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While the theory of dwarf and giant stars and the measurements just described afford no direct evidence bearing on Laplace's explanation of the formation of planets, they show that stars exist which are comparable in diameter with our solar system, and suggest that the sun must have shrunk from vast dimensions.
I read with much interest Laplace's report to the Senate, and must confess I was very glad to see the Gregorian calendar again acknowledged by law, as it had already been acknowledged in fact. Frenchmen in foreign countries experienced particular inconvenience from the adoption of a system different from all the rest of the world.
Lingard, from your Oriental Shasters, and Talmuds, and Korans, with Cassini's Siamese fables, and Laplace's Mecanique Celeste, down to Robinson Crusoe and the Belfast Town and Country Almanack, are familiar to him, we shall say nothing: for unexampled as it is with us, to the Germans such universality of study passes without wonder, as a thing commendable, indeed, but natural, indispensable, and there of course.
Of this nature was Laplace's theory, whether weak or not, as to the origin of the earth and planets. The following is the way. But in others of the same class, the permanency of the effect is only the permanency of a series of changes.
Since its promulgation, however, knowledge has transgressed many boundaries, and set at naught much ingenious theorising. How has it fared with Laplace's sketch of the origin of the world? It has at least not been discarded as effete. The groundwork of speculation on the subject is still furnished by it. It is, nevertheless, admittedly inadequate.
The first reason is that, accepting Laplace's theory of the origin of the planetary system from a series of rings left off at the periphery of the contracting solar nebula, Mars must have come into existence earlier than the earth, because, being more distant from the center of the system, the ring from which it was formed would have been separated sooner than the terrestrial ring.
Outside the orbit of Mars, solar tidal friction can scarcely be said to possess at present any sensible power. But it is far from certain that this was always so. It seems not unlikely that its influence was the overruling one in determining the direction of planetary rotation. M. Faye, as we have seen, objected to Laplace's scheme that only retrograde secondary systems could be produced by it.
Before considering more recent theories than Laplace's, let us see what the nature of the photographic revelations is. The vast celestial maelstrom discovered by Lord Rosse in the ``Hunting Dogs'' may be taken as the leading type of the spiral nebulæ, although there are less conspicuous objects of the kind which, perhaps, better illustrate some of their peculiarities.
Halley discovered in 1693, by examining the records of ancient eclipses, that the moon was going faster then than 2,000 years previously so much faster, as to have got ahead of the place in the sky she would otherwise have occupied, by about two of her own diameters. It was one of Laplace's highest triumphs to have found an explanation of this puzzling fact.
This has an important bearing on the nebular hypothesis of the origin of the solar system, and at one time was thought to furnish a convincing argument against that hypothesis; but it has been shown that by a modification of Laplace's theory the peculiar behavior of Uranus and Neptune can be reconciled with it. Very little is known of the surfaces of Uranus and Neptune.
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