[31] An extensive mountain-range on the E. by S. limb.
[32] 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.
[33] Chambers, in his ‘Descriptive Astronomy,’ 4th edition, 1889, devotes a chapter to the discussion of facts having reference to Vulcan; and the reader desiring full information will find it here.
[34] This period was probably derived erroneously by Bianchini. It includes 25 periods of 23h 22m, which corresponds with the times of rotation by Cassini and others given in the table.
[35] Schröter’s final result. In 1788 he had derived a period of 23h 28m from observations of faint dark spots, and in 1789-91 irregularities in the S. horn of Venus gave him a period of 23h 20m 59s.
[36] 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.”
[37] 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.
[38] Deduced from observations extending over 15 years only, at Bristol.
[39] The question of periodicity is an extremely interesting one as affecting the disposition, form, and colours of the markings on Jupiter. Certain features visible in 1869-70 were unmistakably reproduced in 1880, and it has been suspected that the cycle of these changes accords with the length of the Jovian year. Future observations must be compared with old drawings and records for the identification of similar features if they are recurrent.