HISTOBY OF PHYSICAL ASTRONOMY,

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nil these qualities in so high a degree, that he stands without a rival ;among ancient or modern philosophers. His discovery of Universal Gravitation , beyond all comparison the greatest achievement that thehuman mind can boast of, affords abundant illustration of the truth of fthis remark. Throughout the magnificent train of investigations which ;that discovery suggested to his mind, we see him constantly uniting the tsagacious and comprehensive views of the genuine interrogator of nature jwith the fertility of invention, the skilful research, the profundity and jelegance, of the consummate mathematician. We have, in fact, presented ;to us the unexampled combination in one individual of all those attributesof genius which ennoble the human intellect, and which have thrown thehalo of immortality around the names of Kepler and Leibnitz —of Galileo 1and Descartes —of Bradley and Laplace.

The transcendent powers of Newton ’s intellect are equally discerniblein his inductive ascent to the principle of gravitation, and in his sub-sequent developement of its numberless consequences. Notwithstandingthe sagacity he exhibited in connecting the fall of a stone at thesurface of the earth with the motion of the moon in her orbit, andboth of these phenomena with the motions of the planets round the sun,he would inevitably have failed in establishing this sublime conception asa physical truth, if he had not also possessed sufficient mathematicalgenius to solve the problem of central forces for an orbit of variable cur-vature. To those who are acquainted with the state of mechanical sci-ence in Newton ’s time it would be superfluous to mention that the highestpowers of invention were indispensable for this purpose. When we reflecton the fact that Kepler spent a considerable part of his life in vain efforts ito establish a connexion between the motions of the planets and the con- jtinual agency of some physical principle, that the question entirely escaped ithe sagacity of Galileo , and that Huygens , although in complete posses-sion of the laws of motion, was unable to advance in its solution beyondthe case of a circular orbit, we may well imagine the obscurity in which jjit was enveloped, and the mathematical difficulties which the investigation jmust have offered. Even when Newton had succeeded in this research, 1he merely established the mutual gravitation of the planets, accord- ?ing to the law of the inverse square of the distance, but he was not ,also enabled to extend the same principle to the ultimate particles jjof which the masses of the planets are composed. In order to effectthis object, and thereby to establish the law' of gravitation in itswidest generality, he was compelled to determine the effect of theattraction of a spherical agglomeration of particles. This problem isof a totally opposite nature to the one already referred to; for here ivehave an infinite number of particles in juxtaposition, all attracting thebody with unequal intensities and in different directions. Its intricacy ismanifest at first sight; nor was this circumstance compensated by any 1preliminary hints calculated to facilitate its solution, for the mere con- Jception of such a problem had not yet occurred to any mathematician. 3Newton , however, again triumphed over opposing difficulties, and thus 3succeeded in riveting, with the bonds of demonstrative reasoning, all the ;links of his magnificent generalization.

In redescending from the principle of universal gravitation, and pur-suing it into its remoter consequences, he displays even more astonishingforce of genius than he does in the course of his inductive ascent. Itmight be supposed that when once the highest step of generalization was