serve, in some measure, as a specimen of the apparent motions of all the other superior planets. In the following figure S represents the sun; A B C D the planet Mars in four different positions in its orbit; E F G HIK, the orbit of the earth; and L M N OP, a segment of the starry heavens. Suppose Mars at A and the earth at E, directly between it and the sun, then all the planet's enlightened hemisphere will be turned towards the earth, and it will appear like the full moon. When the planet is at B it will be gibbous, like the moon a few days before or after the full. At C it would again appear wholly enlightened, were it not in the same part of the hea

vene with the sun. At D it is again gibbous, as seen from E, and will appear less gibbous as it advances towards A. At A it is said to be in opposition to the sun, being seen from the earth at E among the stars at N, while the sun is seen in the opposite direction, E C. When the planet is at C and the earth at E, it is said to be in conjunction with the sun, being in the same part of the heavens with that luminary. in regard to all the superior planets, there is but one conjunction with the sun during the course of their revolution; whereas the inferior planets, Mercury and Venus, have two conjunctions, as formerly explained. Let us new attend to the apparent motions of this planet. Suppose the earth at F, and the planet at rest in is orbit at A, it will be projected or seen by a ray of ligh. among the s.ars at L; when the earth arrive at G, the planet will appear at M, by the ay GM nd in the same manner, when the

earth is at H, I, and K, the planet will be seen among the stars at N, O, and P; and, therefore, while the earth moves over the large part of its orbit, F H K, the planet will have an apparent motion from L to P among the stars, and this motion is from west to east, in the order of the signs, or in the same direction in which the earth moves; and the planet is then said to be direct in motion. When the earth is at K and the planet appears at P, for a short space of time it appears stationary, because the ray of light proceeding from P to K nearly coincides with the earth's orbit and the direction of its motion. But when the earth moves on from K to E, the planet will appear to return from P to N; and while the earth moves from E to F, the planet will still continue to retrograde from N to L, where it will again appear stationary as before. From what has been now stated, it is clear that, since the part of the orbit which the earth describes in passing through FHK is much greater than the arch K E F, and the space LP which the planet describes in its direct and retrograde motion is the same; therefore, the direct motion is very slow from L to P, in comparison of the retrograde motion from P to L, which is performed in much less time.

In the above description I have supposed the planet at rest in its orbit at A, in order to render the explanation more easy and simple, and the diagram less complex than it would have been had we traced the planet through different parts of its orbit, together with the motions of the earth. But the appearances are the same, whether we suppose the planet to be at rest or in motion. The only dif ference is in the time when the retrograde or direct motions happen, and in the places of the heavens where the planet will be at such times situated. What has now been stated in regard to the apparent motions of Mars will apply to Jupiter, Saturn, and all the superior planets, making allowance for the difference of time in which their direct and retrograde motions are performed. All the superior planets are retrograde in their apparent motions when in opposition, and for some time before and after; but they differ greatly from each other, both in the extent of their arc of retrogradation, in the duration of their retrograde movement, and in its rapidity, when swiftest. It is more extensive and rapid in the case of Mars than of Jupiter, of Jupiter than of Saturn, and of Saturn than of Uranus. The longer the periodic time or anual revolution of a superior planet, the more frequent are its stations and retrogradations; they ar: les, in quantity, but continue a longer time. The inean arc of retrogradation of Mars, or from ? te ï, Fig. 34, is sixteen de

grees, twelve minutes, and it continues about seventy-three days; while the mean arc of retrogradation of Jupiter is only nine degrees, fifty-four minutes, but its mean duration is about 121 days. The time between one opposition of Saturn and another is 378 days, or one year and thirteen days. The time between two conjunctions or oppositions of Jupiter is 398 days, or one year and thirty-three days. But Mars, after an opposition, does not come again into the same situation till after two years and fifty days. It is only at and near the time of the opposition of Mars that we have the best telescopic views of that planet, as it is then nearest the earth; and, consequently, when it has passed its opposition for any considerable time, a period of two years must elapse before we see it again in such a conspicuous situation. Hence it is that this planet is seldom noticed by ordinary observers, except during a period of three or four months every two years. At all other times it dwindles to the apparent size of a small star.

