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From the time that men first began to speculate about the earth, one of the principal questions was how it was held in its position in the universe, and the ancients had many curious theories regarding this subject.
One of the oldest beliefs was that the earth was supported by Atlas, the Titan, who had rebelled against the authority of the gods and was punished by being made to stand in the centre of the Western Ocean and bear the world on his shoulders.
Still another theory was that the earth rested on the back of an enormous tortoise; and a third belief, which was held by some of the Eastern nations, was that the world was carried by a large whale, whose sudden movements caused earthquakes and other such calamities. Another philosopher declared that the world floated in the ocean like an egg, the half that was above the water being the part that was inhabited.
But these different speculations failed to satisfy even the minds of the early students of nature, and as time passed and scientific knowledge increased, it was found necessary to account in some other way for the earth's support.
The establishment of the Copernican system made the problem all the more perplexing, as it was more difficult to imagine a support for a world that was whirling through space than for one at rest, and after the discovery of Kepler's laws the subject became more interesting than ever, and received a larger share of attention.
Kepler himself had suggested that the motion of the planets might be caused by spokes radiating from the sun and pushing the planets with them as they rotated. And absurd as this theory seems, Kepler spent considerable time in trying to verify it, and it was regarded as highly plausible by many other astronomers.
Descartes, the great French philosopher, invented the theory that all space was filled with air, in which there were innumerable whirlpools and vortices. One great vortex was supposed to exist around the sun, which carried the planets around, and just as the centre of a whirlpool in a river revolves more rapidly than its outer circles, so those planets near the sun would be carried around faster than those farther away. This theory accounted for the movement of the moons around the planets by supposing that they were carried by smaller vortices around their individual centres, while the elliptical figure of the orbits was explained by imagining the planets pushing one another a little out of a circular path.
But although the name Descartes was celebrated enough to cause his theory to be received with great respect, and although it was supported by some of the most eminent scientific men, it was never fully accepted, as it was thought impossible that nature, whose known laws were so simple and harmonious, should have so blundered in describing the orbits of the planets as to make it possible for them to push one another out of their paths.
Those philosophers who combined scientific experiment with a belief in astrology and the supernatural, still held the old belief of the crystal spheres in which the planets were borne around, and which has a mystic relation to the ten heavens and the atmospheres of air and fire; while a more common and simpler theory, which was admitted by some of the most learned men, was that each planet was carried through its orbit by an angel.
But the sixteenth century was a time of earnest thought, and of great men whose achievements had already made it famous in the history of science, and it was felt that whatever problem might vex the human mind would be solved at last, if painstaking labor and devotion to knowledge were of any avail.
The Copernican system had set the current of speculation in new directions, and on the day of the death of Galileo, its most famous supporter, there was born in Woolsthorp, Lincolnshire, England, a child whose name now stands as the greatest in the history of science, and whose work it was to perfect the great theory and prove its truth by means of the most splendid discovery that the world has ever known.
This was Isaac Newton, the descendant of a line of English farmers, who passed the uneventful years of his boyhood in a quiet country home, whose humdrum life gave no hint of the brilliant future in store for him.
A mile from his home was the little hamlet of Stoke, where he attended day school, and where he learned to read and write; and with this first knowledge of books, he displayed also that love of mechanics which showed even at that early age the bent of his mind. He was always making little models of machines, finding hints for them in his plays, and in the suggestions of the world of nature with which he was so early familiar; and the little water-clocks and sun-dials which he made served a still greater purpose than an hour's amusement, for they developed a sense of observation and accurate reasoning which were of the greatest service later on.
When he was twelve years old he entered the grammar school at Grantham, but attracted no attention for any especial talent, and had it not been for the books which he read at home, his school life might have passed without leaving any particular mark upon his character.
But it was during this period that Newton was attracted by some works on chemistry, alchemy, and magnetism, and the reading of these books made an impression upon his mind which was never lost, and which went far toward determining his career.
From this time a new world was opened to the thoughtful lad who, as he wandered over the meadows around his home, or through the pleasant English lanes, puzzled his head over the questions that had occupied the gravest thinkers of all ages, and wondered if ever the answers would be reached.
