While in their rotation period, Mercury accomplishes 47 kilometers (293⁄8 miles) per second, and the Earth 291⁄2 (181⁄8 miles), Neptune rolls along his immense orbit at a rate of only 51⁄2 kilometers (about 31⁄4 miles) per second.
The vast distance that separates us prevents our distinguishing any details of his surface, but spectral analysis reveals the presence of an absorbent atmosphere in which are gases unknown to the air of our planet, and of which the chemical composition resembles that of the atmosphere of Uranus.
One satellite has been discovered for Neptune. It has a considerable inclination, and rotates from east to west.
And here we have reached the goal of our interplanetary journey. After visiting the vast provinces of the solar republic, we feel yet greater admiration and gratitude toward the luminary that governs, warms, and illuminates the worlds of his system.
In conclusion, let us again insist that the Earth,—a splendid orb as viewed from Mercury, Venus, and Mars,—begins to disappear from Jupiter, where she becomes no more than a tiny spark oscillating from side to side of the Sun, and occasionally passing in front of him as a small black dot. From Saturn the visibility of our planet is even more reduced. As to Uranus and Neptune, we are invisible there, at least to eyes constructed like our own. We do not possess in the Universe the importance with which we would endow ourselves.
Neptune up to the present guards the portals of our celestial system; we will leave him to watch over the distant frontier; but before returning to the Earth, we must glance at certain eccentric orbs, at the mad, capricious comets, which imprint their airy flight upon the realms of space.