3 i4 THE POPULAR SCIENCE MONTHLY 



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 over- 

 come 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 prob- 

 lem which found its final answer in JSTewton's Law of Universal Gravi- 

 tation. This law may be stated in the following familiar terms: 

 " Every p article 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 



