"The stately march of the planets in their orbits becomes slower the farther they are from the sun. The velocity of Mercury in its orbit is thirty, that of Jupiter is eight, and that of Neptune is only three miles a second. On the other hand, the inner planets, as a rule, take some twenty-four hours, and the outer only ten hours to spin round their axis. The inner planets are small in comparison with the outer. If we represent the sun by a gourd, 20 inches in diameter, Mercury will seem a bilberry (⅟₁₆ inch) Venus, a white currant, the Earth a black currant (¼ inch), Mars a red currant (⅛ inch), the planetoids as fine seed, Jupiter an orange or peach (2 inches), Saturn a nectarine or greengage (1 inch), Uranus a red cherry (¾ inch), and Neptune a white cherry (barely 1 inch in diameter). By putting the sun and planets in a row, and drawing a contour of the whole, we obtain the figure of a dirk, a bodkin, or an Indian club, in which the sun stands for the knob (disproportionately big), the inner planets for the handle, and the outer for the blade or body. Again, the average density of the inner planets exceeds that of the outer by nearly five to one, but the mass of any planet is greater than the combined masses of all which are smaller than it. The inner planets derive all their light and heat from the sun, and have few or no satellites; whereas the outer, to all appearance, are secondary suns, and have their own retinue of worlds. On the similitude of a clan or house we may regard the inner planets as the immediate retainers of the chief, and the outer as the chieftains of their own septs or families." "How do you account for the symmetrical arrangement?" I enquired.