If we may adopt this view, then we must regard the sphere as subject to the tidal influence of the sun. It was much hotter, and therefore more yielding, than the present earth; it was also rotating much faster, probably once in about four or five hours. It would be contracting as a consequence of cooling, and the contraction would lead to instability (gravitational instability); its rapid rotation would also tend toward instability (rotational instability). It is difficult to say which of these two, gravitational or rotational instability, would be the most effective; but the combined result would be to give a pear-shaped form to the rotating mass, and eventually to deepen the constriction between the narrow and the broad end, till the smaller protuberance became completely dissevered from the larger mass, and so entered on an independent existence as the moon. This final step in the process would probably depend on the tide-producing power of the sun; the larger mass remained behind as the earth, whose individual existence may be said to date from this event. [Sidenote: How the Moon Broke Away] The young earth would be subject to very much the same conditions after as before the ejection of the moon, and might very possibly again pass into a pear-shaped form, but without proceeding further through those subsequent changes, which would have led to the formation of another satellite; and while possessing some such form as this, she might very well have consolidated. With advancing years she would lose, as we have seen, the activity of her youth, the drag of the tides would cause her to spin ever more slowly on her axis, till the day would become prolonged to the twenty-four hours of the present. With this diminished rate of spin, the earth, if free to yield, would lose the pear-shaped form and become an oblate spheroid, and the oblateness of this spheroid would continually diminish, so that it would continually approach towards a true sphere. Suppose, however, that the earth as it cooled lost its power of readily yielding--and at present it is more rigid than a globe of steel--then it would pass from form to form, not by a flowing movement, but by a series of ruptures, and its form at any moment might be a little in arrear of that which it would have possessed if it had been in the fluid state.