebula which the telescope reveals to us, and that stars other than the sun may have been formed in a similar way.

We owe infinitely much to the thinkers who have spent their lives in striving to make out the secrets of the universe; yet, in view of all that still remains hidden from us, any one of these may say with Newton: "To myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."

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CHAPTER XVIII

MAN DISCOVERS THE EARTH

UST as we find, however far we go back in the history of the human race, certain communities

producing men whose minds were filled with an intense curiosity about the universe, so we find in similar communities in every age men intensely curious about the earth. They were men of a different order, no doubt, those who, aided only by their will and intelli

ANCIENT BRITISH DUG-OUT BOAT

gence, were resolved to penetrate the secrets of the sun and moon and the starry tracts of sky, and those whose imaginations took fire at the sight of strange faces, strange garb, strange wares, or at the far-off wonders described in some traveller's tale. If these latter were men of a less rare type, great qualities were demanded of them when the desire to know became the will to know. The man who launched the first dug-out canoe upon a river or lake was a pioneer of extraordinary worth. Oak and triple brass," says the Roman poet, 66 were about the heart of him who first committed his frail bark to the implacable sea.' And if the wanderer Ulysses has a more enduring attraction for us than the

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warriors of Greece and Troy, it is because in nearly every one of us there exists that curiosity which in some leads to adventure and discovery, even if in most it is no more than a desire to see what is round the curve of a strange road, or over the crest of an unclimbed hill.

It would be impossible ever to write a full history of exploration. Lands have been discovered and lost and rediscovered, perhaps with a new civilization and a new race inhabiting them. Columbus' discovery of America was a rediscovery of the continent by Europeans, since the Norsemen had made their way there from Iceland about A.D. 1000, and the Norsemen may well have been preceded by Asiatic explorers. Alexander the Great, after overthrowing the Persian Empire in the fourth century B.C., pushed eastward, conquering, exploring, colonizing, as far as the borders of North-west India, yet to a medieval Englishman these lands were hardly more than legendary. The Roman Empire, stretching from the Rhine to the Euphrates, and from the Danube to the African deserts, broke up the isolation of the tribes and nations which came under its rule, and the Roman army system made it an ordinary thing for a Briton, for instance, to see service in Spain, and a Spaniard to keep the wall which Hadrian built as a barrier against the unconquered barbarians of North Britain. When the Empire fell to pieces under the attacks of the Teutonic tribes darkness settled again upon Europe, and light penetrated again but slowly, first mainly as a result of Christian missions, pilgrimages, and the wars of the Cross. The growth of men's knowledge of one another and of the earth on which they live has been like a gradual inundation that now advances, now recedes, but in the end covers all. The means to explore and the scope of exploration have