_Bright Equatoreal Spots._—The bright spots near the equator rotated in 9^h 50^m 6^s in 1880; but in subsequent years the time slightly increased, for in 1882 I found it 9^h 50^m 8^s·8, and in 1883 9^h 50^m 11^s·4. The bright spots therefore perform a rotation in 5½ minutes less time than the red spot. The former move so much more swiftly than the latter that they pass it at the rate of 260 miles per hour, and in 44½ days have effected a complete circuit of Jupiter relatively to it. Thus a brilliant white spot, if noticed in the same longitude as the red spot on one night, will, on subsequent nights, be observed to the W. of it, and, after an interval of about 44½ days, the same objects will again occupy coincident longitude.
_Dark Spots in N. hemisphere._—In the autumn of 1880 there was a confluent outbreak of dark spots from a belt in about 25° N. latitude, and these exhibited a rotation-period of only 9^h 48^m, so that they travelled more rapidly than the white spots on the equator. Some short dusky belts were also remarked slightly S. of the latitude of the red spot, and these indicated a period of 9^h 55^m 18^s. It is clear from these various results that the motion of the Jovian markings does not decrease according to their distance from the equator.
_Rotation-Period._—Below are given the times of rotation ascertained by some previous observers:—
h m s 1665. J. D. Cassini 9 55 58 1672. ” 9 55 50 1692. ” (A)9 50 1708. J. P. Maraldi 9 56 48 1713. ” 9 56 1773. Sylvabelle 9 56 1779. (B)W. Herschel 9 54 53} to 9 55 40} 1779. ” (A)9 50 48 } to 9 51 45·6} 1786. (C)J. H. Schröter 9 55 33·6 ” ” 9 55 17·6 ” ” (A)9 50 27 1835. J. H. Mädler 9 55 26·5 ” G. B. Airy 9 55 21·3 1836. J. H. Mädler 9 55 23·5 1862. J. F. J. Schmidt 9 55 25·7 1866. ” 9 55 46·3 1873. O. Lohse 9 55 19·6 1880. J. F. J. Schmidt 9 55 34·4 ” ” (A)9 50 1881. W. F. Denning 9 48 ” ” 9 55 17·9 1883. ” (A)9 50 8·7 1885. G. W. Hough 9 55 37·4 ” ” (A)9 50 9 1886. A. Marth 9 55 40·6 1887. A. S. Williams:— Spots in 12° N. lat. 9 55 36·5 ” 4° N. lat. (A)9 50 40·1 ” 8° S. lat. (A)9 50 22·4 ” 30° S. lat. 9 55 17·1 1890. (D)W. F. Denning 9 55 39
A: Bright spots near the equator of Jupiter.
B: Herschel’s observations embraced few rotations, and the periods he derived differed considerably.