PLANETARY MACHINE S.
THE machines contrived for exhibiting the motions of the planetary bodies, in different ages of the world, have been mechanical representations of the particular systems that prevailed at the respective times of their construction, and the names given to them seem to have been dictated by circumstances of an arbitrary nature. The appearance of certain wandering bodies traversing the regions of space, by motions sometimes direct and sometimes retrograde, in very early ages attracted the notice equally of shepherds and philosophers ; and for centuries before telescopes were invented, continued series of observations on the compa rative changes of place had determined the various periods of seven primary bodies, with a degree of accuracy that has excited the admiration of succeeding generations. The different suppositions on which the phenomena arising out of these varying motions were attempted to be account ed for, gave rise to different planetary systems; and the principal difficulty to be solved was, whether the sun or the earth was to be considered as the planet next to Mars; or, in other words, whether the duration of the year de pended on the revolution of the sun round the earth, or of the earth round the sun. And also, whether the moon, and two inferior planets, revolved round the body that pro duced the change of seasons, or round the centre of the system. But whatever opinion predominated in any age, it is pretty certain that the mechanism contrived to illus trate that opinion produced corresponding motions. His tory informs us that astronomy was cultivated by the Chi nese, Brahmins, Chaldeans, and Egyptians, many centuries before the Christian era ; and there can be no doubt but that celestial mechanism, of one construction or another, formed one of the many images which superstition, if not philosophy, pointed out as a proper representative of ce lestial objects. The three spheres, made more than 2000 years before our Saviour by the Chinese sages Yu•chi, Zuni, and Chun, appear to have contained each seven pla nets, viz. the Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn, with the earth in the centre, round which they probably revolved in different periods of time by some species of mechanism not sufficiently described. The
idea that the sun was the fourth planet continued a part of the Ptolemaic system, and the astronomers of the present day, though greatly enlightened, and advocating a system that accounts for all the various changes among the celestial phenomena, yet retain a variety of terms that are calculated to confuse rather than rectify the natural notions of man in his attainment of correct information on the subject of planetary motions, while they continue to bear testimony to the former existence of a system which is now exploded. We still speak of the sun's longitude, latitude, right as cension, declination, motion in the ecliptic, &c. as if we still believed him to he actually a planet, and the earth at rest ; and these expressions having become familiar, will probably be transmitted to future generations as appropriate terms in the science of astronomy.
The veneration in which works of genius have been held in all ages has been marked in no instance more clearly than in cases where the celestial sphere has been imitated by human ingenuity. The sphere of Archimedes formed a subject for the Roman poet Claudian, about two hundred years before the Christian epoch ; and Cicero made the invention of Possidonius' sphere the theme of his admira tion in his second book a De .Vaturd Deorum," in which he entertains no doubt but that the barbarous Scythian or Britain would acknowledge its workmanship to be a proof of the perfection of reason.
We do not find that Ptolemy, who, in the second cen tury, rejected the Egyptian system that gave rise to the Latin names of our week, and who substituted his system of cycles and epicycles, ever attempted the mechanical construction of a machine that should represent these mo tions, though, in his Almagest, he has given an account of a sphere on which he depicted the constellations, and to which he could refer the apparent paths of the plane tary bodies, as Eudoxus had done nearly 500 years before him.