Mars, the fourth planet in order of distance from the sun, and the nearest to us of the superior planets, that is, of the planets whose orbits lie outside that of the earth. Mars travels around the sun in a mean sidereal period of 686-9797 days, on an orbit inclined 1° 51' to the plane of the ecliptic, at a mean distance of 139,311,000 m. from the sun; but this orbit is considerably eccentric, insomuch that his greatest distance, 152,304,000 m., exceeds his least, 126,318,000 m., by more than 26,000,000 m. He returns to opposition at intervals separated by a mean period of 779.936 days, which is therefore the planet's mean synodical period. The earth's mean distance being 91,430,000 m., the mean distance of Mars from the earth at the time when the two planets are in conjunction is about 48,000,000 m. But at a conjunction when Mars is near his perihelion, the distance is much reduced, more indeed than by the 13,000,000'm. by which the perihelion distance of Mars is less than his mean distance.

For the perihelion of the orbit of Mars lies in lon. 333 1/2°, while the aphelion of the earth's orbit lies in Ion. 280 1/2°, so that the earth is about 53° from aphelion when in the same longitude as the perihelion of Mars, and is therefore at a distance from the sun considerably exceeding her mean distance. Without entering at any length, however, into niceties of the sort (which would be idle, since conjunctions do not happen exactly when Mars is in perihelion), we may say that at a conjunction near the perihelion of Mars the distance between the two planets amounts to about 35,000,000 m., while at a conjunction near the aphelion of Mars the two planets are separated by about 61,000,000 m. Accordingly, Mars is studied under very different conditions when he is in opposition (to the sun - that is, in conjunction with the earth) near perihelion and near aphelion. For not only is his apparent disk larger in the former than in the latter case in the proportion of about (61)2 to (35)2, or more than 3 to 1, but furthermore the planet is more brilliantly illuminated (and will therefore better bear telescopic magnifying) in the former case in the proportion of about (152)2 to (126)2, or nearly as 3 to 2. On the whole, therefore, the planet can be more favorably studied in the former case than in the latter in the proportion of about 9 to 2, measuring the conditions by the amount of magnifying due to proximity and to the telescopic powers practically available.

To ordinary observation, the effect of the considerations just pointed out is that Mars when in opposition near perihelion looks about 4 1/2 times brighter than when in opposition near aphelion. The opposition of the year lb77 will illustrate this; for during the oppositions which have recently occurred Mars has been far from perihelion, the oposition of 1869 occurring when Mars was nearly in aphelion, and those of 1871 and 1873 being little more favorable. In the opposition of 1875 he will be nearer, but still some 05° from his perihelion. But in 1877 the place of opposition will have been carried more nearly to the perihelion (somewhat past that point) than previously for more than 30 years. He will therefore'present a very distinguished appearance in the heavens, being little inferior in brightness to. the planet Jupiter. It is indeed a noteworthy circumstance that theoretically Mars should then be brighter than Jupiter. That is, comparing the size of his disk with that of Jupiter, and the amount of light received by the two planets from the sun, Mars should be brighter when he is in opposition near perihelion than Jupiter ever is; but owing either to the inferior reflective quality of the surface of Mars, or more probably to the fact that most of the light of Jupiter is reflected from cloud masses, and that a portion of that light is inherent, Jupiter in opposition is always brighter than Mars ever is. - The diameter of Mars is variously estimated by different observers, but is probably about 4,400 m.

The volume of his globe is therefore less than the earth, in the proportion of about 168 to 1,01 0, or the earth is nearly six times as large as Mars. The mass of the planet is however even smaller in proportion; for his density is estimated at seven tenths of the earth's, and his mass though less than hers in the proportion of about 118 to 1,000, is nearer one ninth than one eighth of the earth's mass. This small planet rotates on an axis inclined about 28° to the orbit, so that the seasons have a greater range than those of the earth, whose inclination to her orbit is but 23 1/2°. The rotation has been determined with a near approach to accuracy. Cassini, who was one of the first to study'the telescopic aspect of Mars, assigned to the planet a rotation period of 24h. 40m. Sir W. Herschel in 1777 attacked the problem with less success than usually attended his work as an observer. He unfortunately missed count of one rotation in a synodical revolution of Mars, and this error, distributed among all the rotations of the revolution (corresponding to about 24h. divided into 750 parts), amounted to nearly two minutes, so that his estimate of the rotation period, 24h. 39m. '25s., was about two minutes too fast.

Madler, from observations extending over the years 1830-'37, deduced a period of 24h. 37m. 23.8s. Kaiser of Leyden, combining his own observations with those by Madler, Sir W. Herscliel, and Huygens in 1G72, deduced a rotation period of 24h. 37m. 22'62s. Lastly, the present writer, by combining observations made in 1873 with Hooke's observations in 1060, deduced the period 21h. 37m. 22.73s. As this differed more from Kaiser than could bo explained if neither Iluygens's observations nor Hooke's had been misunder-stood, Kaiser went afresh over his work, and obtaining his former result expressed the opinion that Hooke's observation was untrustworthy. But the present writer, having carefully examined Kaiser's work, found that Kaiser had apparently counted the years 1700 and 1800 as leap years; at any rate, he had somehow counted two days too many in the interval of about 200 years. This excess of one day per century really corresponded to a defect of 37m. 23s., since a Martial rotation too many had been of course introduced along with the extra day, and a Martial rotation requires 24h. 37m. 23s. instead of 24h., so that the additional terrestrial day fell short by 37m. 23s. of what was really required to provide (so to speak) for the added Martial day.

