No. 8.

On the Origin of the Solar System.

I have already shewn that the rotations and the enormous heat of the solar system may be due to partial impact, la this letter I shall attempt to shew that the planets may have originated at the birth of the sun, and not subsequently by the release of zones and their subsequent condenaation as supposed by Laplace. I shall attempt, in a future letter, to show that that hypothesis appears inconsistent with the present energy of the rotation of the sun, and on several other dynamical grounds, as well as with the molecular theory of gases. As far and the subject has been discussed, there does not appear to be a single motion in the whole solar system inconsistent with the hypothesis of partial impact, and those very points which tell most against the zone hypothesis, are some of the strongest points in favour of the present hypothesis. Of course, in a short letter I can only give the faint outlines of such a wide subject. I am of opinion that the balance of evidence at present tells in favour of our solar system having been formed by the impact of two very Urge bodies of unequal sizes; that the coalesced part did not form a large ratio of the whole, and that the velocity was such that a considerable proportion of the material struck off the small body probably escaped into space, and in all probability neither of the larger pieces of the original bodies returned again but also escaped into space. I have already shown that a long spindle shaped mass, with an axis of rotation at right angles to its length, will be generally produced as the middle body, resulting from a case of partial impact. The ends of this mass will generally have resultant molar motion and be thrown off, often passing completely away from the attractive influence of the general mass; but some of the pieces would frequently return again. It is certain that these masses would be in advance of the gas. At first, the orbits of these bodies would be extraordinarily eccentric. On passing away on their first journey they would be in advance of the expelled gap, but would meet it on returning. As the motion of this gas would be directly outward the energy of its motion would act in a direction exactly opposed to gravitation, the path of the planets would thus be rendered straighter, and the perhelion distance would be increased; also the resistance of the gas would be greatest at perhelion, and this would lesson aphelion distance. Thus both of these forces tend to render the orbit more circular. Again, on each of its orbits the attraction of gravitation would be greater on its outward journey than on its return, in consequence of the expelled matter passing outside its orbit into space. This fact would both tend to render the orbit more circular and neutralise the action of the gaseous resistance in causing the body to approach the sun. It is a well-known fact that if a projectile revolves on an axis at right angles to the line of motion, there is a tendency to move in a curve. In the case of the planets this action would tend to straighten the curve, and, at it would not act until the return of the planet, as the planet would be in advance of the gas, it would tend to increase the perhelion's distance. All these facts, therefore, tend to render the orbits more circular, but not as an average result to considerably alter their mean distance from the sun. The larger masses would suffer less resistance in proportion than the smaller once, and the general result must be that, if all started at the same distance, the smaller bodies would be brought nearer the sun. It is easy to see that the centrifugal force, and the attraction of the nebulous mass, would cause all the planets to travel approximately in the plane of the eoliptic; also why the sun's equator so nearly approaches it, and generally why the rotations of the planets on their axes should be in the same directions. On the other hand, the pressure due to heat, the extreme wast of symmetry of such a case of partial impact, combined with the original motion of rotation of the colliding bodies (if they had any), must all tell in the ultimate resultant motion, both orbital and axial. Almost certainly these forces would produce slightly inclined orbital planes, with inclination of polar axes to these planes, and may, as en extreme case, produce a retrogade motion. It is also easy to see that the enormous atmosphere of those early days would effectually clear the bodies of all but very large masses of cosmical dust. This fact seems to suggest that the production of the asteroids and of Saturn's ring were secondary phenomena, having probably been produced when the nebulous matter had shrunk within their respective orbits. I shall probably discuss their origin in a future letter.