In 1865 Zöllner showed that the rapid motions of the cloud-belts on both Jupiter and Saturn indicated a high internal temperature. At the distance of Jupiter sun-heat is only one twenty-seventh as great as on the Earth, and would be quite incapable of forming clouds many times denser than those on the Earth. In 1871 Zöllner drew attention to the equatorial acceleration of Jupiter, analogous to the same phenomenon on the Sun. In 1870 these opinions of Zöllner’s were adopted and supported by Proctor in his ‘Other Worlds than Ours.’ In his subsequent volumes Proctor did much to popularise the idea, which is now accepted all over the astronomical world. During the century many valuable observations on Jupiter were made by numerous observers, among them Airy, Mädler, Webb, Schmidt, and others. Much time was devoted to the accurate determination of the rotation period, which was fixed at 9 hours 55 minutes 36·56 seconds by Denning in observations from 1880 to 1903. No really important discovery was made till 1878, when Niesten at Brussels discovered the “great red spot,” a ruddy object 25,000 miles long by 7000 broad, attached to a white zone beneath the southern equatorial belt. This remarkable object has been observed ever since. In 1879 its colour was brick-red and very conspicuous, but it soon began to fade, and Riccó’s observation at Palermo in 1883 was thought to be the last. After some months, however, it brightened up, and, notwithstanding changes of form and colour, it is still visible, a permanent feature of the Jovian disc. In 1879 a group of “faculæ,” similar to those on the Sun, was observed at Moscow by _Theodor Alexandrovitch Brédikhine_ (1831-1904), and at Potsdam by _Wilhelm Oswald Lohse_ (born 1845). It was soon observed that the rotation period, as determined from the great red spot, was not constant, but continually increasing. A white spot in the vicinity completed its rotation in 5½ minutes less, indicating the differences of rotation on Jupiter. The great red spot has been observed since its discovery by Denning at Bristol and _George Hough_ (born 1836) at Chicago. Twenty-eight years of observation have not solved the mystery of its nature. The researches made on it, in the words of Miss Clerke, “afforded grounds only for negative conclusions as to its nature. It certainly did not represent the outpourings of a Jovian volcano; it was in no sense attached to the Jovian soil—if the phrase have any application to the planet; it was not a mere disclosure of a glowing mass elsewhere seethed over by rolling vapours.”