For the same reason, nothing can be ascertained respecting the physical and geographical conditions of Neptune, the last planet of our solar system, which was discovered in our time by M. Le Verrier, thanks to the simple force of calculation, thereby affording the most brilliant proof ever given of the utility of the mathematical sciences. Neptune is so small and so far from us, that it is probable mere observation of the heavens would never have detected its existence. In this case mathematical analysis was more powerful than the telescope. It would be impossible to give particulars analogous to those which we have supplied concerning the foregoing planets, in reference to a star only 105 times larger than the earth, which revolves at the distance of _one milliard 150 millions of leagues_ from the sun, and the duration of whose year is 164 times that of the terrestrial year, so that if the ages of the Christian era were counted according to the Neptunian chronology, instead of being in the 19th century, we should be in the 12th year of that era. All we can say about Neptune, therefore, is that it forms the boundary of the domain of our visible world. We cannot, however, state positively that our solar world terminates at this limit. No doubt the range of our astronomical glasses goes no farther, but assuredly they do not sweep the boundaries of the empire of the sun. It is known, in fact, that comets return to us after having (as indicated by their geometrical curve), swept over the depths of space to a distance of thirty-two _milliards of leagues_. Thus the distance of one milliard 150 millions of leagues, which is that of Neptune from the sun, by no means represents the confines of our solar world, but simply defines the limits of the range of our telescopes. [Illustration: Asteroids. Jupiter. Saturn. Uranus. Neptune. Fig. 5.--Size of the Planets Jupiter, Saturn, Uranus, and Neptune compared with the Earth.] This rapid glance at our solar system in its entirety, proves that the earth is not in possession of any privilege. The part which she plays in the economy of the universe is equally fulfilled by other stars, and there is nothing to justify the pre-eminence assigned to her by the ancients. She is not the largest, the warmest, or the brightest of the planets. She simply forms a portion of a group of stars, and is but one individual of that group. These considerations tend to lead us to a very important deduction. Since the earth is in no way distinguished from the other planets of our solar system, there must exist in other planets the things which are found on our globe; air, water, a hard soil, rivers and seas, mountains and valleys. Even vegetation and forests ought to be there, regions covered with verdure and with shade. So there surely ought to exist in the other planets, animals, and even men, or at least creatures superior to animals, corresponding to our human type.