Newton entered Trinity College, Cambridge, in 1661, and almost immediately, attracted the attention of his teachers by his extraordinary talent for mathematics. Subjects which his fellow-students found most difficult he grasped with apparent ease, and he soon became known as one from whom great achievements might be expected. And this expectation was not disappointed, for before leaving college Newton gave proof of the originality of his mind by making certain discoveries in mathematics which at once attracted the attention of scientific men, and promised a future of renown.
It was in the same year that he left college—1665—that Newton conceived the great idea that won him eternal fame, and, strange as it may seem, this idea was something quite apart from the studies in mathematics and light which had hitherto occupied his mind.
The great question of the motion of the earth was ever before men's minds, and Newton's experiments in light and his mathematical discoveries still left room for thoughts of the problem that had not yet been solved by ancient or modern philosophers, although from time to time some hint of the meaning had been given. The old Greeks had claimed that all motion in the universe was caused by the action of two forces which they called love and hate, and the alchemists had taught that all nature was pervaded by a subtle power which could not only change base metals to gold and give man an infinite existence on the earth, but also held sway over the remotest regions of space, and bound the stars and planets in its mystic rule.
Newton's early studies had made him familiar with the older theories, and also with the laws of chemistry, which demonstrated the close relation which existed between different forms of matter. Kepler's laws exactly describe the motions of the heavenly bodies which Galileo's telescope had first proven, but the question still remained—what gave the planets their motion, and carried them around the sun—and Newton, in is twenty-fourth year, gave evidence of the masterly powers of his mind by offering an explanation so clear and yet so simple as to perfectly harmonize with the known laws of nature, and place its probability beyond a doubt.
Experiments had shown that magnetism, or the power of attraction, existed between certain bodies, but the nature and power of this force were quite unknown. The ancients were content to say that certain bodies had a breath, or life, which attracted other bodies, and so let the mysterious power alone; and in later times, while it was known that this power of attraction existed in a far greater degree than had formerly been supposed, it was still an almost unknown subject. Kepler and other astronomers even went so far as to say that the planets attracted one another, but how great this attraction was and what result it would have were not demonstrated.
Still the subject was one of intense interest to philosophers, and was ever present in their thoughts, and as the smallest incident often leads to great results, so in the case of Newton, the simple circumstance of an apple falling from a tree in the garden in which he was sitting, suggested a train of thought which finally led to the discovery of the great law which holds the planets in their courses and governs the remotest stars.
It was an accepted fact in philosophy, that all objects on the earth were held there by magnetism, or the force of attraction, and that in fact the earth was a great magnet which held all things upon it in their places, and kept them from flying off into space, just as surely as the loadstone attracted steel.
The fall of the apple from the tree led Newton to the thought that the magnetic power of the earth must also extend to things beyond its surface, and not in actual contact with it, and this suggested the still greater idea that, if the earth had any attractive power at all, this power must be felt to the farthest limit of the solar system, though in a much less degree. Newton at once perceived that if this were true the earth would exert an attraction over the moon, and he immediately undertook to see if this were so.
Ever since the establishment of the Copernican system, astronomers had been trying to find out what power kept the moon revolving around the earth; for it was evident that, according to the laws of motion, the moon would fly off into space were it not for the action of some powerful but unknown force.
Newton decided that whatever this power was, it must also exist between Jupiter and his moons in order to agree with the harmonious working of the universe, and he therefore made a calculation which proved that Jupiter's moons revolved around him and were kept in their orbits by the same power which the earth exerted over all objects on and near it, and that this power was greater or less according to the distance of the satellite from the planet; or that Jupiter exerted a certain power over the nearest moon, less power over the next in order, and so on. This being established, it was an easy matter to determine if the earth kept her moon in place in the same way. But the most accurate calculations failed to prove the truth of the theory, and Newton was obliged to own to himself that his reasoning had been at fault. He therefore said nothing of his hope or disappointment, resolving to keep both secret until time should have given better opportunities for a study of the problem.
