be obvious at her transits, as, in some of them at least, it is probable that the satellite would be projected near the primary on the sun's disk; but later astronomers have searched in vain for any appearances of the kind, and the inference is, that former astronomers were deceived by some optical illusion.

LETTER XXI.

SUPERIOR PLANETS MARS, JUPITER, SATURN, AND URANUS.

"With what an awful, world-revolving power,
Were first the unwieldy planets launched along
The illimitable void! There to remain

Amidst the flux of many thousand years,

That oft has swept the toiling race of men,

And all their labored monuments, away."-Thomson.

MERCURY AND VENUS, as we have seen, are always observed near the sun, and from this circumstance, as well as from the changes of magnitude and form which they undergo, we know that they have their orbits within that of the earth, and hence we call them inferior planets. On the other hand, Mars, Jupiter, Saturn, and Uranus, exhibit such appearances, at different times, as show that they revolve around the sun at a greater distance than the earth, and hence we denominate them superior planets. We know that they never come between us and the sun, because they never undergo those changes which Mercury and Venus, as well as the moon, sustain, in consequence of their coming into such a position. They, however, wander to the greatest angular distance from the sun, being sometimes seen one hundred and eighty degrees from him, so as to rise when the sun sets. All these different appearances must naturally result from their orbits' being exterior to that of the earth, as will be evident from the following representation. Let E, Fig. 58, page 244, be the earth, and M, one of the superior planets, Mars, for example, each body being seen in its path around the

Fig. 58.

M'

E

sun.

M

At M, the planet would be in opposition to the sun, like the moon at the full; at Q, and Q', it would be seen ninety degrees off, or in quadrature; and at M', in conjunction. We know, however, that this must be a superior and not an inferior conjunction, for the illuminated disk is still turned towards us; whereas, if it came between us and the sun, like Mercury, or Venus, in its inferior conjunction, its dark side would be presented

to us.

The superior planets do not exhibit to the telescope different phases, but, with a single exception, they always present the side that is turned towards the earth fully enlightened. This is owing to their great distance from the earth; for were the spectator to stand upon the sun, he would of course always have the illuminated side of each of the planets turned towards him; but so distant are all the superior planets, except Mars, that they are viewed by us very nearly in the same manner as they would be if we actually stood on the sun. Mars, however, is sufficiently near to appear somewhat gibbous when at or near one of its quadratures. Thus, when the planet is at Q, it is plain that,

of the hemisphere that is turned towards the earth, a small part is unilluminated.

MARS is a small planet, his diameter being only about half that of the earth, or four thousand two hundred miles. He also, at times, comes nearer to the earth than any other planet, except Venus. His mean distance from the sun is one hundred and forty-two millions of miles; but his orbit is so elliptical, that his distance varies much in different parts of his revolution. Mars is always very near the ecliptic, never varying from it more than two degrees. He is distinguished from all the planets by his deep red color, and fiery aspect; but his brightness and apparent magnitude vary much, at different times, being sometimes nearer to us than at others by the whole diameter of the earth's orbit; that is, by about one hundred and ninety millions of miles. When Mars is on the same side of the sun with the earth, or at his opposition, he comes within forty-seven millions of miles of the earth, and, rising about the time the sun sets, surprises us by his magnitude and splendor; but when he passes to the other side of the sun, to his superior conjunction, he dwindles to the appearance of a small star, being then two hundred and thirty-seven millions of miles from us. Thus, let M, Fig, 58, represent Mars in opposition, and M', in the superior conjunction, while E represents the earth. It is obvious that, in the former situation, the planet must be nearer to the earth than in the latter, by the whole diameter of the earth's orbit. When viewed with a powerful telescope, the surface of Mars appears diversified with numerous varieties of light and shade. The region around the poles is marked by white spots, (see Fig. 56, page 237,) which vary their appearances with the changes of seasons in the planet. Hence Dr. Herschel conjectured that they were owing to ice and snow, which alternately accumulate and melt away, according as it is Winter or Summer, in that region. They are greatest and most conspicuous when that part of the planet has just emerged from a long Winter, and

they gradually waste away, as they are exposed to the solar heat. Fig. 56, represents the planet, as exhibited, under the most favorable circumstances, to a powerful telescope, at the time when its gibbous form is strikingly obvious. It has been common to ascribe the ruddy light of Mars to an extensive and dense atmosphere, which was said to be distinctly indicated by the gradual diminution of light observed in a star, as it approaches very near to the planet, in undergoing an occultation; but more recent observations afford no such evidence of an atmosphere.

By observations on the spots, we learn that Mars revolves on his axis in very nearly the same time with the earth, (twenty-four hours thirty-nine minutes twenty-one seconds and three tenths,) and that the inclination of his axis to that of his orbit is also nearly the same, being thirty degrees eighteen minutes ten seconds and eight tenths. Hence the changes of day and night must be nearly the same there as here, and the seasons also very similar to ours. Since, however, the distance of Mars from the sun is one hundred and forty-two while that of the earth is only ninety-five millions of miles, the sun will appear more than twice as small on that planet as on ours, (see Fig. 53, page 236,) and its light and heat will be diminished in the same proportion. Only the equatorial regions, therefore, will be suitable for the existence of animals and vegetables.

The earth will be seen from Mars as an inferior planet, always near the sun, presenting appearances similar, in many respects, to those which Venus presents to us. It will be to that planet the evening and morning star, sung by their poets (if poets they have) with a like enthusiasm. The moon will attend the earth as a little star, being never seen further from her side than about the diameter under which we view the moon. To the telescope, the earth will exhibit phases similar to those of Venus; and, finally, she will, at long intervals, make her transits over the solar disk. Mean-while, Venus will stand to Mars in a relation similar to that of Mercury