_Occultations of Mars._—The most ancient account of a planetary occultation is probably that given by Aristotle, who refers to a lunar obscuration of Mars that occurred on April 4, 357 B.C., according to the calculations of Kepler. Another occultation of Mars appears to have been recorded by the Chinese on Feb. 14, 69 B.C. Tycho Brahe observed a repetition of the event on Dec. 30, 1595. Mr. Baily describes a phenomenon of this kind which occurred on Feb. 18, 1837, when “the planet appeared of a fine yellow colour both at its ingress and egress. No projection was observed.” Mr. Snow, of Ashhurst, saw the occultation of March 12, 1854, and he states the planet “was of almost precisely the same colour as the Moon, and he could not help comparing it to a spangle on the face of the sky. Whilst it was slowly and solemnly vanishing, it gave for several seconds the notion of its being the summit of a lunar mountain, but melted gradually away.” As Mars emersed, “nothing whatever was to be seen of the two bodies, clinging together, as it were, by threads of light; nothing of the pear-shaped appearance often recorded as put on by planets under similar circumstances.” Mr. J. Tebbutt, of Windsor, N. S. W., watched an occultation of Mars in full daylight on Aug. 12, 1875, when “the rapid disappearance of the planet’s disk was an exceedingly interesting phenomenon, its extinction taking place at a considerable distance from the Moon’s illuminated disk. The line marked by the Moon’s dark limb across the disk was well defined.” At the reappearance clouds were prevalent, and “the planet was observed as a small projection on the bright limb;” but he found it difficult to fix the exact time of last contact, owing to the ill-defined character of the planet’s gibbous limb. An occultation of Mars was also seen by Prof. Grant at Glasgow on June 3, 1878. FOOTNOTES: [30] This was believed by Sir J. Herschel to be due to “an ochrey tinge in the general soil, like what the Red-Sandstone districts on the Earth may possibly offer to the inhabitants of Mars, only more decided.” [31] Herschel’s earlier observations were made in 1777-79, and his period, like that of his predecessors, is about 2 min. in excess of the correct value; but Mädler pointed out that, by giving Mars an additional rotation on his axis, Herschel’s value will agree within 2 sec. of his own. Herschel appears to have adopted 768 rotations instead of 769, and may have been led to this by the excessive periods of Cassini and Maraldi and by the want of intermediate data between his own observations in April 1777 and May-June 1779. His second determination, made in 1784, is more correct. [32] Deduced from observations extending over 15 years only, at Bristol. CHAPTER X. _THE PLANETOIDS._ Number.—History of their Discovery.—Dimensions and Brightness.—Occultation of Vesta.