But no uncertainty in the data on which these calculations proceed, andno consequent error in the inferred rate at which the Sun is expending hisreserve energy, militates against the general proposition that this reserveof energy is being expended, and must in time be exhausted.
Thus while the Solar System, if evolved from diffused matter, has illustratedthe law of equilibration in the establishment of a moving equilibrium; andwhile, as at present constituted, it illustrates the law of equilibrationin the perpetual balancing of all its movements; it also illustrates thislaw in these processes which astronomers and physicists infer are still goingon. That motion of masses produced during Evolution, is being slowly rediffusedin molecular motion of the ethereal medium; both through the progressiveintegration of each mass, and the resistance to its motion through space.
Infinitely remote as may be the state when all the relative motions of itsmasses shall be transformed into molecular motion, and all the molecularmotion dissipated; yet such a state of complete integration and completeequilibration, is that towards which the changes now going on throughoutthe Solar System inevitably tend. §172. A spherical figure is the one which can alone equilibrate theforces of mutually-gravitating molecules. If an aggregate of such moleculesrotates, the form of equilibrium becomes a spheroid of greater or less oblateness,according to the rate of rotation; and it has been ascertained that the Earthis an oblate spheroid, diverging just as much from sphericity as is requisiteto counterbalance the centrifugal force consequent on its velocity roundits axis. That is to say, during the evolution of the Earth, there has beenreached an equilibrium of those forces which affect its general outline.
The only other equilibration which the Earth as a whole can exhibit, is theloss of its rotation; and that any such loss is going on we have no directevidence. It has been contended, however, by Prof. Helmholtz and others,that inappreciable as may be its effect within known periods of time, thefriction of the tidal wave must be diminishing the Earth's motion round itsaxis, and must eventually destroy it. Now though it seems an oversight tosay that the axial motion can thus be destroyed, since the extreme effect,to be reached only in infinite time, would be an extension of the Earth'sday to the length of lunation; yet it seems clear that this friction of thetidal wave is a real cause of decreasing rotation. Slow as its action is,we must recognize its retarding effect as exemplifying, under another form,the universal progress towards equilibrium.(*)
It is needless to show in detail how those movements which the Sun's raysgenerate in the air and water on the Earth's surface, and through them inthe Earth's solid substance,(*) oneand all teach the same general truth. Evidently the winds and waves and streams,as well as the denudations and depositions they effect, illustrate on a grandscale, and in endless modes, that gradual dissipation of motions describedin the first section, and the consequent tendency towards a balanced distributionof forces. Each of these sensible motions, produced directly or indirectlyby integration of those insensible motions communicated from the Sun, becomesdivided and subdivided into motions less and less sensible; until by gradualor sudden arrest of each, and production of its equivalent in molecular motion,there is an escape of it into space in the shape of thermal undulations.
In their totality, these complex motions constitute a dependent moving equilibrium.
As we before saw there is traceable throughout them an involved combinationof rhythms. The unceasing circulation of water from the ocean to the landand from the land back to the ocean, is a type of these various compensatingactions which, in the midst of all the irregularities produced by their mutualinterferences, maintain an average. And in this, as in other equilibrationsof the third order, we see that the energy ever in course of dissipation,is ever renewed from without: the rises and falls in the supply being balancedby rises and falls in the expenditure; as witness the variations of meteorologicactivity in northern zones caused by changes of the seasons. But the factit chiefly concerts us to note is that this process must go on bringing thingsever nearer to complete rest. These mechanical movements, meteorologic andgeologic, which are continually being equilibrated, both temporary by counter-movementsand permanently by the dissipation of such movements and counter-movements,will slowly diminish as the quantity of force received from the Sun diminishes.
As the insensible motions propagated to us from the centre of our systembecome feebler, the sensible motions here produced by them must decrease;and at that remote period when the solar heat has ceased to be appreciable,there will no longer be any appreciable re-distributions of matter on thesurface of our planet.