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been able to see it. In our latitude and climate, we can generally easily detect it if we watch for it at tlie time of its greatest elongation or quadrature, as given in the almanac.
Motion In Space.—It revolves about the sun at a mean distance of 35,000,000 miles. Its orbit is the most eccentric (flattened) of any among the eight principal planets, so that although when in perihelion it approaches to within 28,000,000 miles, in aphelion it speeds away 15,000,000 miles farther, or to the distance of 43,000,000 miles. Being so near the sun, its motion in its orbit is correspondingly rapid—viz., 30 miles per second. At this rate of speed, we could cross the Atlantic Ocean in two minutes. The Mercurial year comprises only about 88 days, or nearly three of our months. Mercury revolves upon its axis in about the same time as the earth, so that the length of the Mercurial day is nearly the same as that of the terrestrial one. Though Mercury thus completes a sidereal revolution around the sun in 88 days, yet to pass from one inferior or superior conjunction to the same again (a synodic revolution) requires 116 days. The reason of this is, as already explained, that when Mercury comes around to the same spot in its orbit again, the earth has gone forward, and it requires 28 days for the planet to overtake us.
Distance From The Earth.—This varies still more than its distance from the sun. At inferior conjunction it is between the earth and the sun, and its distance from us is tlie difference between the distance of the earth and the planet from the sun: at superior conjunction it is the sum of these distances. Its apparent diameter in these different positions varies in the same proportion as the distances, or as three to one. The greatest and least distances vary according as either planet may happen to be in aphelion or perihelion. If at inferior conjunction Mercury is in aphelion and the earth in perihelion, its distance from us is only 90,000,000 - 43,000,000 = 47,000,000 miles. If at superior conjunction Mercury is in aphelion and the earth in aphelion also, its distance from us is 93,000,000 + 43,000,000 = 136,000,000 miles.
Dimensions.—Mercury is about 3,000 miles in diameter. Its volume is about -£$ that of the earth— L e., it would require twenty globes as large as Mercury to make one the size of the earth, or 25,000,000 to equal the sun. Yet as it is £ denser than the earth, its weight is nearly -fa that of the earth, and a stone let drop upon its surface would fall 7| feet the first second. Its specific gravity is about that of tin. A pound weight removed to Mercury would weigh only about seven ounces.
Seasons.—As Mercury's axis is much inclined from a perpendicular, its seasons are peculiar. There are no distinct frigid zones; but large regions near the poles have six weeks continuous day and torrid heat, alternating with a night of equal length and arctic cold. The sun shines perpendicularly upon the torrid ^one only at the equinoxes, while he sinks far toward the southern horizon at one solstice, and as far toward the northern horizon at the other. The equatorial regions, therefore, modify their temperature during each revFig. 23
olution from torrid to temperate, and the tropical heat is experienced alternately toward the north and south of what we call the temperate zones. There is no marked distinction of zones as with us, but each zone changes its character twice during the Mercurial year, or eight times during the terrestrial one. An inhabitant of Mercury must be accustomed to the most sudden and violent vicissitudes of temperature. At one time the sun not only thus pours down its vertical rays, and in a few weeks after sinks far down toward the horizon, but, on account of Mercury's elliptical orbit, when in perihelion the planet approaches so near the sun that the heat and light are ten times as great as that we receive, while in aphelion it recedes so as to reduce the amount to four and a half times (the average, however, is seven times),—a temperature sufficient to turn water to steam, and even to melt many of the metals. This entire round of transitions is swept through four times during one terrestrial year. The relative length of the days and nights is much more variable than with us. The sun, apparently seven times as large as it seems to us, must be a magnificent spectacle, and illumine every object with insufferable brilliancy. The evening sky is, however, lighted by no moon.
Telescopic Featukes.—Under the telescope, Mercury presents all the phases of the moon, from a slender crescent to gibbous, when its light is lost in that of the sun. These phases prove that Mercury is spherical, and shines by the light reflected from the sun. When in quadrature, it can sometimes be detected with a telescope in daylight. Being an inferior planet, we can never see it when full, and hence the brightest, nor when nearest the earth, as then its dark side is turned toward us. Owing to the dazzling light, and the vapors almost always hanging around our horizon, this planet has not received much attention of late; the cuts here given, and the remarks concerning its physical features, are based upon the observations of the older astronomers. It is thought by some to have a dense atmosphere loaded with clouds, which would materially diminish the intensity of the sun, and perhaps make Mercury quite habitable. Sir W. Herschel, however, emphatically denies this, and asserts that the atmosphere is too insignificant to be detected. There are some dark bands about its equator. It has lofty mountains, which intercept the light of the sun, and deep valleys plunged in shade. One mountain has been ascertained to be about ten miles in height, which is ^^ of the diameter of the planet. The height of the Dhawalaghiri of the Himalayas is less than 29,000 feet, or T¥V-?r part of the earth's diameter.
The Queen of Beauty. Sign f, a looking-glass.
Desobiption.—Venus, the next in order to Mercury, is the most brilliant of all the planets. "When visible before sunrise, she was called by the ancients Phosphorus, Lucifer, or the Morning Star, and when she shone in the evening after sunset, Hesperus, Vesper, or the Evening Star. She presents the same appearances as Mercury. Owing, however, to the greater diameter of her orbit, her apparent oscillations