Note 15 — PHYSICAL GEOGRAPHY.[14]

"Nor are there," writes Humboldt, "any constant relations between the distances of the planets from the central body round which they revolve, and their absolute magnitudes, densities, times of rotation, eccentricities and inclinations of orbit and of axis. We find Mars, though more distant from the sun than either the earth or Venus, inferior to them in magnitude; Saturn is less than Jupiter, and yet much larger than Uranus. The zone of the telescopic planets, which are so inconsiderable in point of volume, viewed in the series of distances commencing from the sun, comes next before Jupiter, the greatest in size of all the planetary bodies. Remarkable as is the small density of all the colossal planets which are farthest from the sun, yet neither in this respect can we recognise any regular succession. Uranus appears to be denser than Saturn, and (though the inner group of planets differ but little from each other in this particular) we find both Venus and Mars less dense than the earth, which is situated between them. The time of rotation increases, on the whole, with increasing solar distance, but yet it is greater in Mars than in the earth, and in Saturn than in Jupiter." After other remarks of the same character, he adds, "The planetary system, in its relation of absolute magnitude, relative position of the axis, density, time of rotation, and different degrees of eccentricity of the orbits, has, to our apprehension, nothing more of natural necessity than the relative distribution of land and water on the surface of our globe, the configuration of continents, or the elevation of mountain chains. No general law, in these respects, is discoverable either in the regions of space or in the irregularities of the crust of the earth."