The only practical evidence which has been adduced to prove the resistance of a medium, namely, a very slight diminution in the period of that nearly evanescent body, Encke's Comet, is very far from being definite and satisfactory. The mass of the moon being enormously greater, it is probable that many millions of years will pass before a diminution of her orbital period from this cause will be perceptible. The immense periods of time attributed to the past processes of geological evolution, and to the supposed metamorphoses of organic life, are therefore very brief when compared with those required for the returns of satellites to their parent orbs, admitting, as theoretical considerations seem to require, that such returns are ultimately inevitable.
The eccentricity being diminished by the resistance of a medium, the moon's orbit would eventually become, and afterward continue, circular, so that final contact would be unaccompanied by violent collision. But, before the time of actual contact, changes of form would be induced both in planet and satellite by mutual attractions, exemplified in the production of daily terrestrial tides. The investigations of Hopkins, Thomson, and recently of Barnard, in regard to tidal and precessional influences, indicate that, even at the present distance of the moon, they must cause elongations and contractions of the solid materials of the earth, which are quite appreciable. A considerable diminution of the distance between the earth and moon would give rise to changes in the form of the earth, and hence to bendings to and fro of its external shell even if the earth were solid throughout. This would be accompanied by earthquakes and kindred disturbances far exceeding in magnitude and destructiveness any thing of the kind now known to man. The frequency of these occurrences would be the same as that of the moon's meridian passage.
Resistances to this tidal action, however, would be developed, in consequence of which the molar motion of rotation would be converted into molecular motion, so long as the angular motion of rotation in either body was different from that of the moon's revolution, until the rotations became synchronous with the revolution, a condition already arrived at in the case of the moon. Synchronism once attained would be permanent, acceleration both of revolution and rotation occurring as the distance diminished, and both at the expense of the potential energy of gravity between the two bodies. Each body presenting the same face to the other, no meridian passage could take place, and hence no tidal action.
But there yet remains to be considered a continually increasing tendency to distortion of form consequent upon approach. This effect would be produced very gradually, being spread over such enormous durations of time. The curious and complicated foldings of the rocks in the Appalachian regions indicate that the solid materials of the earth are sufficiently plastic to allow it to take on any form toward which forces of sufficient magnitude direct it, provided the times be