Distance, Motion, and Orbit of Mars.This planet is ascertained to be about 145 millions of miles from the sun. From what we have stated above it is obvious that, in the course of its revolution, it is at very different distances from the earth. When at its greatest distance, as when the earth is at E, and the planet at C, Fig. 34, it is 240 millions of miles from the earth. This will appear from an inspection of the figure. The distance, E S, from the earth to the sun is 95 millions of miles; the distance, S C, of Mars from the sun is 145 millions. These distances added together amount to the whole distance from E to C, or from the earth to Mars when in conjunction with the sun. When nearest the earth, as at A, it is only 50 millions of miles distant from us. For as the whole distance of the planet from the sun, A S, is 145 millions, subtract the distance of the earth from the sun, E S=95 millions, and the remainder will be the distance of the planet, E A=50 millions of miles from the earth. Small as this distance may appear compared with that of some of the other planets, it would require more than 285 years for a steam-carriage, moving without intermission at the rate of twenty miles an hour, to pass over the space which intervenes between the earth and Mars at its nearest distance.

From what has been now stated, it is eviuent that this planet will present a very different aspect as to size and splendour in Jifferent parts of its orbit. When nearest to the earth, it appears with a surface twentyfive times larger than it does at its greatest distance, and seems to vie with Jupiter in apparent magnitude and splendour. But, when

verging towards its conjunction with the sun, it is almost imperceptible. And this is one proof, among others, of the truth of the Copernican system. All its motions, stations, and direct and retrograde movements, and the times in which they happen, exactly accord with its position in the system and the motion of the earth, as a planet between the orbits of Venus and Mars. Whereas, were the earth supposed to be the centre of this planet's motion, according to the Ptolemaic hypothesis, it would be impossible to account for any of the phenomena above stated.

The orbit of Mars is 901,064,000, or more than 900 millions of miles in circumference. Through this space it moves in one year and 322 days, or in 16,488 hours. Consequently, its rate of motion is 54,640 miles every hour, which is more than a hundred times the greatest velocity of a cannon ball when it leaves the mouth of the cannon. The diurnal rotation of this planet, or its revolution round its axis, is accomplished in twenty-four hours, thirty-nine minutes, twenty-one secopds, which is about two-thirds of an hour longer than our day. This period of rotation was first ascertained by Cassini, from the mo tion of certain spots on its surface, which I shall afterward describe. Its axis is inclined to the plane of its orbit in an angle of thirty degrees, eighteen minutes, which is nearly seven degrees more inclined from the perpendicular than that of the earth. This motion is in the same direction as the rotation of the earth, namely, from west to east. The incli nation of the orbit of Mars to that of the earth is one degree, fifty-one minutes, six seconds, so that this planet is never so much as two degrees either north or south of the ecliptic. The orbit of Mars is considerably eccentric. Its eccentricity is no less than 13,463,000 miles, or about 1-21 of its diameter, which is more than eight times the eccentricity of the orbit of the earth. Hence it follows, that Mars, when in opposition to the sun, may be nearer the earth by a considerable number of millions of miles at one time than at another, when he happens to be about his perihelion, or nearest distance from the sun at such opposition. On the 27th of August, 1719, this planet was in such a position, being in opposition within two and a half degrees of its perihelion, and nearer to the earth than it had been for a long period before; so that its magnitude and brightness were so much increased that, by common spectators, it was taken for a new star.

Appearance of the Surface of Mars when viewed through Telescopes.-It was not be fore the telescope was brought to a certain degree of perfection that spots were discovered. on the surface of Mars. This instrument was

I have, however, distinctly perceived its sur-
face as delineated in Figures 43 and 44.
Fig. 43.
Fig. 44.

ec

These observations were made in November and December, 1832, and in January, 1837, and the appearances were very nearly the same; but the spots as represented in the two figures were seen at different times, and were evidently on different hemispheres of the planet, which were presented in succession by its motion of rotation. The instrument used in the observations was a 44 inch achromatic telescope, with magnifying powers of 150 and 180 times.

