historically, the foundation of that sublime superstructure which in a subsequent age was reared by Newton, and which, by reason of the magnitude of its proportions and the multiplicity of its details, all pervaded and determined by the most admirable unity, now stands and in all probability will ever stand, as the most imposing monument ever erected by the human intellect.
Although Kepler's theory, that bodies terrestrial mutually attracted each other, met with ready reception, more than thirty years elapsed after the publication of this work before the idea was entertained, at least favorably, of accounting for the revolutions of the heavenly bodies on the theory of the universality of the attraction of gravitation. Kepler indeed, as we have remarked above, alludes to such an hypothesis only however to expose, as he imagined, its fallacy. The motions of the heavenly bodies being curvilinear, whilst the motions of bodies under the influence of gravity were rectilinear, it was taken for granted as a thing self-evident that the two phenomena must be due to entirely different physical causes. Familiar as we are with the fact, that by the two laws of motion above mentioned, the hypothesis of an attractive force of the sun, combined with the hypothesis of a tendency of the planets to move in a straight line in virtue of an original impulse communicated to them, would satisfactorily and readily account for their curvilinear motion, it cannot but be a matter of surprise that the truth should have remained so long unrecognized.
The credit of having been the first to generalize the idea of gravity, and refer the revolutions of the heavenly bodies to the attraction of matter for matter, appears to be due to Borelli, an Italian philosopher, a pupil of Galileo. It is announced in a work which he published "On the Satellites of Jupiter,' in 1666, although, as we shall have occasion to notice subsequently, Newton had conceived the same idea at least as early as 1665. Both Newton and Huyghens, however, attributed to Borelli the honor of having been the first to announce the important truth.
The idea, having been suggested, was at once accepted by many with favor and immediately led to the investigation of a hitherto unexplored field in the department of mechanical philosophy. Whilst the labors of others in this field were not unimportant, particularly those of Wallis, the name which is especially deserving of honorable mention in this connection is that of Huyghens. In a work published in 1672, we meet for the first time with a scientific discussion of the doctrine of Central Forces. His investigations were remarkably satisfactory and complete as to the phenomena of circular motion, the attractive force being at the center and contributing largely to the success of the labors of subsequent inquirers.
