Hypothesis aside, the principle of equilibration is still perpetuallyillustrated in those minor changes of state which the Solar System undergoes.
Each planet, satellite, and comet, exhibits at its aphelion a momentary equilibriumbetween that force which urges it further away from its primary, and thatforce which retards its retreat. In like manner at perihelion a converseequilibrium is momentarily established. The variation of each orbit in eccentricity,and in the position of its plane, has similarly a limit at which the forcesproducing change in the one direction, are equalled by those antagonizingit; and an opposite limit at which an opposite arrest takes place. Meanwhile,each of these simple perturbations, as well as each of the complex ones resultingfrom their combination, exhibits, besides the temporary equilibration ateach of its extremes, a certain general equilibration of compensating deviationson either side of a mean state. That the moving equilibrium thus constitutedtends, in the course of indefinite time, to lapse into a complete equilibrium,by the gradual decrease of planetary motions and eventual integration ofall the separate masses composing the Solar System, is a belief suggestedby certain observed cometary retardations -- a belief entertained by someof high authority. The received option that the appreciable diminution inthe period of Encke's comet, implies a loss of momentum caused by resistanceto the ethereal medium, commits astronomers who hold it, to the conclusionthat this same resistance must cause a loss of planetary motions -- a losswhich, infinitesimal though it may be in such periods as we can measure,will, if indefinitely continued, bring these motions to a close. Even shouldthere be, as Sir John Herschel suggests, a rotation of the ethereal mediumin the same direction with the planets, this arrest, though immensely postponed,would not be absolutely prevented. Such an eventuality, however, must inany case be so inconceivably remote as to have no other than a speculativeinterest for us. It is referred to here, simply as illustrating the still-continuedtendency towards complete equilibrium, through the still-continued dissipationof sensible motion, or transformation of it into insensible motion.
But there is another species of equilibration going on in the Solar System,with which the human race is less remotely concerned. The tacit assumptionthat the Sun can continue to give off an undiminished amount of light andheat through all future time, is now abandoned. Involving as it does, undera disguise, the conception of power produced out of nothing, it is of thesame order as the belief which misleads perpetual-motion schemers. The spreadingrecognition of the truth that whatever force is manifested under one shapemust previously have existed under another shape, implies recognition ofthe truth that the force known to us in solar radiations, is the changedform of some other force of which the Sun is the seat; and that, by the emissionof these radiations, this other force is being slowly exhausted. The forceby which the Sun's substance is drawn to his centre of gravity, is the onlyone which physical laws warrant us in concluding to be the correlate of theforces emanating from him: the only assignable source for the insensiblemotions constituting solar light and heat, is the sensible motion which disappearsduring the concentration of the Sun's mass. We before saw it to be a corollaryfrom the nebular hypothesis, that there is such a progressing concentrationof the Sun's mass. And here remains to be added the further corollary, thatjust as in the case of the small members of the Solar System, the heat generatedby concentration, once escaping rapidly, has in each left a central residuewhich escapes but slowly; so in the case of that immensely larger mass formingthe Sun, the immensely greater quantity of heat generated and still in processof rapid diffusion, must, as the concentration approaches its limit, diminishin amount, and eventually leave but a relatively small internal remnant.
With or without the accompaniment of that hypothesis of nebular condensationwhence it naturally follows, the doctrine that the Sun is gradually losinghis heat, has now gained general acceptance; and calculations have been made,both respecting the amount of heat and light already radiated, as comparedwith the amount that remains, and respecting the period during which activeradiation will continue. Prof. Helmholtz estimates that since the time when,according to the nebular hypothesis, the matter composing the Solar Systemextended to the Orbit of Neptune, there has been evolved by the arrest ofsensible motion, an amount of heat 454 times as great as that which the Sunstill has to give out. He also makes an approximate estimate of the rateat which this remaining 1/464th is being diffused: showing that decreaseof the Sun's diameter to the extent of 1/10,000 would produce heat, at thepresent rate, for more than 2000 years; or in other words, that a contractionof 1/20,000,000 of his diameter, suffices to generate the light and heatannually emitted; and that thus at the present rate of expenditure, the Sun'sdiameter will diminish by something like 1/20 in the lapse of the next millionyears.* Of coursethese conclusions are but rude approximations to the truth. Until quite recently,we have been totally ignorant of the Sun's chemical composition, and evennow have obtained but a superficial knowledge of it. We know nothing of hisinternal structure; and it is quite possible that the assumptions respectingcentral density, made in the foregoing estimates, are wrong.
