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in which they are illustrated. In Fig . XXIV., for instance,we have a copy, upon a reduced scale, of a diagram of theapparent path of Mercury, relatively to (or, in other words, asseen from) the Earth , between the years 1708 and 1715, whichwas presented to the French Academy of Sciences by J. D.Cassini, on August 7th, 1709 ; the dotted line, about half-waybetween the greatest and least distances of the planet, beingintended to represent the apparent annual path of the Sun. Itis of course to be understood that by the apparent path of anyplanet we refer to its movements relatively to the fixed stars,amongst which it appears to journey, as the result of its onwardprogress in its orbit, quite apart from, and in addition to, theapparent daily motion which it has around the Earth from eastto west, in common with the Sun, the Moon , and all the stars,as the result of the Earth ’s rotation upon its axis from westto east.
In Fig . XXV., which is a copy of a portion of another byCassini, and which, so far as it extends, is drawn to the samesize as the original, we see a similar representation of theapparent paths of the two great planets, Jupiter and Saturn .The Earth , which, in order to save space, is not represented in thediagram, must be supposed to be situated some way beneath it,so that it would be in the centre of the series of loops, in thecomplete figure, which Jupiter would appear to describe aroundit in about 12 years, and Saturn in about 29| years. So far aswe have copied Cassini’s drawing, the apparent path of Jupiter is shown for about three years, beginning with the middle ofthe year 1716; and that of Saturn for about four years, fromthe middle of the year 1708; the monthly places of the planetsbeing indicated by small dots.
Whether, then, we observe a plauet whose orbit lies betweenthe Earth and the Sun, such as Mercury (technically called aninferior planet), or one such as Jupiter or Saturn , whose orbitis outside that of the Earth (technically called a superiorplanet), it is found, as these diagrams show, that, in either case,the planet appears to pursue a lengthened onward course; thenit pauses, in order, as it were, to double back for a shorterperiod upon its previous path; after which, having made