From another class of observations we are also taught the important truth that Jupiter is not, superficially at least, a solid body. The period of rotation of the planet around its axis is derived from the observation of certain marks, which present sufficient definiteness and sufficient permanence to be suitable for the purpose. Suppose one of these objects to lie at the centre of the planet's disc; its position is carefully measured, and the time is noted. As the hours pass on, the mark moves to the edge of the disc, then round the other side of the planet, and back again to the visible disc. When it has returned to the position originally occupied the time is again taken, and the interval which has elapsed is called the period of rotation of the spot. If Jupiter were a solid, and if these features were engraved upon its surface, then it is perfectly clear that the time of rotation as found by any one spot must coincide precisely with the time yielded by any other spot; but this is not observed to be the case. In fact, it would be nearer the truth to say that each spot gives a special period of its own. Nor are the differences very minute. It has been found that the time in which the red spot (the latitude of which is about 25 deg. south) is carried round is five minutes longer than that required by some peculiar white marks near the equator. The red spot has now been watched for about twenty years, and during most of that time has had a tendency to rotate more and more slowly, as may be seen from the following values of its rotation period:-- In 1879, 9h. 55m. 33.9s. In 1886, 9h. 55m. 40.6s. In 1891, 9h. 55m. 41.7s. Since 1891 this tendency seems to have ceased, while the spot has been gradually fading away. Generally speaking, we may say that the equatorial regions rotate in about 9h. 50m. 20s., and the temperate zones in about 9h. 55m. 40s. Remarkable exceptions are occasionally met with. Some small black spots in north latitude 22 deg., which broke out in 1880 and again in 1891, rotated in 9h. 48m. to 9h. 49-1/2m. It may, therefore, be regarded as certain that the globe of Jupiter, so far as we can see it, is not a solid body. It consists, on the exterior at all events, of clouds and vaporous masses, which seem to be agitated by storms of the utmost intensity, if we are to judge from the ceaseless changes of the planet's surface. [Illustration: PLATE XI. Feb. 2nd. Feb. 4th. Feb. 12th. Feb. 28th. THE PLANET JUPITER. 1897.] [Illustration: Fig. 57.--The Occultation of Jupiter (1).] [Illustration: Fig. 58.--The Occultation of Jupiter (2).] [Illustration: Fig. 59.--The Occultation of Jupiter (3).] [Illustration: Fig. 60.--The Occultation of Jupiter (4).]