Pagina:Scientia - Vol. VII.djvu/253

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the origin and nature of comets 245

would take some hours, in which case the rotation of the planet would greatly alter the direction of projection. I do not think there is much in this objection, as if the whole mass of a small comet were concentrated into a compact form, it would not occupy many cubic miles, and its ejection might be practically instantaneous. In fact there in less difficulty in this than in supposing that if the comet came from regions outside the planetary system, and was merely captured by the planet, it should be sufficiently small and compact for all parts of it to suffer the same perturbations. For a very close appulse is required for capture, and if the comet were a few thousand miles in diameter, the perturbations of different parts would be sensibly different.

In discussing the solar origin of comets, we pointed out the difficulty of conceiving how the ejected matter could form such large masses of iron as we find in meteors. The difficulty would be less in the case of the planets, the temperature being probably low enough for iron to assume a solid form quite close to their surfaces.

I conclude from all this that the hypothesis of the planetary origin of short-period comets deserves consideration, and should not be dismissed so summarily as it is by many astronomers.

Bredichin has given a different hypothesis for the origin of the short-period comets in his pamphlet «Sur l’origine des comètes périodiques» Moscow, 1889. He supposes that they have simply arisen from the splitting up of larger comets, on the analogy of Biela’s comet, that of Liais in 1860, of the great comet of 1882, and Brooks’s comet in 1889. But in none of these cases is there evidence of notable alteration in the period, nor does the theory explain in any way the relation of the orbits to those of the four giant planets.

The third hypothesis of the origin of comets is that they are unattached outlying fragments of the nebula, which is conjectured to have been the parent of the solar system. I consider that we are practically driven to this theory by exclusion in the case of those comets whose perihelion distance is large, and which do not belong to the planetary families. Seeing that comets have a large amount of meteoric matter associated with them, we must assume a meteoric, rather than a purely gaseous nebula.