Page:Popular Science Monthly Volume 28.djvu/266

256 We find Mars, though more distant from the Sun than either the Earth or Venus, inferior to them in magnitude; being, indeed, that one of the long-known greater planets which most nearly resembles in size Mercury, the nearest planet to the solar orb. 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; and yet the disks of these small planets (whose apparent diameters scarcely admit of measurement) are less than twice the size of France, Madagascar, or Borneo. Remarkable as is the small density of all the colossal planets which are farthest from the Sun, yet neither in this respect can we recognize any regular succession. Uranus appears to be denser than Saturn; and we find both Venus and Mars less dense than the Earth, which is situated between them. The time of rotation decreases on the whole with increasing solar distance, but yet it is greater in Mars than in the Earth, and in Saturn than in Jupiter. Among all the planets, the elliptic paths of Juno, Pallas, and Mercury have the greatest eccentricity, and Venus and the Earth, which immediately follow each other, have the least, while Mercury and Venus (which are likewise neighbors) present in this respect the same contrast as do the four smaller planets, whose paths are so closely interwoven. The eccentricities of Juno and Pallas are nearly equal, but are each three times as great as those of Ceres and Vesta.

I will not prolong the quotation, but will add the following sentences, which contain the result which I wish to enforce:

The planetary system in its relations of absolute magnitude, relative position of the axes, 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. They are facts of Nature which have arisen out of the conflict of various forces acting under unknown conditions.

In other words, from the point of view now under consideration there is no such thing as the uniformity of Nature.

Nevertheless, the instinct of seeking uniformity in other departments, when it has been discovered in one, and that an important department, is not only intelligible but is of the highest value as a help in the pursuit of knowledge. Professor Huxley, as we have seen, describes the principle as a working hypothesis, which has never failed him; and, so regarded, it can lead to no error, and it may lead to the discovery of new truth. If uniformity be wrongly assumed, the results obtained may be erroneous, or they may not; examination and experiment will show which they are; a working hypothesis may always be freely granted to an investigator, but it must not be confounded with a postulate upon which the whole body of science rests.

Let me illustrate the character of a working hypothesis by a second