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Page:Popular Science Monthly Volume 4.djvu/61

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51
THE RINGED PLANET.

that, whereas there is nothing inexplicable or even very surprising in supposing that Saturnian cloud-layers, resulting from the action of intense Saturnian heat, alter greatly at times in level, the observations we have described become altogether inexplicable, and cannot, in fact, be rejected, if we adopt the theory that Saturn resembles the earth on which we live.

It may be asked whether Jupiter, to which planet the same reasoning may be applied, has ever shown signs of similar changes. To this it may first be replied, that we should not expect Jupiter to be affected to the same degree, simply because the chief disturbing causes—his satellites and the sun—are always nearly in the same level, owing to the peculiarity in Jupiter's pose to which attention has already been directed. But, secondly, such briefly-lasting changes as we might expect to detect have occasionally been suspected by observers of considerable skill; and among others by the well-known Schröter, of Lilienthal. Such changes have consisted, for the most part, merely in a slight flattening of a part of Jupiter's outline. But on one occasion a very remarkable phenomenon, only (but very readily) explicable in this way, was witnessed by three practised observers—Admiral Smyth, Prof. Pearson, and Sir T. Maclear—at three different stations. Admiral Smyth thus describes what he saw: "On Thursday, June 26, 1828, the evening being extremely fine, I was watching the second satellite of Jupiter as it gradually approached to transit Jupiter's disk. It appeared in contact at about half-past ten, and for some minutes remained on the edge of the disk, presenting an appearance not unlike that of the lunar mountains coming into view during the moon's first quarter, until it finally disappeared on the body of the planet. At least twelve or thirteen minutes must have elapsed, when, accidentally turning to Jupiter again, to my astonishment I perceived the same satellite outside the disk! It remained distinctly visible for at least four minutes, and then suddenly vanished!" For our own part, we can conceive of no possible explanation of this remarkable phenomenon, unless it be admitted that the change was in the apparent outline of Jupiter. Of course, to suppose that even a cloud-layer rose or fell, in a few minutes, several thousand miles (about 8,000, if the stated times be correct), is as inadmissible as to suppose the solid crust of a globe to undergo so vast a change of level; but nothing of this sensational description is required. All that would be necessary would be, that an upper cloud-layer should for a few minutes be dissipated into vapor, either by warm currents, or more probably by a temporary increase of the heat supplied by Jupiter's fiery globe within the cloud-envelopes, and that a few minutes later the clouds should form again by the condensation of the vaporized matter. The changes in the aspect of the Jovian belts are often sufficiently rapid to indicate the operation of precisely such processes.

Associated with such phenomena as we have mentioned is the evi-