CHAPTER VIII.

MARS-THE PLANETOIDS-METEORS, AND METEOR PLANETS

Exterior Planets-Mars-Ruddy Colour-Varying Appearance-Elements of the Planet - Analogy to the Earth-The Planetoids - Great Gulf between Mars and Jupiter-Inference of Kepler-Bode's Law-Meeting of German Astronomers-Discovery of Ceres, Pallas, Juno, and VestaRecent Discoveries of Small Planets-The Planet-Finders - Characteristics of the Small Planets - Hypothesis of Olbers - Meteors and Meteorites - Shooting-Stars-Leading Features of their ApparitionLoud Explosions-Meteoric Showers - Medieval Chronicles - Grand Shower of 1799-Extraordinary Display of 1833-Conclusions relative to it-Fall of Bodies to the Earth-Examples of Aerolites-Their Chemical Analysis - Presumed Volcanic, Lunar, and Atmospheric Origin — Planetary Theory.

We now come to the superior or exterior planets, those which never pass between us and the sun, but wander to the greatest angular distance from the central sphere, being sometimes seen on the meridian at midnight, rising at sunset, and setting at sunrise. MARS, the nearest of these bodies, is readily distinguished by a ruddy appearance, noticed from remote antiquity; and probably this fiery hue originated the name, derived from the god of war, in the ancient mythology. The dark red light reflected is generally imputed to a very dense and extensive atmosphere, which, like our own, is penetrated by the yellow and red rays more easily than by the blue and violet. But some consider the cause to be geological, conceiving the general soil to be thus coloured, like the red sandstone districts of the earth, but more decidedly. The planet is remarkable for striking changes of aspect, sometimes shining as a dazzling orb with a large diameter, and then gradually dwindling to an almost imperceptible speck. This is

owing to the great change in its distance from us, for it varies from under fifty millions of miles to nearly five times that extent, or to upwards of two hundred and thirty millions. Thus, when the planet and the earth are on the same side of the sun, it is clear they will be nearer by the entire diameter of the terrestrial orbit, than when the two bodies are in opposition to each other. Hence the change from a faint hazy appearance to a lustre which, under the most favourable circumstances, rivals the glow of Jupiter. On the 27th of August 1719, when the planet was at the nearest point to the earth, and at the same time in perihelion, or at the nearest point to the sun, its brightness was so great as to alarm the ignorant; and by some it was mistaken for a comet or a new star.

Mars has a diameter of 4300 miles, nearly one-half less than that of the earth. He revolves round the sun at the mean distance of 140,000,000 of miles, and accomplishes a revolution in 687 terrestrial days, equal to nearly two years. By observation of spots upon the surface, his rotation upon a fixed axis has been nicely ascertained to be performed in rather more than twentyfour hours and a-half. Thus intervals of day and night are experienced similar in length to our own; and as the axis has about the same inclination to the plane of the orbit, as in the case of the earth, there is the same succession of seasons. Under powerful telescopic examination, well-defined seas, of a greenish hue, and continents, are observed, with the geographical lines of demarcation beautifully distinct. But having no attendant moon to attract the waters, the oceans must be almost tideless. Bright appearances at the poles, changing in magnitude and luminosity, are supposed to be the light reflected from masses of snow and ice

accumulated in the polar regions. The changes exhibited are obviously connected with the summer aud winter seasons as their cause. When emerging from the darkness of their long winter, the bright spots are the most distinct, and at the close of the long summer they are the least perceptible, as if the ice and snow had partially melted under the dissolving action of the solar rays. These are the chief facts known respecting our "celestial cousin-german," as Mars has been called, owing to the strong analogy between him and the earth.

THE PLANETOIDS.

It was long remarked with surprise, that on proceeding outwardly in the system, the interplanetary spaces are proportionate to each other till we come to the orbit of Mars, where an apparent interruption to the general order occurs. Thus Mercury and Venus are separated by about 31 millions of miles; Venus and the Earth by 25 millions; the Earth and Mars by 48 millions. But the orbit of Jupiter, the next of the primary planets, is nearly 340 millions of miles beyond that of Mars, or considerably more than twice the whole distance of Mars from the sun. This seeming anomaly arrested the attention of Kepler, who sagaciously inferred the existence of an unknown planet in the enormous gap, boldly predicted its discovery, and assumed its reality in theorizing upon the system. The idea was strongly revived towards the close of the last century by the following curious but purely empirical law, detected by Professor Bode of Berlin :

If the third sum, 10, be taken to represent the earth's distance from the sun, the remaining terms of the series will represent very nearly the distances of the other planets. Thus,

The fifth term in the series is blank, and marks the immense interval between the orbits of Mars and Jupiter, where it was supposed a planet ought to be, and would be discovered. Bode's law teaches, that if, proceeding through the system outwards from the sun, the interval between planet and planet is measured, it will be found that each successive interval is about twice as great as the one before, and only half that of the succeeding one, subject to the striking exceptional case referred to. But as this numerical harmony is not fulfilled with absolute precision, it is regarded as a striking coincidence, rather than an astronomical law.

So strongly did the great leap from Mars to Jupiter impress the German astronomers with the idea of a planet revolving in the intermediate region, that, at a meeting held in Lilienthal in the year 1800, it was determined to raise the "hue and cry" regularly after the long-suspected world. Accordingly, an association of twenty-four practical observers was formed; and the whole zodiac was divided into twenty-four zones. Each zone was committed to a particular observer, whose duty it was to scrutinize the province assigned him, note the telescopic fixed stars in it, and detect, if possible, any moving body among them. The eminent Schroeter was appointed president of this association, and Baron de Zach secretary. It is obvious, that by noting down with the utmost precision the number, magnitude, and relative places of all the stars in a given

region, any subsequent changes might be easily recognised upon the region being re-examined. It would be exactly the same as if a daguerreotype picture of that part of the heavens had been taken, which, being compared with another picture taken twelve months afterwards, the comparison of the two would immediately show if any object had wandered from its place, or any stranger had intruded.

Piazzi, the Italian astronomer, though not a member of the planet-seeking fraternity, was the first to justify the anticipations indulged. This was at Palermo, in Sicily, on the new-year's day of 1801, the first of the nineteenth century. While carefully examining a part of the constellation Taurus, he observed a small star not marked in any catalogue, which, on the following evening, had sensibly changed its place, and which he at first announced as a comet, misled by a hazy appearance. It was then approaching the sun, and soon ceased to be visible. But on extricating itself from the solar rays, its planetary character was revealed. The new body received the name of CERES, after the titular goddess of Sicily. King Ferdinand wished to commemorate the event by a gold medal bearing the astronomer's likeness; but Piazzi suggested that the sum to be expended might be more usefully applied to the purchase of a large telescope. The orbit of Ceres exactly answered to the place where a planet was presumed to be absent. But while the whole astronomical world was delighted with this verification of its conjectures, the result was somewhat disappointing and perplexing. The new planet proved an anomalous object in the system, being of excessively minute size, and having an orbit very considerably exceeding in its inclination to the ecliptic that of the old planets.