INTRODUCTION.

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was the centre, and the satellites revolved in smaller vortices roundtheir respective primaries. This system offered a plausible explana-tion of the motions of the planets and satellites in one common jdirection; but it was inconsistent with the motions of comets, and a jmultitude of other phenomena, and, besides, was nothing else than a jmere gratuitous assumption. Some writers commend the Cartesian sys- jtem of vortices, as the earliest attempt to explain the motions of the Iplanets by mechanical principles; but Delambre has justly remarked, that, jby misleading men’s minds from nature, this fiction of the imaginationretarded rather than promoted the progress of true science. Descartes ,however, deserves honourable mention in the history of Astronomy onaccount of the vigorous efforts he made to overthrow the Aristotelian phi-losophy, and especially for his important discoveries in the pure mathe-matics. By his happy innovation of expressing the fundamental propertyof a curve by means of an equation between two variable co-ordinates, heextended incalculably the powers of analysis, besides thereby preparingthe way for the discovery of the infinitesimal calculus, and its applicationto the vast domain of Celestial Dynamics .

Nearly about the same time with Descartes flourished Huygens , a phi-losopher endued with equal genius, but exhibiting greater caution in hisresearches. Posterity is indebted to him for one of the most admirableinventions of modem times—the application of the pendulum to clocks.Mechanical constructions moved by weights had been employed to measuretime as early as the thirteenth century, and Galileo had already conceivedthe idea of using the pendulum for a similar purpose. The Italian phi-losopher failed, however, in all his attempts to construct an accurate time-keeper, because he constantly sought to apply the pendulum as the primemover. Pluygens accomplished this object with the most compdete suc-cess, by simply making the pendulum to regulate the descent of the weightin the ancient clocks. It would he difficult to say whether the ordinaryconcerns of life, or the more refined purposes of science, have gainedmost, by this valuable improvement. Huygens is distinguished by histelescopic discoveries in the heavens. He it was who first established thereal character of the appendage with which Saturn is furnished, havingfound it to consist of a luminous ring, encompassing the body of theplanet, at an appreciable distance from his surface. He also discoveredthe most conspicuous of the satellites of that planet; but he forgot hishabitual caution when he asserted that as his discovery made the numberof satellites equal to that of the planets, no others of a similar kind wouldbe made in the solar system. Huygens discovered the principal theoremsrelative to the motion of a body compelled to revolve in a circular orbit,under the influence of a force acting constantly at the centre. Thesetheorems were announced at the end of his treatise, “ De Horologio Oscil-latorio,” published in the year 1671 ; but no demonstration was given ofthem. By his elegant speculations on the evolutes of curves he also faci-litated the application of the same principles to orbits of variable curva-ture. This philosopher is indeed universally admitted to he one of themost original geniuses who flourished in the seventeenth century. In hisintellectual character there appears the rare union of all those qualitieswhich form the mathematician and the experimental philosopher. In thisrespect he approaches more nearly to the illustrious Newton than anyother individual of modern times.

Cassini, the contemporary of Huygens , was one of the greatest astro-