When it was discovered that the old planets in certain important particulars resembled the earth, being opaque and solid bodies, having similar motions round the sun and on their own axis, some accompanied by satellites, and all having arrangements producing day and night, summer and winter, who could help wondering whether they must not also have inhabitants, reckoning and regulating their lives and employments by days, months, and years? This was, at most, however, a mere guess or conjecture; and whether it is now more probable than then, depends on the intervening progress of astronomy and science in general. Have subsequent discoveries strengthened or impugned the validity of the conjecture? The limits of our system have been since vastly extended by the discovery of Uranus and Neptune; and the planetary sisterhood has also increased in number by thirty little and very eccentric ones. Now, as to NEPTUNE, says the Essayist, in substance, what reason has a sensible person for believing it peopled, as the earth is, by human beings—_i. e._ consisting of body and soul? He is thirty times further than we are from the sun, which will appear to it a mere star—about the size of Jupiter to us; and Neptune’s light and heat will be nine hundred times less than ours![83] _If_ it, nevertheless, contain animal and intellectual life, we must try to conceive how they get on with such a _modicum_ of those useful elements! But have we general grounds for assuming _all_ the planetary bodies inhabited? Beginning with the moon, we have encountered a decided negative. If any planet, however, have sufficient light, heat, clouds, winds, and a due adjustment of gravity, and the strength of the materials of which organisation consist, there _may_ be life of some sort or other. Now we can measure and weigh the planets, exactly, by the law of gravitation, which embraces every particle of matter in our system, and find the mass of our earth to be only five times heavier than water. Comparing it with JUPITER—the bulk of which is 1331 times greater than that of the Earth—his density is, as a whole, only a quarter of that of the Earth—not greater than it would be as a sphere of water; and he is _conjectured_ to be such, and the existence of his belts to be lines of clouds, fed with vapours raised by the sun’s action on such a watery sphere—the lines of such clouds being of so steady and determined a character, in consequence of his great rotatory velocity. Equal bulk for equal bulk, he is lighter than the Earth, but of course much heavier altogether; and as he is five times the Earth’s distance from the Sun, he must get a proportionally smaller amount of light and heat, and even that diminished by the clouds enveloping him to so great an extent. What a low degree of vitality, and what kind of organisation must animal existence possess, to suit such physical conditions, especially with reference to gravity, which, at his surface, is nearly