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THE POPULAR SCIENCE MONTHLY.

very beginning he was interested in this particular problem of gravity, but it was only one of many. Moreover, his was not a mind to stop short of the full and complete solution. This force of gravity—if it be this force that governs the universe—must be expressed by an exact law; he sees more clearly than any one else what is involved. So he ponders over the problem, and one day—would that we knew the date—he is seated in his garden studying, beside him his books and papers. In the sky is the pale moon and as Newton gazes upon her he ponders

PSM V78 D344 Solar system after newton.png
Solar System after Newton.

on the world-old problem, this riddle of the ages. What force holds that moon forever circling about the earth? An apple falls to the ground beside him and its thud awakens a train of thought in his mind. Can it be the same force in the two cases—is the moon but a larger apple forever falling toward the earth, urged by the force of gravity, and forever receding from the earth, urged by her own uniform motion? Fascinated by the thought, Newton sets to calculating. He has studied out the laws of uniform motion and of accelerated motion—now he combines the two and gets the law of circular motion. He draws a