The first great cosmological speculation which has been raised quite above the plane of guesswork by making no other assumption than that of the uniformity of nature, is the well-known Nebular Hypothesis. Every astronomer knows that the earth, like all other cosmical bodies which are flattened at the poles, was formerly a mass of fluid, and consequently filled a much larger space than at present. It is further agreed, on all hands, that the sun is a contracting body, since there is no other possible way of accounting for the enormous quantity of heat which he generates. The so-called primeval nebula follows as a necessary inference from these facts. There was once a time when the earth was distended on all sides away out to the moon and beyond it, so that the matter now contained in the moon was then a part of our equatorial zone. And at a still remoter date in the past, the mass of the sun was diffused in every direction beyond the orbit of Neptune, and no planet had an individual existence, for all were indistinguishable parts of the solar mass. When the great mass of the sun, increased by the relatively small mass of all the planets put together, was spread out in this way, it was a rare vapour or gas. At the period where the question is taken up in Laplace's treatment of the nebular theory, the shape of this mass is regarded as spheroidal; but at an earlier period its shape may well have been as irregular as that of any of the nebulae which we now see in distant parts of the heavens, for, whatever its primitive shape, the equalization of its rotation would in time make it spheroidal. That the QUANTITY of rotation was the same then as now is unquestionable; for no system of particles, great or small, can acquire or lose rotation by any action going on within itself, any more than a man could pick himself up by his waistband and lift himself over a stone wale So that the primitive rotating spheroidal solar nebula is not a matter of assumption, but is just what must once have existed, provided there has been no breach of continuity in nature's operations. Now proceeding to reason back from the past to the present, it has been shown that the abandonment of successive equatorial belts by the contracting solar mass must have ensued in accordance with known mechanical laws; and in similar wise, under ordinary circumstances each belt must have parted into fragments, and the fragments chasing each other around the same orbit, must have at last coalesced into a spheroidal planet. Not only this, but it has also been shown that as the result of such a process the relative sizes of the planets would be likely to take the order which they now follow; that the ring immediately succeeding that of Jupiter would be likely to abort and produce a great number of tiny planets instead of one good-sized one; that the outer planets would be likely to have many moons, and that Saturn, besides having the greatest