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53) in elevating the waters; this produces the highest or Spring tide. In quadrature (as in Fig. 5i), the sun tends to diminish the height of the water: this is called Neap-tide. When the moon is in perigee her attraction is stronger; hence the flood-tide is higher and the ebb-tide lower than at other times. This reFig. 54.
mark applies also to the sun. The height of the tide also varies with the declination of the sun and moon, —the highest or equinoctial tides taking place at the equinoxes, if, when the sun is over the equator, the moon also happens to be very near it: the lowest occur at the solstices. The force and direction of the winds, the shape of the coast, and the depth of the sea wonderfully complicate the explanation of local tides.
Height of the tide at different places.—In the open sea the tide is hardly noticeable, the water sometimes rising not higher than a foot; but where the wave breaks on the shore, or is forced up into bays or narrow channels, it is very conspicuous. The difference between ebb and flood neap-tide at New York is over three feet, and that of spring tide over five feet; while at Boston it is nearly double this amount. A headland jutting out into the ocean will diminish the tide; as, for instance, off Cape Florida, where the average height is only one and a half feet. A deep bay opening up into the land like a funnel, will converge the wave, as at the Bay of Fundy," where it rolls in, a great roaring wall of water sixty feet high, frequently overtaking and sweeping off men and animals. The tide sets up against the current of rivers, and often entirely changes their character; for example, the Avon at Bristol is a mere shallow ditch, but at flood-tide it becomes a deep channel navigable by the largest Indiamen.
Differential effect.—The whole attraction of the moon is only T^ that of the sun: yet her influence in producing the tides and precession is greater, because that depends not upon the entire attraction either exerts, but upon the difference between their attraction upon the earth's centre and upon the earth's nearest surface. For the moon, on account of her nearness, the proportion of the distance of these parts is treble that of the sun, and hence her greater effect.
The god of war. Sign, t, shield and spear.
Description.—Passing outward in our survey of the solar system, we next meet with Mars. This is the first of the superior planets, and the one most like the earth. It appears to the naked eye as a bright red star, rarely scintillating, and shining with a steady light, which distinguishes it from the fixed stars. Its ruddy appearance has led to its being celebrated among all nations. The Jews gave it the appellation of "blazing," and it bore in other languages a similar name. At conjunction its apparent
DIABlETEIt OP MAltS AT EXTREME, LEAST, AND MEAN DISTANCES.
diameter is only about 4"; but once in two years it comes into opposition with the sun, when its diameter increases to 30". At intervals of 8yr. 7 mo. this occurs when the planet is also in perihelion and perigee. Mars then shines with a brilliancy rivalling that of Jupiter himself.
Motion In Space.—Mars revolves about the Sun at a mean distance of about 140,000,000 miles. Its orbit is sufficiently flattened to bring it at perihelion 26,000,000 miles nearer that luminary than when in aphelion. Its motion varies in different portions of its orbit, but the average velocity is about fifteen
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.