Yet there is an answer on the part of the Earth to these solar changes. The Earth itself is a kind of magnet, possessing a magnetism of which the intensity and direction is always changing. To watch these changes, very sensitive magnets are set up, and a slight daily to-and-fro swing is noticed in them; this swing is more marked in summer than in winter, but it is also more marked at times of the sunspot maximum than at minimum, showing a dependence upon the solar activity. Yet more, from time to time the magnetic needle undergoes more or less violent disturbance; in extreme cases the electric telegraph communication has been disturbed all over the world, as on September 25, 1909, when the submarine cables ceased to carry messages for several hours. In most cases when such a "magnetic storm" occurs, there is an unusually large or active spot on the Sun. The writer was able in 1904 to further prove that such "storms" have a marked tendency to recur when the same longitude of the Sun is presented again towards the Earth. Thus in February 1892, when a very large spot was on the Sun, a violent magnetic storm broke out. The spot passed out of sight and the storm ceased, but in the following month, when the spot reached exactly the same apparent place on the Sun's disc, the storm broke out again. Such magnetic disturbances are therefore due to streams of particles driven off from limited areas of the Sun, probably in the same way that the long, {77} straight rays of the corona are driven off. Such streams of particles, shot out into space, do not spread out equally in all directions, like the rays of light and heat, but are limited in direction, and from time to time they overtake the Earth in its orbit, and, striking it, cause a magnetic storm, which is felt all over the Earth at practically the same moment. JUPITER is, after the Sun, much the largest member of the solar system, and it is a peculiarly beautiful object in the telescope. Even a small instrument shows the little disc striped with many delicately coloured bands or belts, broken by white clouds and dark streaks, like a "windy sky" at sunset. And it changes while being watched, for, though 400,000,000 miles away from us, it rotates so fast upon its axis that its central markings can actually be seen to move. This rapid rotation, in less than ten hours, is the most significant fact about Jupiter. For different spots have different rotation periods, even in the same latitude, proving that we are looking down not upon any solid surface of Jupiter, but upon its cloud envelope--an envelope swept by its rapid rotation and by its winds into a vast system of parallel currents.