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 FEATURES.-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 part of the earth's diameter.

VENUS.

The Queen of Beauty. Sign ?, a looking-glass.

DESCRIPTION.-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

are nearly 48° east and west of the sun,* or about 18° more than those of Mercury. She is therefore seen much earlier in the morning and much later at night. She is "morning star" from inferior to superior conjunction, and "evening star" from superior to inferior conjunction. She is the most brilliant about five weeks before and after inferior conjunction, at which time the planet is bright enough to cast a shadow at night. If, in addition, at this time of greatest brilliancy, Venus is at or near her highest north latitude, she may be seen with the naked eye in full daylight. This occurs once in eight years, in which interval the earth and planet return to the same situation in their orbits; eight complete revolutions of the earth about the sun occupying nearly the same time as thirteen of Venus. This happened last in February, 1862. A less degree of brilliancy is attained once in twenty-nine months, under somewhat the same circumstances..

MOTION IN SPACE.-Unlike Mercury, Venus has an orbit the most circular of any of the principal

* This distance varies but little, owing to the slight eccentricity of Venus's orbit.

Arago relates that Bonaparte, upon repairing to the Luxembourg, when the Directory was about to give him a fête, was much surprised at seeing the multitude paying more attention to the heavens above the palace than to him or his brilliant staff. Upon inquiry, he learned that these curious persons were observing with astonishment a star which they supposed to be that of the Conqueror of Italy. The emperor himself was not indifferent when his piercing eye caught the clear lustre of Venus smiling upon him at midday.

planets. Her mean distance from the sun is about 66,000,000 miles, which varies at aphelion and perihelion within the limits of a half million miles against 15,000,000 miles in the case of the former planet. She makes a complete revolution around the sun in about 225 days, at the mean rate of 22 miles per second; hence her year is equal to about seven and one half of our months. This is a sidereal revolution, as it would appear to an observer at the sun, but a synodic revolution is 584 days. Mercury, we remember, catches up with the earth in 28 days after it reaches the point where it left the earth at the last inferior conjunction. But it takes Venus nearly two and a half revolutions to overtake the earth and come into the same conjunction again. This grows out of the fact that Venus has a longer orbit to travel through, and moves only about one-fifth faster than the earth, while Mercury travels nearly twice as fast. The planet revolves upon its axis in about 24 hours; so the day does not differ in length essentially from ours.

DISTANCE FROM THE EARTH.-The distance of Venus from the earth, like that of Mercury, when in inferior conjunction, is the difference between the distances of these two planets from the sun, and when in superior conjunction the sum of these distances.

* Let the pupil calculate the distances of the earth and Venus from each other, when in perihelion and aphelion, as in the case of Mercury, (See tables in Appendix.)

The figure represents its apparent dimensions at the extreme, mean, and least distances from us. The variation is nearly as the numbers 10, 18, and 65. It would be natural to think that the planet is the brightest when the nearest, and thus the largest,

Fig. 24.

EXTREME, MEAN, AND LEAST APPARENT SIZE OF VENUS.

but we should remember that then the bright side is toward the sun, and the unillumined side toward us. Indeed, at the period of greatest brilliancy of which we have spoken, only about one-fourth of the light is visible. At this time, however, many observers have noticed the entire contour of the planet to be of a dull gray hue, as seen in the cut.

DIMENSIONS.-Venus is about 7,500 miles in diameter. The volume of the planet is about four-fifths that of the earth, while the density is about the same. A stone let fall upon its surface would fall 14 feet in