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THE LAW OF GRAVITATION.

THE DISCOVERY OF THE LAW OF GRAVITATION.
By the late Professor JOHN T. DUFFIELD,

PRINCETON UNIVERSITY.

THE law of gravitation, that all matter attracts all matter, directly as the mass and inversely as the square of the distance, whether we consider the extent of its reach, or the number and variety and peculiar interest of the problems of which it furnishes the solution, or the grandeur of many of those problems by reason of the magnitude of the elements involved; whether we consider the power which it gives us to anticipate nature, and predict with the minutest accuracy and certainty of a mathematical demonstration celestial phenomena for ages to come; we cannot but regard it as the most important truth in the whole book of nature and its discovery as the most interesting event in the history of physical science. As there is but one material universe and the law of gravitation solves the enigma of its structure, no other problem of equal interest and importance can ever occupy the attention of the student of nature.

Kepler has remarked that: "The occasions by which men have acquired a knowledge of celestial phenomena, are not less admirable than the discoveries themselves." If this be so, the history of the discovery of that great law of nature by which all celestial phenomena are determined can never cease to be a matter of peculiar interest.

In the account which we propose to give of the discovery we shall select as our chronological starting point the beginning of the seventeenth century. At that period the theory in regard to the structure of the material universe which, with few exceptions had been held from time immemorial, still prevailed. The earth was regarded as the center of the universe, about which the sun, moon, planets and stars performed their ceaseless revolutions. More than half a century before (in 1543) Copernicus, in his memorable work, 'De Orbium Cœlestium Revolutionibus,' had, indeed, announced the true system of the universe, yet as he was led to the adoption of the theory he proposed, not so much by positive evidence in its favor as by the difficulty of reconciling certain phenomena with the Ptolemaic theory; moreover, as the objections to this theory were from their very nature such that few could appreciate their force, whilst in the apparent motions of the heavenly bodies every one could see what seemed to be an ocular demonstration of its truth, it is not strange that the doctrine of Copernicus should have been for so long a time generally regarded as an