THE ASTRONOMICAL HISTORY OF WORLDS. 581 



extraneous disturbance. A tidal force more than a thousand times great- 

 er 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 plan- 

 ets, 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 ar- 

 rangement, so necessary for security against excessive tides in a terra- 

 queous 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 ex- 

 perience 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 bod- 

 ies, the lunar waters pressing during nearly fourteen days to the points 

 nearest and most distant from the earth, and then retiring to other lo- 

 calities. 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 sup- 

 posed case, may in the vicinity of great central orbs, and in the absence 

 of great periodical disturbances, become potent enough to impress pe- 

 culiar 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 in- 

 debted 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 con- 

 siderable 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 impos- 

 sible for satellites to hold their parts together if revolving close to its 



