This planet was the first of the asteroids, and in the century that has elapsed hundreds of them have been discovered, while at the present time no year passes by without several more being added to the number. While some of these are nearer to the sun than is the first one discovered, and others are farther from it, their average distance is fairly represented by the number 2.8. Why Bode's law should hold true, or even so nearly true as it does, is an unexplained riddle, and many astronomers are inclined to call it no law at all, but only a chance coincidence--an illustration of the "inherent capacity of figures to be juggled with"; but if so, it is passing strange that it should represent the distance of the asteroids and of Uranus, which was also an undiscovered planet at the time the law was published. 135. THE PLANETS COMPARED WITH EACH OTHER.--When we pass from general considerations to a study of the individual peculiarities of the planets, we find great differences in the extent of knowledge concerning them, and the reason for this is not far to seek. Neptune and Uranus, at the outskirts of the solar system, are so remote from us and so feebly illumined by the sun that any detailed study of them can go but little beyond determining the numbers which represent their size, mass, density, the character of their orbits, etc. The asteroids are so small that in the telescope they look like mere points of light, absolutely indistinguishable in appearance from the fainter stars. Mercury, although closer at hand and presenting a disk of considerable size, always stands so near the sun that its observation is difficult on this account. Something of the same kind is true for Venus, although in much less degree; while Mars, Jupiter, and Saturn are comparatively easy objects for telescopic study, and our knowledge of them, while far from complete, is considerably greater than for the other planets. Figs. 84 and 85 show the relative sizes of the planets composing the inner and outer groups respectively, and furnish the numerical data concerning their diameters, masses, densities, etc., which are of most importance in judging of their physical condition. Each planet, save Saturn, is represented by two circles, of which the outer is drawn proportional to the size of the planet, and the inner shows the amount of material that must be subtracted from the interior in order that the remaining shell shall just float in water. Note the great difference in thickness of shell between the two groups. Saturn, having a mean density less than that of water, must have something loaded upon it, instead of removed, in order that it should float just submerged. JUPITER