There are also indirect resistances, the effects of tidal friction, on all bodies which, like the earth, have portions of their free surfaces covered by liquids, which, so long as these bodies move relatively to neighboring bodies, must keep drawing off energy from their relative motions. "Thus, if we consider the action of the moon on the earth, with its oceans, lakes, and rivers, we perceive that it must tend to equalize the period of the earth's rotation on its axis, and of the revolution of the two bodies about their centre of inertia; because, so long as these periods differ, the tidal action of the earth's surface must keep subtracting energy from their motions."[160] As the tidal wave sweeps over the oceans and rushes into the numerous bays and estuaries, the motions which it produces in the waters necessarily involve an expenditure of power or _vis viva_ in overcoming the resistance from friction. The energy of motion thus expended must be drawn from the set of machinery which produces the motions--that is, from the motion of revolution of the moon, and the motion of rotation of the earth. It can not be returned to the machinery, because all that is not spent in triturating the sand and other materials composing the ocean-bed, is transformed into heat and radiated into space. It is true that in the present state of science we have not exact data for estimating the relative importance of tidal friction, and of the resistance of the interstellar medium; but, whatever it may be, there can be, says Thomson, "but one ultimate result for such a system as that of sun and planets if continuing long enough under existing laws.... That result is _the falling together into one mass, which, although rotating for a time, must in the end come to rest relatively to the surrounding medium_."[161]