Page:Popular Science Monthly Volume 13.djvu/357

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ON THE FORMATION OF NEBULÆ.

to the present time, the path pointed out by the illustrious Laplace will be closely followed.

More recent writers have suggested other modes for the formation of such systems, some of which, however, would seem to be the exception instead of the rule here presented, as the sequel will show.

No definite explanation will be given in this paper as to how the out-thrown masses of nebulous matter—which will hereinafter be described—may be utilized in forming other systems; and no suggestion given as to the formation or structure of the globular or other clusters of stars, which are so bewildering to investigate, and yet so interesting to look upon; but the formation of these, and of that great galaxy of stars of which our sun seems to be a member, may be accounted for on the same dynamical principles that are employed in this discussion.

Following Laplace, it is assumed, then, that our planetary system began its evolutions as a great nebulous sphere of perfectly dissociated matter of almost inconceivable rarity, and of nearly uniform density.

At this time it was not quiescent, but most probably was agitated by the movement of internal currents and counter-currents, the resultant motion of which caused the entire mass to revolve around one of its diameters.

From the nature of the case, this rotation must have been very slow indeed, probably one rotation in millions of years. If its diameter were 500 times that of Neptune's present orbit, it would require more than 36,000,000 years to make one revolution; even this velocity would slightly flatten it at the poles.

During these revolutions it was radiating heat, not only from its surface, but, on account of its extreme rarity,[1] from regions far below its surface. This radiation, combined with the gravitation of its parts, caused contraction; and this contraction, by well-known mechanical laws, increased the rate of rotation, and a consequent further flattening at the poles, while a correspondingly greater increase of density in the central parts took place.

This process continued till the rate of rotation of the bulging equatorial belt generated sufficient tangential force, by virtue of the rapid rotation of those particles, to counterbalance the gravitating force of the disk-like mass within, at which time further contraction of the outer edge of this nebulous belt, or beginning of a ring, ceased, except that infinitesimal portion due to resistance alone; for, at this time, each particle in this outer belt revolved in its own orbit around the central mass.

As radiation continued, contraction also continued, thereby conferring a planetary character on each particle of the successive layers of the equatorial ring of particles thus left out by the contracting nebulous spheroid.

Let us now confine our attention, to this out-left ring, for its conduct

  1. It should be here remarked that the heat thus radiated from the nebula is not lost, but continues as wave-motion in the ether, until it is again converted into molecular motion in ponderable matter.