Ten years afterward a French mathematician announced that the accepted theory of the moon's distance from the earth was incorrect, and that the moon was in reality farther from the earth than had been supposed. This discovery at once led Newton back to his old theory in regard to the attractive power of the earth, for, since the degree of attraction depended upon the distance, he saw that his former hope might still be realized. He therefore began another calculation based upon the new value of the moon's distance, and so great was his joy on finding that the numbers were coming out as he wished, that his excitement prevented him from finishing the calculation, and he had to ask the aid of a friend. This success was immediately followed up by calculations on the satellites of Saturn, and the same result was obtained. Newton then extended his observations to the revolution of the planets around the sun, and to the motion of comets; and finally, after innumerable experiments and calculations, gave to the world his great law of attraction, viz., that every particle of matter in the universe attracts every other particle with a force depending upon the weight and the distance&—a body twice as heavy as another body exerting twice the force, and a body at twice the distance exerting one-fourth the force.
This law, which is generally known as the law of gravitation, is considered the greatest discovery ever made by the human mind.
Not only did it solve the question of the means by which the planets were carried around the sun, but it proved that the planets had this motion simply because of their mutual attraction, and the attraction of the sun; and that the whole universe was governed by the same law, which kept the planets in their orbits, governed the movement of comets, and controlled the entire mechanism of the heavens.
Newton also deduced from this law the correct figure of the earth, proving that gravitation, which caused the earth to rotate on its axis, would also give it a spheroidal shape, and not make it the perfect sphere which it had been supposed to be; the simple experiment of a circular elastic hoop made to rotate around a fixed axis being sufficient to prove that a rotating body always tends to assume a spheroidal form, and to be flattened at its poles in proportion to the rapidity of movement. And although at this time there was no means of finding out the figure of the earth by actual measurement, later on it was proven by conclusive experiment that Newton's theory in regard to it was so correct as to approach very nearly to the actual amount of oblateness.
Newton also proved that tides were caused by the attraction exerted by the sun and moon upon the earth, the moon exerting much more force than the sun, because of its nearness to the earth. When the sun and moon are both on the same side of the earth their force is united, and they draw the water away from the earth toward them, and the earth away from the water at the point directly opposite; and when the sun and moon are on opposite sides of the earth the same thing happens; so that at these times—at new and full moon—the highest tides occur; the lowest tides occurring when the sun and moon are at right angles, for then their forces do not act together, one drawing in one line and the other in a line perpendicular to it, so that much of the attraction is lost.
These and many other phenomena were explained by Newton as having their origin in the attraction of gravitation, and the results of his investigations, together with his work on other subjects, were finally summed up in his great work called the "Principia," which was published in 1687, the cost of the printing being born by Halley, the astronomer, as Newton himself could not afford the expense.
Although it might have been supposed that the grand, yet simple, principles laid down in the"Principia" would appeal to every scientific mind, yet such was not the case, and Newton had to suffer from that misapprehension and prejudice which fall to the lot of every original thinker. But few people were capable of understanding the new ways of reasoning which Newton introduced, and some of the most celebrated astronomers of the day derided the conclusions as absurd and false. Books were written to prove that the phenomena of the heavens could be explained on entirely different principles from those laid down in the "Principia," and it was even said that the Newtonian philosophy was simply another form of the old superstition of the ancients, who believed in the presence of mysterious agents, working in undiscoverable ways, and holding all the universe in their subtle power. But the new thought made its way surely, if slowly, and during the next century was accepted by the whole world of science.
The mystery which had baffled the ages was unfolded at last, and the old dreams of the "world-secret," the faith of Copernicus, the vision of Galileo, and the inspiration of Kepler, were triumphantly shown to have been, not idle play, but divine leadings toward the discovery of the greatest truth of nature that has ever been revealed to man.
What this mysterious power is which binds the universe together in one harmonious whole, we do not know. We can only see its workings, and define its results, and the rest is unknown. Nature holds her grandest secrets close, and even Newton, her greatest interpreter, after a long life of research, could only sum up his experience in these significant words: "I have been but as a child playing on the sea-shore; now finding some pebble rather more polished, and now some shell more beautifully variegated than another, while the immense ocean of truth extended before me unexplored."