Besides the dark spots here delineated, there is a small portion of the globe of Mars, round its south pole, which has, at least occasionally, a much brighter appearance than the other parts. Maraldi, who made observations on Mars about the year 1719, says that this bright spot had been noticed for sixty years before that period, and that it is more permanent than any of the other spots of Mars; that this segment or zone is not all of equal brightness, more than one half of it being brighter than the rest; that the part which is least bright is subject to great changes, and has sometimes disappeared; and that there has sometimes been seen a similar luminous zone round the north pole of Mars, which has appeared of different brightness in different years. The bright spot at the polar point is represented at a, Figures 41 and 42. These white spots have been conjectured to be snow, as they disappear when they have been long exposed to the sun, and are greatest when just emerging from the long night of the polar winter in that planet. This is the opinion of Sir W. Herschel, in his paper on this subject in the Philosophical Transactions. "In the year 1781," says this astronomer, "the south polar spot was extremely large, which we might well expect, as that pole had but lately been involved in a whole twelvemonth's darkness and absence of the sun; but in 1783 I found it considerably smaller than before, and it decreased continually from the 20th of May till about the middle of September, when it seemed to be at a stand. During this last period the south pole had already been about eight months enjoying the benefit of summer, and still continued to re

ceive the sunbeams, though, towards the latter end, in such an oblique direction as to be but little benefited by them. On the other hand, in the year 1781, the north polar spot, which had then been its twelvemonth in the sunshine, and was but lately returning into darkness, appeared small, though undoubtedly increasing in size." Hence he concludes, "that the bright polar spots are owing to the vivid reflection of light from frozen regions, and that the reduction of those spots is to be ascribed to their being exposed to the sun."

Atmosphere of Mars. From the gradual diminution of the light of the fixed stars when they approach near the disk of Mars, it has been inferred that this planet is surrounded with an atmosphere of great extent. Although the extent of this atmosphere has been much overrated, yet it is generally admitted by astronomers that an atmosphere of considerable density and elevation exists. Both Cassini and Roemer observed a star, at six minutes from the disk of Mars, become so faint before it was covered by the planet that it could not be seen even with a three feet telescope; which, in all probability, was caused by the light of the star being obscured by passing through the dense part of the atmosphere of the planet. It is doubtless owing to this circumstance that Mars presents so ruddy an appearance, more so than any other planet or star in the nocturnal sky. When a beam of light passes through a dense medium, its colour inclines to red, the other rays being partly reflected or absorbed. Thus the morning and evening clouds are generally tinged with red, and the sun, moon, and stars, when near the horizon, either rising or setting, uniformly assume a ruddy aspect, because their light then passes through the lower and denser part of our atmosphere. When the light of the sun passes through the atmosphere of Mars, the most refrangible colours, such as the violet, will be partly absorbed; and before the reflected rays reach the earth, they must again pass through the atmosphere of the planet, and be deprived of another portion of the most refrangible rays; and, consequently, the red rays will predominate, and the planet assume a dull red colour. This I conceive to be the chief reason why I could never perceive Mars in the daytime, even when in the most favourable position, so distinctly as Jupiter, although the quantity of solar light which falls on this planet is more than eleven times greater than what falls on Jupiter; which seems to indicate that Jupiter is surrounded with a less dense and more transparent atmosphere. Sir W. Herschel, though he questions the accuracy of some of the observations of the dimness caused by the appulses of the fixed stars to this planet, yet

admits that it has a considerable atmosphere. "For," says he, "besides the permanent spots on its surface, I have often noticed occasional changes of partial bright belts, and also once a darkish one in a pretty high latitude: and these alterations we can hardly ascribe to any other cause than the variable disposition of clouds and vapours floating in the atmosphere of the planet."

