256 THE POPULAR SCIENCE MONTHLY, 



"We find Mars, though more distant from the Sun than cither the Earth or 

 Venus, inferior to them in magnitude ; being, indeed, tliat one of the long-known 

 greater planets which most nearly resembles in size Mercury, the nearest planet 

 to the solar orb. Saturn is less tlian Jupiter, and yet much larger than Uranus. 

 The zone of the telescopic planets, which are so inconsiderable in point of vol- 

 ume, 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. Kemarkable 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.t 



I will not prolong the quotation, but will add the following sen- 

 tences, 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 difterent degrees of eccentricity of the 

 orbits, has to our apprehension nothing more of natural necessity than the rela- 

 tive 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 depart- 

 ments, 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 al- 

 ways be freely granted to an investigator, but it must not be con- 

 founded with a postulate upon which the whole body of science rests. 



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



* This was written when only four asteroids were known. While this article is 

 passin^r through the press the discovery is announced of the 249th asteroid ! 

 f " Cosmos," vol. 1. (Sabine's translation). % Ibid. 



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