THE DISSIPATION OF ENERGY. 431 



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 con- 

 siderations 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 col- 

 lision. But, before the time of actual contact, changes of form would 

 be induced both in planet and satellite by mutual attractions, exem- 

 plified 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 mate- 

 rials of the earth, which are quite appreciable. A considerable dimi- 

 nution 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 



