of the most beautiful discoveries ever. made in astronomy, the aberration of the fixed stars, and the nutation of the axis of the Earth.

When the application of the pendulum to clocks, and of telescopes to quadrants, had rendered the slightest changes in the position of the celestial bodies perceptible. to observers, they endeavoured to determine the annual parallax of the fixed stars; for it was natural to suppose, that so great an extent as the diameter of the terrestrial orbit, would be sensible even at the distance of these stars. Observing them carefully, at every season of the year, there appeared slight variations; sometimes favorable, but more frequently contrary to the effects of parallax. To determine the law of these variations, an instrument of great radius, and divided with extreme precision, was requisite. The artist who executed it, deserves to partake of the glory of the astronomer who owed his discovery to him. Graham, a famous English watch-maker, constructed a great

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sector, with which Bradley discovered the aberration of the fixed stars, in the year 1727. To explain it, this great astronomer conceived the fortunate idea of combining the motion of the Earth with that of light, which Roemer had discovered at the end of the last century, by means of the eclipses of Jupiter's satellites. should be surprised that none of the distinguished philosophers who then existed, and who knew the motion of light, should have paid any attention to the very simple effects which result from it, in the apparent position of the fixed stars. But, the human mind, so active in the formation of systems, has almost always waited till observation and experience have acquainted it with important truths, which its powers of reasoning alone might have

discovered.

It is thus that the invention of telescopes has followed by more than three centuries that of lenses, and even then was only due to accident.

In 1745, Bradley discovered by obser

vation, the nutation of the terrestrial axis. In all the apparent variations of the fixed stars, observed with extraordinary care, he perceived nothing which indicated a perceptible parallax. The measures of the degrees of the terrestrial meridian, and of the pendulum, multiplied in different parts of the globe, of which France gave the example, by measuring the whole arc of the meridian, which crosses it, and by sending the academician to the north and to the equator, to observe the magnitude of these degrees, and the intensity of the force of gravity. The arc of the meridian, comprised between Dunkirk and Barcelona, determined by very precise observations, and forming the base of the most natural and simple system of measures; the voyages undertaken to observe the two transits of Venus over the Sun's disk, in 1761 and 1769, and the exact knowledge of the dimensions of the solar system, which has been derived from these voyages; the invention of achromatic telescopes, of chronometers, of the sextant

and repeating circle, the discovery of the planet Uranus, by Herschel, in 1781; that of its satellites, and of two new satellites of Saturn, due to the same observer, all the astronomical theories being brought to perfection, and all the celestial phenomena, without exception, being referred to the principle of universal gravitation. These, with the discoveries of Bradley, are the principal obligations which astronomy owes to our century, which, with the preedino will always be considered as the most glorious epoch of the science.

CHAP. V.

Of the Discovery of universal Gravitation.

AFTER having shewn by what successive

efforts the human mind has attained the knowledge of the celestial motions, it only remains to consider the means by which it has arrived at the general principle, on which these laws depend. Descartes was the first who endeavoured to reduce the motions of the heavenly bodies to some mechanical principle. He imagined vortices of subtle matter, in the centre of which he placed these bodies. The vortex of the Sun forced the planet into motion; that of the planet, in the same manner, forced its satellite to revolve round it. The motion of comets traversing the heavens in all directions, destroyed these vortices, as they had before destroyed the