13. We must avoid drawing inferences too boldly, on a subject where our means of knowledge are so obscure as they are with regard to the interior of the Earth; but yet, perhaps, we may be allowed to say, that the result which we have just stated, that the Earth is so light, suggests to us the belief that the interior consists of the same materials as the exterior, slightly condensed by pressure.[4] We find no encouragement to believe that there is a nucleus within, of some material, different from what we have on the outside; some metal, for instance, heavier than lead. If the earth were of granite, or of lava, to the centre, it would, so far as we can judge, have much the same weight which it now has. Such a central mass, covered with the various layers of stone, which form the upper crust of the Earth, would naturally make this globe of at least the weight which it really has. And therefore, if we were to learn that a planet was much lighter than this, as to its materials,--much less dense, taking the whole mass together,--we should be compelled to infer that it was, throughout, or nearly so, formed of less compact matter than metal and stone; or else, that it had internal cavities, or some other complex structure, which it would be absurd to assume, without positive reasons. 14. Now having decided these views from an examination of the Earth, let us apply them to other planets, as bearing upon the question of their being inhabited; and in the first place, to Jupiter. We can, as we have said, easily compare the mass of Jupiter and of the Earth; for both of them have Satellites. It is ascertained, by this means, that the mass of weight of Jupiter is about 333 times the weight of the earth; but as his diameter is also 11 times that of the earth, his bulk is 1331 times that of the earth: (the _cube_ of 11 is 1331); and, therefore, the density of Jupiter is to that of the earth, only as 333 to 1331, or about 1 to 4. Thus the density of Jupiter, taken as a whole, is about a quarter of the earth's density; less than that of any of the stones which form the crust of the earth; and not much greater than the density of water. Indeed, it is tolerably certain, that the density of Jupiter is not greater than it would be, if his entire globe were composed of water; making allowance for the compression which the interior parts would suffer by the pressure of those parts superincumbent. We might, therefore, offer it as a conjecture not quite arbitrary, that Jupiter is a mere sphere of water.