[19] Mösting, Lalande, and Herschel form a fine triangle when the Sun has attained a great altitude. Mösting is a ray-centre. [20] A ring-plain 37½ miles in diameter, with very irregular terraced walls. [21] A range of mountains, with intervening valleys. [22] Mädler describes this as a square enclosure with rampart-like boundaries, which “throw the observer into the highest astonishment.” [23] A great walled plain, 91 miles in diameter. [24] A walled plain, 55 miles in diameter, in which Schröter suspected changes. [25] An extensive mountain-range on the E. by S. limb. [26] A walled plain, 95 miles in diameter, and probably the deepest in the N.E. quadrant, for the S.E. side of its wall rises to nearly 17,000 feet. After the full the same objects should be re-examined under the reversed illumination. CHAPTER VII. _MERCURY._ Supposed planet, “Vulcan.”—Visibility of Mercury.—Period &c.—Elongations.—Amateur’s first view.—Phases.—Atmosphere of Mercury.—Telescopic observations.—Schiaparelli’s results.—Observations of Schröter and Sir W. Herschel.—Transits of Mercury.—Occultations of Mercury. “Come, let us view the glowing west, Not far from the fallen Sun; For Mercury is sparkling there, And his race will soon be run. With aspect pale, and wav’ring beam, He is quick to steal away, And veils his face in curling mists,— Let us watch him while we may.” _Supposed planet “Vulcan.”_—Mercury is the nearest known planet to the Sun. It is true that a body, provisionally named Vulcan[27], has been presumed to exist in the space interior to the orbit of Mercury; but absolute proof is lacking, and every year the idea is losing strength in the absence of any confirmation of a reliable kind. Certain planetary spots, observed in motion on the solar disk, were reported to have been transits of this intra-Mercurial orb. Some eminent astronomers were thus drawn to take an affirmative view of the question, and went so far as to compute the orbital elements and predict a few ensuing transits of the suspected planet. But nothing was seen at the important times, and some of the earlier observations have been shown to possess no significance whatever, while grave doubts are attached to many of the others. Not one of the regular and best observers of the Sun has recently detected any such body during its transits (which would be likely to occur pretty frequently), and there is other evidence of a negative character; so that the ghost of Vulcan may be said to have been laid, and we may regard it as proven that no major planet revolves in the interval of 36,000,000 miles separating Mercury from the Sun.