Now if we divide 37m. 23s. by the number representing all the Martial rotations in a century, or roughlv if we divide 2,250 seconds by 35,000, we obtain about 0.065 of a second; and adding this to Kaiser's value of the rotation period, we obtain 24h. 37m. 22.685s. This is near enough to the writer's value (24h. 37m. 22-735s.) to show that both Huygens's picture and Hooke's can be relied upon, the difference resulting merely from such errors in drawing as might be expected. We may therefore assign a part of the difference to error in Hooke's picture, and the rest to error in Huygens's. After a critical examination of the two pictures for this purpose, the present writer finds that the most probable value of the rotation period is 24-h. 37m. 22715s., the true value almost certainly lying between 21h. 37m. 22.7s. and 24h. 37m. 22:73s. This is the only case in which the rotation of a planet has been determined, or probably can be determined, with so great an approach to ex-actness. The marks on Mercury and Venus are too uncertain to be trusted, and the planets Jupiter and Saturn do not in all probability show their real surface to us.

The determination of a planet's rotation period is not a mere matter of curiosity; for when the period has been determined with considerable accuracy, a planet may be regarded thenceforth as a sort of celestial chronometer, by which changes in the rate of other motions may be inferred if not gauged. - The surface of Mars has been carefully studied by many skilful observers. Hooke Cassini, Huygens, and Fontana were the first to recognize any of those markings which are now known to belong to the real surface. They noticed that the polar parts of the planet appeared to be occupied by white matter; the idea does not seem to have presented itself that this matter might be like the snow and ice which are found in the polar regions of our earth. On the contrary, when Maraldi in 1720 studied these white regions, and found that one of them had diminished in size, he predicted its entire disappearance. It was not until Herscliel had carefully examined them for a considerable time, and found their variations to correspond to the progress of summer and winter in the northern and southern hemispheres of the planet, that the resemblance between the white spots and our arctic and antarctic snows was recognized, and that Herschel, ever on the watch for analogies of the kind, expressed the opinion that these spots are the snows of Mars. The other parts of the planet present two chief colors, a faint ruddy tint, apparently representing the continents of the planet, and a still fainter indigo-green tint, which from an early period has been regarded as indicating the presence of seas and oceans upon that distant world.

It was for a long time impossible, however, as Dr. Whewell pointed out, to be certain that this interpretation of the white spots and of the greenish markings was correct, or that water existed in any form on the surface of Mars. But recently the spectroscopic analysis of the light of Mars has shown beyond question that at times the vapor of water exists in the planet's atmosphere, since the same bands are seen which appear in the solar spectrum when the sun is low down and shining through the denser and more moisture-laden parts of our atmosphere. Nevertheless, it is not easy to understand how the condition of Mars as to temperature can so nearly resemble the earth's, as we should have to believe if we considered only the relative extent of the snowy polar regions in the two planets. Mars is so much further from the sun that the solar radiation is reduced, as compared to that to which our earth is exposed, in the ratio of about 1 to 2 when Mars is in perihelion, and of about 1 to 3 when he is in aphelion. Moreover, being a much smaller planet, we should expect his atmospheric envelope to be much less dense, since if reduced as his volume it would be reduced as the cube of his linear dimensions; whereas it would extend over a surface reduced only as the square of those dimensions.

On this assumption there would be less air per square mile of the planet in the proportion of about 11 to 20; and as gravity at the surface of Mars is less than gravity on the earth in the ratio 387 to 1,000, the atmospheric pressure would be less as about 4,257 to 20,000, or would be little more than one fifth that of the earth's sea level. Of course, we have no certain assurance that the assumption here made is even approximately correct. But since, to make the climate of Mars as warm as our earth's, the atmosphere should be much denser than ours, whereas the assumption which must be regarded as the most probable would make the atmospheric density barely one fifth of ours, it seems difficult to regard the climate of Mars as probably like that of our earth. There are reasons, therefore, for viewing as at least worthy of consideration the theory of Mattieu Williams, that the climate of Mars is really unlike that of our earth, notwithstanding the similarity of the 'snow regions in extent. Without entering into the details of his theory, or accepting the relations which he somewhat speculatively exhibits, we may thus far provisionally adopt his views as not improbable. - Owing to the much smaller amount of solar radiation at the distance of Mars, and also to the much more limited extent of his oceans, the quantity of aqueous vapor raised into his atmosphere must be very much less in proportion to the extent of his surface; and it is not unlikely that most of the precipitation of such vapor takes place in the form of snow, which would not fall thickly, and would be soon melted during the Martial day in the tropical and subtropical regions.

Thus we may explain the appearances which have hitherto been regarded as due to the dissipation of Martial rain clouds, and also the observed fact that the disk of Mars is whitish near the edge, and the markings invisible there. Thus whatever aqueous vapor, or cloud, was carried to the temperate regions, and whatever ice or snow accumulated in the polar regions, would be much smaller in amount than we should otherwise have inferred from the apparent extent of the polar snow caps, and these would therefore diminish in extent as summer advanced, much more quickly than they would if formed as in the earth's case. The general conclusion to which we should be led if we adopted this view would be that the planet presents conditions unfavorable for the existence of such forms of life as we are familiar with. These questions derive their chief scientific interest, however, as suggesting the careful study of those Martial phenomena which presumably depend on the density of the planet's atmosphere, and its general conditions as respects humidity and so on.