more matter.* The planets, besides their revolution round the Sun, have all a rotatory motion on their own axes, like the bias-bowl as it moves along the green; and this motion the Sun also performs on his axis. That he has, moreover, a motion in infinite space, with all the planets in his train, towards or round some point of the Universe, is now regarded by astronomers as a demonstrated truth. Of the thirteen bodies which, with the Sun in the centre, constitute what is called the Solar or Planetary System, five, besides the Sun and the Earth, were known to the ancients, and distinguished by them from the firmament of fixed stars. The first addition to the number of bodies composing the Solar System as known to the ancients, was made in the year 1781, when Sir William Herschel discovered, far beyond the orbit of Saturn, the Planet at first called Georgium Sidus, or Herschel, but generally known now by the name of Uranus.-This body was regarded as the most remote in the Solar System till the year 1846, when the noblest triumph of astronomical science since the days of Newton and Halley was achieved by the discovery of Leverrier's Planet,† at a * I owe to my distinguished colleague, Professor Forbes, the correction of a numerical blunder, which, in former Class editions of these Outlines, (for they are now for the first time published,) was copied from Professor Robison's Mechanical Philosophy, vol. I. pp. 371 and 553, (Ed. 1804.) This Planet, whose nomenclature can scarcely be considered as yet settled, I have taken the liberty of distinguishing by one of the names proposed-that of Janus-which appears to me preferable to the more accepted one of Neptunus, as being purely classical, and less disturbed by modern and vulgar associations. The new planet is accordingly inserted under the title of Janus, in the sub distance from the Sun nearly twice that of Uranus. What must make the discovery of this new planet for ever memorable as one of the proudest efforts of human intellect, is the fact, that it was the fruit, not of accident or improved instruments, but of that sublime combination of observation and theory which enabled the French philosopher, and his less fortunate but perhaps not less meritorious rival, Mr Adams, of Cambridge University, "not only to point out the spot in the heavens where a Planet hitherto unknown would be found, but to weigh its mass, number the years of its revolution round the Sun, and tell the dimensions of its orbit, before the eye even of the astronomer had singled it out from the thousands of stars that twinkle in the firmament."* joined Tabular View of the Solar System, which I venture to recommend to the reader, as one of the most accurate and comprehensive that has yet been given to the world. I am indebted for it to the high scientific acquirements and great kindness of my colleague, Professor Piazzi Smyth.-See Note C. * See Scotsman Newspaper of 7th October 1846. The paragraph that follows the words quoted in the text is given below, as containing a brief and clear exposition of the remarkable discovery of Leverrier. The article from which the extracts are taken was written by Mr C. Maclaren, whose merits as an Editor and a Geologist are acknowledged by all, but who is not so generally known to be the Author of 'the best Essay that has yet been written on the Topography of the Plain of Troy."*" All who have dipped into the elements of astronomy, know that the planets disturb each other's motions by their mutual attractions. Jupiter and Saturn afford the best example of the perturbations arising from this sort of action and reaction, which produces a cycle of small * I quote these words of Lord Ashburnham, spoken to me many years ago, because, from his Lordship's long residence in Asia Minor, his frequent visits to the spot, and his familiarity with all that has been written on the subject, I regard him as one of the highest authorities that could be referred to. The five small planets, or asteroids, as they are called, which all move in the space between the orbits of Mars and Jupiter, were unknown till the beginning of the present century. Astræa, indeed, the smallest of them, was discovered so late as December 1845.* The asteroids are distinguished from the other eight planets by three peculiarities: 1st, by their comparatively diminutive size, all of them being much smaller than the Moon; † 2dly, by the greater changes in their orbits, only completed in 850 years. The forces exerted by the one planet on the other, may accelerate its motion, or may retard it; may lengthen the major axis, or may shorten it; and in other respects slightly alter the form or the position of its orbit. No permanent derangement of the system, however, results from this cause; for in 850 years the changes in the one direction compensate those in the other. During half that period, for instance, the forces tend to increase the major axis of Jupiter's orbit, and diminish that of Saturn's; and during the other half they tend to increase the major axis of Saturn's orbit, and diminish that of Jupiter's. In the one period the planet's angular motion is less than its average rate; in the other it is greater. These inequalities are, indeed, extremely minute, but modern astronomy can appreciate and measure them. Given the position, mass, and periodic times of two planets, the astronomer is able (though it is no easy task) to calculate the perturbations which either will produce on the other. But the problem which is the counterpart of this given the perturbations, to find the position, mass, and periodic time, of an unknown disturbing body-is one of such infinite difficulty, that certainly few astronomers believed it to admit of a satisfactory solution. It must be kept in mind, that the inequalities of motion which are the effect of the disturbing force are always exceedingly small, and that in the case of Uranus the recorded observations are less numerous and less ancient than for the other planets. Now, this is the problem which M. Leverrier undertook to resolve, and has resolved with triumphant success." * By Hencke, a German observer, who is not to be confounded with Encke, the discoverer of the planetary comet that bears his name. + At railway speed one might make the circuit of the globe Astræa in less than a day. |