Page:Popular Science Monthly Volume 13.djvu/599

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THE ASTRONOMICAL HISTORY OF WORLDS.

extraneous disturbance. A tidal force more than a thousand times greater prevails on the majority of the known secondary planets. If our globe, while keeping its present rotation, could exchange orbits with Jupiter's first satellite, our oceans would feel a periodical disturbance more than 20,000 times as powerful as that which now affects them; and perhaps few mountains would be high enough to escape being covered by the daily swelling of the waters. But by such a violent oceanic movement the earth's rotation should rapidly change until it kept pace with the revolution and then the destructive tides would come to a close, as our planet, having the same hemisphere ever turned to Jupiter, would ever elongate in the same direction, and our oceans would be elevated in the same localities. Now, all the secondary planets, so far as observation has been able to decide, have their movements so adjusted as to keep the same ever turned to the primary. This arrangement, so necessary for security against excessive tides in a terraqueous satellite, would be the inevitable result of tidal friction during past ages. Even in the absence of a liquid envelope, the same result would be produced by the deformation which a solid satellite would experience from the enormous tidal force, if it turned at such a rate as to present its different sides alternately to the primary.

Yet, notwithstanding this arrangement, a satellite would experience tidal oscillation, though on a lower scale, if its path deviated much from a true circle. If a watery envelope had been given to the moon in the same proportion as it has been to our globe, there would be tides occasioned by the periodical change of distance between the two bodies, the lunar waters pressing during nearly fourteen days to the points nearest and most distant from the earth, and then retiring to other localities. The friction in such tidal movements would have the effect of making the moon describe an ellipse somewhat smaller and nearer to a circle in form, but the alterations which it could produce in the lunar distance could not exceed two or three miles in the course of a million years. Yet the agency of permanent changes, so feeble in the supposed case, may in the vicinity of great central orbs, and in the absence of great periodical disturbances, become potent enough to impress peculiar features on the paths of secondary planets. To the tides which rose on their surfaces in past times, as well as to oscillations in their solid matter, the first and second satellites of Jupiter seem mainly indebted for their peculiarity in revolving in true circles; while in the orbit of the third moon there is a slight and in that of the fourth a considerable deviation from a circular figure.

I shall now proceed to the examination of cases in which the cause under consideration becomes far more potent, and under the required conditions makes its effects recognizable by observation, and admitting of no doubt. The powerful tidal action which a great central orb is capable of exerting in its immediate vicinity would render it impossible for satellites to hold their parts together if revolving close to its