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miles per second. The Martial day is about 40 min. longer than ours, and the year contains about 668 Martial days, equal to 687 terrestrial days (nearly two years).

Distance From Earth.—When in opposition, the distance of Mars is (like that of all the superior planets) the difference between the distance of the planet and that of the earth from the Sun: at conjunction it is the sum of these distances. If the orbits were circular, these distances would be the same at every revolution. The elliptical figure, however, occasions much variation. Thus, if it is in perihelion while the earth is in aphelion, the distance is 126,000,000 - 93,000,000 = 33,000,000 miles.

Dimensions.—Its diameter is a little less than 5,000 miles. Its volume is about \ that of the earth, but as its density is only \, it follows that its mass is only \ of the terrestrial mass. A stone let fall on its surface would fall not quite five feet the first second. It is somewhat flattened at the poles, and bulges at the equator like our globe.

Seasons.—The light and heat of the sun at Mars are less than one half that which we enjoy. Its axis is inclined about 28.7°, therefore its zones and seasons do not differ materially from our own: its days, also, as we have seen, are of nearly the same length Since, however, its year is equal to neaily two of our years, the seasons are lengthened in proportion. There must be a considerable difference between the temperature of its northern and southern hemispheres, as the former has its summer when 26,000,000 miles further from the sun than the latter: an increased length of 76 days may, however, be sufficient compensation. It has an atmosphere like our own, loaded with clouds. Mars has no moon. Its nights, therefore, are dark. Our own earth and moon must present in its evening sky a very beautiful pair of planets, showing all the phases which Mercury and Venus present to us, the two always remaining within one half the moon's apparent diameter of each other.

Telescopic Features.—Under the telescope, Mars exhibits slight phases, but by no means to the same

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extent as the inferior planets. Its surface appears covered with dusky patches, which are believed to be continents: these are of a dull red hue. Other portions, of a greenish tint, are considered to be bodies of water. The proportion of land to water on the earth is reversed in Mars. "Here every continent is an island; there every sea is a lake: but these, like our own continents, are chiefly confined to one hemisphere, so that the habitable area of the two globes may not differ so much as the size of the planets." The ruddy color of the planet is thought by Herschel to be due to an ochrey tinge in the soil; by others it is attributed to peculiarities of the atmosphere and clouds. Lambert suggests that it is the color of the vegetation, which, on Mars, may be red instead of green. There are constant changes going on in the brightness of the disk, owing, it is supposed, to the variation of the clouds of vapor in its atmosphere. No mountains have yet been discovered. In the vicinity of the poles are brilliant white spots, which are considered to be masses of snow. The "snow zones" apparently melt and recede with the return of summer in each hemisphere, and increase on the approach of winter. We can thus from the earth watch the formation of polar ice and the fall of snow—in fact, all the vicissitudes of the seasons on the surface of a neighboring planet.

THE MLNOK PLANETS.

V Discovery.—Beyond Mars there is a wide interval which until the present century was not filled. The bold, imaginative Kepler conjectured that there was a planet in this space. This supposition was corroborated by Titius's discovery of what has since been known as Bode's law.

Take the numbers 0, 3, 6, 12, 24, 48, 96, 192, 384, each of which, after the second, is double the preceding one. If we add 4 to each of these numbers, we form a new series:

4, 7, 10, 16, 28, 52, 100, 196, 388.

At the time this law was discovered, these numbers represented very nearly the proportionate distance from the sun of the planets then known, taking the earth's distance as ten, except that there was a blank opposite 28.* This naturally led to inquiry, and a systematic effort to solve the mystery. On the 1st day of January, 1801, the nineteenth century was inaugurated by Piazzi's discovery of the small planet Ceres, at almost the exact distance necessary to fill the gap in Bode's series. This was soon followed by the announcement of other new planets, until (1870) there are one hundred and twelve, and a probability of many more. Indeed, Leverrier has calculated that there may be perhaps 150,000 in all

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Description.—These minor worlds, or "pocket planets," as Herschel styled them, are extremely diminutive. The largest of them is Pallas, whose diameter is perhaps 600 miles. Those recently discovered are so small that it is difficult to decide which is the smallest. A French astronomer recently remarked concerning them, that a "good walker could easily make the tour of one in a day;" a prairie farmer would need to pre-empt a whole one for a flourishing cornfield. They all revolve about the sun in regular orbits, comprising a zone about 100,000,000 miles in width. Their paths are variously inclined to the ecliptic; Massilia's 41"', while that of Pallas rises 34°.

Origin.—One-theory concerning the origin of these small planets is, that they are the fragments of a large planet which, in a remote antiquity, has been shivered to pieces by some terrible catastrophe. "One fact seems above all others to confirm the idea of an intimate relation between these planets. It is this: if their orbits consisted of solid rings, they would be found so entangled that it would be possible, by taking up any one at random, to lift all the rest." Another theory is given under the "Nebular Hypothesis."

Names and signs.—Ceres, the first discovered, received the symbol V, a sickle. This was appropriate, since that goddess was supposed to preside over harvests. Pallas, the second, named from the goddess of wisdom and scientific warfare, obtained the

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