winds, and a due adjustment of the power of gravity, and the strength of the materials of which organized frames consist, there may be, so far as we can judge, life of some kind or other. But yet, even in those cases, we should be led to judge also, by analogy, that the life which they sustain is more different from the terrestrial life of the present period of the earth, than that is from the terrestrial life of any former geological period, in proportion as the conditions of light and heat, and attraction and density, are more different on any other planet, than they can have been on the earth, at any period of its history. 10. Let us then consider the state of these elements of being in the other planets. I have mentioned, among them, the force of gravity, and the density of materials; because these are important elements in the question. It may seem strange, that we are able, not only to measure the planets, but to weigh them; yet so it is. The wonderful discovery of universal gravitation, so firmly established, as the law which embraces every particle of matter in the solar system, enables us to do this, with the most perfect confidence. The revolutions of the satellites round their primary planets, give us a measure of the force by which the planets retain them in their orbits; and in this way, a measure of the quantity of matter of which each planet consists. And other effects of the same universal law, enable us to measure, though less easily and less exactly, the masses, even of those planets which have no satellites. And thus we can, as it were, put the Earth, and Jupiter or Saturn, in the balance against each other; and tell the proportionate number of pounds which they would weigh, if so poised. And again, by another kind of experiment, we can, as we have said, weigh the earth against a known mountain; or even against a small sphere of lead duly adjusted for the purpose. And this has been done; and the results are extremely curious; and very important in our speculations relative to the constitution of the universe.