from the Sun and Mars nearest, occurs ordinarily but once in 15 years, when the diameter of Mars, only 13′′ in reversed circumstances, expands to 23"-5. Every opposition, however, should set the telescope to work; and we will proceed to describe what we may expect to see. 1. The Phases, These are not remarkable: in opposition, a full moon rising through ruddy haze, and, with sufficient power, larger than our Moon to the naked eye: in other situations a dull gibbosity, never sinking to quadrature. M. stated at one time that this phasis is always narrower than it should be by calculation; but in a subsequent publication the remark is not repeated. Pastorff thought he saw a phosphorescence on the dark part: but this was probably a deception. 2. The Dark Spots. The diso, when well seen, is usually mapped out in a way which gives at once the impression of land and water: * the bright part is orange,-according to Secchi, sometimes dotted with red, brown, and greenish points; B. and M. think it much less red than to the naked eye: the darker spaces, which vary greatly in depth of tone, are of a dull grey-green,† or according to Secchi, bluish, possessing the aspect of a fluid absorbent of the solar rays. If so, the proportion of land to water on the Earth is reversed on Mars: on the Earth every continent is an island; on Mars all seas are lakes; so that the habitable area may possibly be much more alike than the diameter of the planets. From the different distribution of the water (if such it be), long narrow straits * Secchi's illustration is strikingly expressed: 'è tutto variegato come una carta geografica.' Herschel II. refers this colour to contrast. Jacob does not detect it. I have seen it with a 9 in. mirror, beautifully blue. The accurate Humboldt has puzzled himself about these colours. (Cosmos, IV. 503.) are more common than on the Earth: Dawes has observed a singular forked shading, as if of two great contiguous estuaries. The dark spots were early seen, and a long series of drawings is extant from Hooke, Cassini, and Campani, in 1666,* to Jacob, Secchi, De La Rue, Lassell, Phillips, Lockyer, Dawes, and others in the present day, with some general correspondence,† but a difference of detail; which seems due in part to differences in telescopes, eyes, climates, and skill in delineation; in part to altered projection owing to the inclination of the axis, shewing us sometimes more of N., sometimes of S. hemisphere; and in part also to changes in the planet's atmosphere. The older observers thought the spots variable: Herschel I. perhaps took the lead in supposing them to be permanent, an idea which Kunowsky, as late as 1822, fancied was due to himself. Schröter's work on Mars, the 'Areographische Fragmente,' which was to have contained 224 figures, was unfortunately left in MS. at his death in 1816,§ but he has stated that he and Olbers found them vary rapidly. B. and M. took up the subject with great spirit at the peculiarly favourable opposition in 1830, recovered some of Kunowsky's spots, and from their further observations in 1832, * Humboldt, following Delambre, says that Cassini does not seem to have discovered the rotation of the spots till after 1670. He must have overlooked the figures in Phil. Trans. No. 14. Kaiser has shewn, from the MS. Journal of Huygens, that the latter discovered the rotation in 1659. Herschel I.'s combined figure in Phil. Trans. 1784, if reversed, will be not unlike B. and M.'s polar projections. 30° 18', according to B. and M. and Herschel II. But is not this a mistake for the complementary angle? Hind gives, after Herschel I., 61° 18' for inclination of axis to orbit of Mars; 59° 42' for do. to Earth's ecliptic; obliquity on Mars, 28° 42': but 90°-59° 42′ = 30° 18'. § This MS. still exists in the hands of his descendants, 1834, and 1837, though the same hemisphere was not always equally visible, inferred their permanence. M., a little shaken as to this in 1839, retrogrades still further at Dorpat in 1841: the drawings, however, of later observers exhibit substantially many of the same forms, and notwithstanding numerous discrepancies, there seems sufficient evidence that most of the spots are really part of the surface. The distant view of the Earth, indeed, might be much of this nature; its outlines at one time distinct, at another confused or distorted by clouds: besides, one affirmative-the re-appearance of a spot-proves more, where there may be hindrances, than can be disproved by many negatives. At present, we can only form an approximate map of Mars; nor shall we ever know the N. so well as the S. hemisphere, as it is turned towards us in the planet's aphelion;-even were its markings equally defined, which B. and M. deny. Under favourable circumstances the dusky spots are not difficult objects; I have repeatedly been able to draw them with my 5 f. achromatic; a much smaller instrument will sometimes shew the darker ones plainly; while on other parts of the globe, they are feeble even in large telescopes. Their motion will be very evident, and as the rotation is completed, according to Proctor, in 24h 37m 22735", they will not vary greatly from night to night at the same hour. These remarkable features, with Proctor's provisional nomenclature, are represented in a design for which I am indebted to his kindness and that of Messrs. Longmans and Co. The expansion of the polar regions, due to the projection of Mercator, must be mentally corrected by the student; and in comparing the portrait with the original, allowance must be made for the perspective of a globe, where the foreshortening is much greater than we are frequently aware of, the eye being probably early biassed by maps of the terrestrial hemisphere, in which it is artificially removed. 3. The Polar Snows. A spot surrounding either pole is so white and luminous as to have occasionally remained visible when a cloud obscured the planet: and frequently to seem, from irradiation, to project beyond the limb, as I have myself noticed. These zones were figured by Maraldi in 1704, who says they had been occasionally seen for 50 years;* in fact they could not long escape the telescope. They were thought to resemble snow before Herschel I.'s time; he gave consis *B. and M. erroneously make him the discoverer in 1716. A figure by Huygens, in 1656, may be meant as a rude representation of them. tency to the idea by ascertaining that they decreased during the summer, and increased during the winter of Mars, and B. and M. have fully confirmed it, with the addition that the S. polar spot has a greater variety of extent, corresponding with its greater variety of climate from the excentricity of the orbit. Each pole comes alternately into sight, and both are sometimes visible on the edge at once, when the opposition of Mars concurs with his equinox. Herschel I. found they were not (or not always) opposite each other, both being sometimes in or out of the disc at the same time. M., and Secchi with the admirable achromatic at Rome, of 9 in. aperture and 15 f. focus, bearing ordinarily a power of 1,000, found the N. zone concentric with the axis, but the S. considerably excentric. It has been suggested by B. and M. that the poles of cold, like those on the Earth, may not coincide with the poles of rotation; -still they should be diametrically opposite. These observers found in 1837 the N. pole surrounded by a conspicuous dark zone, the only well-marked spot in sight, which they thought might possibly be a marsh at the edge of the melting snow : in 1839 M. perceived it had decreased; in 1841 it was no longer visible. About the opposition in 1856 I had interesting views of these zones, which did not seem exactly opposite to each other: the S. was surrounded by a very dark region, never seen by B. and M.; on the intervening limbs were occasionally luminous regions, so bright by contrast as to give an impression of four patches of snow, as in one of Cassini's figures in 1666: these were also seen by Secchi at the same time. In 1845 Mitchell with a great achromatic in America noticed a very dark spot in the centre of the snow, which disappeared the next night : * at another time he saw some * Dawes saw (1847, Nov. 9) a minute black point near the middle of the disc. |