"Credat Judaeus Apella; Non ego." We pass to the next chapter in the history, which professes to explain the gradual formation of a solar system by a process of cooling and shrinking, to which the central orb is exposed. And here we are met by a difficulty at the outset; for the existence of comets with their very eccentric orbits is wholly irreconcilable with the theory. At their perihelion, many of these bodies pass within the orbit of Mercury, while the aphelion of some lies without the path of Uranus. Where were they, when the body of the sun filled up the whole of the vast sphere circumscribed by the orbit of the remotest planet? If we suppose that they are late comers, after the rest of our system was perfected,--that they were generated by themselves in distant regions of space, and, having strayed about, orphan-like, for a while, they accidentally crossed our track, and were taken as adopted children into our family, another question remains to be answered. Why did they not remain in their first position, absorb their full share of nebulous matter, beget a respectable family of planets, and take rank as chiefs of their own clan? These comparatively anomalous bodies are great stumbling-blocks for the _soi-disant_ historians of creation. Again, if an immense orb be formed, the parts of which cohere strongly enough for the whole to turn upon its axis as one body, the process of cooling can go on only from the surface. A crust may finally be formed there; but we see not how the refrigeration and shrinking of the interior parts can then go on separately, until the mass in the centre finally becomes detached from its envelope, like a shrivelled nut from its shell. Our earth is cooling down at this moment, unless the warmth which it receives from the sun exactly counterbalances the loss by radiation of internal heat. But the exterior and interior do not cool by different radiations, nor is there, so far as we know, the least tendency in the central mass to shrink separately, so as to detach itself from the surrounding crust. As deep as we can penetrate towards the centre, we find the heat regularly increase,--just as we might expect, if the only absolute loss of heat be from the surface.