distance as stated above. The probable error of this concluded value is thought not to exceed the 680th part of its value, or 135,000 miles. In 1860, the distance of Mars in opposition was 35,000,000 miles; in 1862 it was 36,000,000; and in 1877 it will be only 34,000,000 miles. At these oppositions, the brilliancy of Mars is so great as to attract the attention of the common observer. In the autumn of 1719, when this planet arrived in opposition, it was only two and a half degrees from its perihelion; and it shone with such uncommon splendor, that many supposed it to be a comet or a new star. The time required for Mars to pass from one opposition to the next succeeding one (called a synodical revolution) is about 780 days, or nearly two years. The first observations on Mars, for the purpose of determining the distance of the sun, were made by M. Richer, in 1672, at Cayenne, South America, whither he was sent for the purpose by the French academy of sciences, and by M. Cassini at the same time in Paris. They deduced a value of the solar parallax equal to 9".50. Notwithstanding the fact that Mars has been closely scrutinized with excellent telescopes, no satellite has yet been discovered. For this reason the mass of the planet, or the relative amount of matter which it contains, has never been very accurately determined. The value of the mass can be ascertained only by the effect which the planet has on the motion of the other members of the solar system. In this way the French calculator, Burckhardt, determined the mass of Mars to be about the 2,680,000th part of the sun's. The diameter of Mars is about 4,000 miles, and its density is a little less than that of the earth. Bodies on the surface of this planet will weigh not quite one-third of what they would on the surface of the earth. Except the moon, we know the most about the surface configuration of Mars of any of the celestial bodies. In examining this planet with a telescope of sufficient power, the first thing that attracts our attention, however, are the phases. In opposition the disk is round, but as it recedes from that point the disk gradually diminishes, till it arrives at quadrature, when the phase is like that of the moon nine days old; so that Mars is generally seen gibbous, but never less than a semicircle. This fact proves that the orbit of this planet is exterior to that of the Earth. The phases of Mars were discovered by Galileo soon after the invention of the telescope. It only requires a moderately powerful telescope to show spots on the surface of Mars. These appearances were first discovered by Fontana, a Neapolitan astronomer.* In the year 1636 he observed a spot on the disk of the planet, which reappeared in 1638. From changes which it apparently underwent, he inferred that the planet was endowed with a rotary motion about an axis. The celebrated Hooke, however, was one of the first astronomers that arrived at this conclusion from reasoning of a strictly legitimate character. In March, 1666, he communicated a paper to the Royal Society of London, in which he shows, from his own observations, that Mars has a rotation which is accomplished either in twelve or twenty-four hours, but in which period he could not decide.† Some observations made contemporaneously, by M. Cassini, in Italy, were more definite, giving twenty-four hours and forty minutes for the period of rotation. In 1704, Maraldi, the elder, concluded, from his own observations, that the period of rotation. was twenty-four hours and thirty-nine minutes; but a more favorable opportunity being presented in 1719, he determined the period, and found it equal to twenty-four hours and forty minutes. In the years 1777 and 1779, Sir William Herschel carefully observed the changes which the spots presented, and concluded from them that the period of rotation is 24 hours 39 minutes and 22 seconds.‡ The more recent observations of Baer and Mädler (1830 to 1837) make the period of rotation 24 hours, 37 minutes and 20 seconds.§ More recently still, the rotation-period has been very accurately determined, by Mr. Richard A. Proctor, to be 24 hours, 37 minutes and 23 seconds. Since Mars revolves around an axis, we should naturally * Grant's Hist. Phys. Astron., p. 235. ↑ Phil. Trans., No. 14. conclude from the theory of rotating bodies, that its polar diameter must be less than its equatorial, and observation seems to show such to be the case. If the density of Mars were homogeneous, or the same throughout, its equatorial diameter would exceed its polar by the 250th part; but observation gives a very different result from this. Sir William Herschel found these diameters in the ratio of 16 to 15,* and M. Arago (1847) as 32 to 31.† Professor Bessel doubted the existence of any observable ellipticity and Professor Hennessy, after examining the whole subject, comes to the conclusion that the form of Mars is nearly spherical.