hand as the planet moves through the field), by a sharply upstanding shoulder or cape. The whole appearance of this bay irresistibly suggests to the observer that it has somehow or other been hollowed out to make room for the spot, which floats, as it were, within it, surrounded generally by a margin of bright material, which divides it from the brown matter of the belt. The red spot, with its accompanying bay and cape, is shown in Fig. 25 and in Plate XXII., which represents the planet as seen by the Rev. T. E. R. Phillips on January 6, 1906. The spot has varied very much in colour and in visibility, but on the whole its story has been one of gradual decline; its tint has paled, and its outline has become less distinct, as though it were being obscured by an outflow of lighter-coloured matter, though there have been occasional recoveries both of colour and distinctness. In 1891 it was a perfectly easy object with 3-7/8 inches; at the present time the writer has never found it anything but difficult with an 18-inch aperture, though some observers have been able to see it steadily in 1905 and 1906 with much smaller telescopes. The continued existence of the bay already referred to seems to indicate that it is only the colour of the spot that has temporarily paled, and that observers may in course of time witness a fresh development of this most interesting Jovian feature. The nature of the red spot remains an enigma. It may possibly represent an opening in the upper strata of Jupiter's dense cloud-envelope, through which lower strata, or even the real body of the planet, may be seen. The suggestion has also been made that it is the glow of some volcanic fire on the body of the planet, seen through the cloud-screen as the light of a lamp is seen through ground-glass. But, after all, such ideas are only conjectures, and it is impossible to say as yet even whether the spot is higher or lower than the average level of the surface round it. A curious phenomenon which was witnessed in 1891 suggested at first a hope that this question of relative height would at least be determined. This phenomenon was the overtaking of the red spot by a dark spot which had been travelling after it on the same parallel, but with greater speed, for some months. It appeared to be quite certain that the dark spot must either transit the face of the red spot or else pass behind it; and in either case interesting information as to the relative elevations of the two features in question would have been obtained. The dark spot, however, disappointed expectation by drifting round the south margin of the red one, much as the current of a river is turned aside by the buttress of a bridge. In fact, it would almost appear as though the red spot had the power of resisting any pressure from other parts of the planet's surface; yet in itself it has no fixity, for its period of rotation steadily lengthened for several years until 1899, since when it has