Conclusions respecting the Physical Constitution of Mars. From the preceding observations and the views we have exhibited of this planet, I presume we are warranted to deduce, with a high degree of probability, the following conclusions: 1. That land and water, analogous to those on our globe, exist in the planet Mars. The dark spots are obviously the water or seas upon its surface, which reflect a much less proportion of the solar light than the land. "The seas," says Sir John Herschel, by a general law in optics, appear greenish, and form a contrast to the land. I have noticed this phenomenon on many occasions, but never more distinct than on the occasion when the drawing was made;" from which the figure of Mars in his "Astronomy" is engraved. It is not improbable, from the size of the dark spots compared with the whole disk of Mars, that about one-third or one-fourth of the surface of that planet is covered with water. If this estimate be nearly correct, it will follow that the quantity of land and water on Mars is nearly in a reverse proportion to that which obtains on our globe, where the quantity of water is nearly four times greater than that of the land. The dark spots in some of the views given above seem to convey the idea of several large gulfs or bays running up into the land. The various appearances of these spots which we have delineated are partly owing to the different relations and positions in which they appear during different periods of the planet's rotation, as I have already shown would happen in the appearance of the earth were it viewed from a distance in the heavens (see page 51.) 2. It is probable, too, that there are strata of clouds of considerable extent occasionally floating in the atmosphere of Mars; for some of the observers referred to above have remarked that some of the spots "changed their form in the course of a month;" and Sir W. Herschel, as above stated, declares that he has noticed "occasional changes of partial bright belts, and also once of a darkish one." These, in all probability, were clouds of greater or less density, which, for the most part, would appear brighter than the seas by the reflection of the solar rays from their upper surfaces; for although the under surface of dense clouds appears dark to us who view them from

below, yet, were we to view their upper sur face from a distance when the sun shines upon them, they would undoubtedly present a bright appearance by the reflection of the solar rays. It is doubtless owing to the occas sional interposition of such clouds in the atmosphere of Mars that the permanent spots sometimes appear to vary their form and aspect. 3. A variety of seasons, somewhat similar to ours, must be experienced in this planet. The diversity of seasons on our globe arises chiefly from the inclination of its axis to the plane of the ecliptic. Now, in reference to Mars, the axis of rotation is inclined to its orbit at even a greater angle than that of the earth; and, therefore, the contrast between its opposite seasons is probably more marked and striking than on the earth. The seasons will also continue for a much longer period than with us, as the year in Mars is nearly double the length of ours, so that summer and winter will be prolonged for a period. of eight or nine months respectively. If the opinion of Sir W. Herschel be correct, that the white spots at the poles of Mars are caused by the reflection of the sun's rays from masses of ice and snow, it will afford an additional proof of the existence of a diversity of seasons on this planet, and that its inhabitants are subjected to a winter of great severity and of long duration. 4. This planet bears a more striking resemblance to the earth than any other planet in the solar system. Its distance from the sun, compared with that of the other superior planets, is but a little more than that of the earth. The distinction of land and water on its surface is more strikingly marked than on any of the other planets. It is encompassed with an atmosphere of considerable extent. It is probable that large masses of clouds are occasionally formed in that atmosphere, such as sometimes hover over the whole of Britain, and even of Europe, for several weeks at a time. The length of the day is nearly the same as ours, and it has evidently a succession of different seasons. Were we warranted from such circumstances to form an opinion respecting the physical and moral state of the beings that inhabit it, we might be apt to conclude that they are in a condition not altogether very different from that of the inhabitants of our globe.

Magnitude and Extent of Surface of Mars.-This planet is now estimated to be about 4200 miles in diameter, which is only a little more than half the dian.eter of the earth. It contains 38,792,000,000, or more than 38 thousand millions of solid miles; and the number of square miles on its surface is 55,417,824, or more than fifty-five millions, which is about six millions of square miles