Page:Popular Science Monthly Volume 78.djvu/324

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age have the full benefit of all preceding ages, could man in very truth be "the heir of all the ages," there would be no lost arts and the world would not have to learn over and over again lessons once mastered.

A last impediment may be mentioned, peculiar perhaps to problems "like the discovery of the law under discussion. It is hard for a worker in any field not to attempt to reach forward to the final solution of his problem. This is true even if the data for generalization be most meager. Thus it has happened time after time that ill-formed theories have been advanced even by great minds. Indeed the very greatness of the man whose name the theory bears proves an added obstacle. Thus the dicta of Aristotle held sway for centuries and even Galileo's brilliant experiments at the leaning tower of Pisa could scarce overcome the false Aristotelian theory of falling bodies. Another example is Newton's theory of light which survived at least a hundred years, simply because it was Newton's,

We shall in the present paper seek to trace the history of the problem which found its final answer in Newton's Law of Universal Gravitation. This law may be stated in the following familiar terms: "Every particle of matter attracts every other particle of matter with a force proportional to the mass of each and to the inverse square of the distance between them."

In tracing the history of how the race came into a clear knowledge of this law we find two streams with their headwaters far back in history, slowly gathering volume age by age and finally uniting and bearing the world on to the long-sought-for goal. The first of these streams may be called the study of pure motion, or of kinematics, the second the study of the causes of motion, or of dynamics. The first is best illustrated by the history of astronomy as developed down to the seventeenth century, while the second is best illustrated by the history of mechanics during the same period.

In astronomy we shall pay attention only to those persons whose work has a bearing upon the present problem. The first name of worth seems to be that of Thales of Miletus (640-546 B.C.). With remarkable clearness he maintained the sphericity of the earth, the present theory of lunar eclipses, and the correct view regarding the source of the light received from the earth's satellite. He also suggested that the stars may be regarded as being of the same material as the earth. Thales was followed by his disciple, Anaximander (611-547), who was the first of the ancients to view the heavens with the eye of a philosopher. His name should be immortal, for it was he who first suggested that the earth moves about the sun as a center, a doctrine which became one of the tenets of the Ionian school.

Perhaps rapid progress might have been made in the explanation by natural causes of the phenomena of the heavens, but soon the jealous