§ Considerable uncertainty yet remains to be cleared up. As soon as telescopes of considerable power were turned upon the planet Mars, spots of various forms were seen upon its surface. Robert Hook made a drawing of the appearance of this planet on the 12th of March, 1666, which compares very favorably with our best modern representations of the surface of the martial world. He used a telescope 36 feet in length. We have already mentioned that Cassini saw spots so that he was able to determine roughly the period of rotation of the planet. This he did in the beginning of March, 1666, and used for the purpose a 16-foot telescope, though it was not equal in power to a modern telescope of a quarter that length. At the end of the same month some observers at Rome constructed a drawing of Mars, which aroused Cassini's indignation; "For," said he, "these observers represent the spots they saw as small, far apart, remote from the middle of the disk, and the eastern spot less than the western, whereas, by observations made at Bonomia on the same day, I know that there were two very large spots close to each other, in the midst of the disk, and the eastern larger than the western." During the opposition of Mars in 1704, Maraldi observed the planet for the purpose, as we have already stated, of determining the period of rotation, and found upon its surface some spots that seemed to be permanent. The opposition of 1719 being much more favorable for that purpose, and also for find + lbid. * Phil. Trans., 1784, p. 273. † Cosmos, vol. iv., p. 503. ing its period of rotation, because the planet was unusually near the earth at that time, he succeeded in making out some things, which, as shown in two of his drawings, are now easily recognizable when compared with the best representations of the martial surface. What is now called the Hourglass Sea is a triangular or funnel-shaped spot running nearly north and south, as depicted in one of Maraldi's drawings. In the other there is an elbow-shaped spot since discovered by means of the more powerful modern telescopes (Maraldi's was thirty-four feet long, but not achromatic) to be two seas.* Sir William Herschel was the first that attempted a systematic examination of the surface features of this planet, which he did during the years 1777 and 1779. The results of his labors are contained in a Memoir, published in the "Philosophical Transactions" for 1781. His object was to decide whether the diurnal rotation of the earth remains always the same. He thought, by watching the rotation of some of the planets, he might find some trace, if any existed, of the variability of our day. He soon ascertained that Mars is the only planet available for this purpose. With this view he constructed a series of pictures of this planet; but owing to the defects in the figure of his telescopes, he was not very successful. His representations have been called caricatures of Mars. Herschel gave a truthful representation of what he saw, but he did not see the planet as it was. Maraldi discovered, in 1716, near the poles of Mars, white spots, which Sir William Herschel was the first to show were dependent on climatic changes.† As winter returned to either pole the bright spot at that pole was found by him to increase in extent till it had reached its maximum, near spring, on the planet in that hemisphere when, soon after, as warm weather returned, the spot gradually diminished till it became quite small during the summer season of the martial year in that hemisphere. Thus, in the year 1781, the spot about the south pole was very large, as it should have been according to Herschel's explanation, since that pole had recently been involved * See Mém. Acad. des Sciences, for 1720. † See Phil. Trans, for 1784, vol. lxxiv. in darkness during a space of nearly twelve months. In 1783 it had become much smaller, and it continued to decrease from the 20th of May of that year till the middle of September. The south pole had now been enjoying the sunshine for more than eight months, and still continued to receive it, but in so oblique a direction as to be little benefitted by it. On the other hand, the north polar spot appeared small, since it had been enjoying the sunshine for a year, and was but lately returning to darkness.* Herschel's explanation, which refers the spots to the accumulation of snow and ice about the poles, is the one generally adopted by astronomers. The reader will at once perceive that it implies the existence of an atmosphere surrounding Mars, and water on the surface of the planet. Afterward his father, Sir John Herschel, made a series of observations from this planet, from which he has supplied us with a drawing of our hemisphere of the martial world. He says that what may be the outlines of continents and seas can be seen with perfect distinctness. Of these the former are characterized by that ruddy appearance which the light of this planet presents. This red appearance arises, without doubt, from the color of the soil, being, it is thought, much like the red sandstone districts in some parts of the earth. The seas, on the other hand, appear greenish. "These spots, however, are not always to be seen equally distinct, but when seen they offer the appearance of forms considerably definite and highly characteristic."+ The next series of observations which deserves especial attention is that taken in the years 1830-1837, by Baer and Mädler, two German astronomers. Their telescope was not large-four inches aperture and a little more than five feet focal length; but the experience of the observers compensated in a great measure for the want of power in their instrument. Their observations furnish some forty views of hemispheres of this planet, formed by planes passing nearly through the poles of rotation of the planet. From these observations it was possible Ibid., p. 261. † Outlines, art. 510. See, also, art. Nachr., No. 191, and in 349. |