THE RINGED PLANET. 45 



mentioned — the small mean density of the giant planets — we have at 

 once the strongest possible evidence to show that the condition of 

 these bodies must be unlike that of the earth. Of course, if we as- 

 sume that Saturn's substance (to limit our attention to this planet) is 

 composed of materials altogether unlike any which exist on earth, a 

 way out of our difficulty is found, though not an easy one. In that 

 case, however, we are only substituting one form of complete dissimi- 

 larity for another. And all the results of spectroscopic analysis, as 

 applied to the celestial bodies, tend to show the improbability that 

 such differences of elementary constitution exist — we will not say in 

 the solar system only, but in the sidereal universe itself. If, however, 

 we admit that Saturn is in the main constituted of elements such as 

 we are familiar with, we find it extremely difficult, or rather it is ab- 

 solutely impossible, to suppose that the condition of his substance is 

 like that of the earth's. There are certain unmistakable facts to be 

 accounted for. There is the mighty mass of Saturn, exceeding that 

 of the earth ninety-fold. That mass is endued with gravitating en- 

 ergy, precisely in the same way as the earth's mass. There must be 

 from the surface toward the centre a continually increasing pressure. 

 This pressure is calculable, 1 and enormously exceeds the internal 

 pressures existing within the earth's interior. There is no possibility 

 of cavities, as Brewster and others have opined ; for there is no known 

 material, not the strongest known to us, iron, or platinum, or adamant, 

 which could resist the pressures produced by Saturn's internal gravi- 

 tation. Steel would be as yielding as water under these pressures. 

 There must be compression with its consequent increase of density, 

 such compression exceeding many million-fold the greatest with which 

 terrestrial experimenters have dealt. That, with these enormous forces 

 at work, the actual density of Saturn as a whole should be far less than 

 that of water is utterly inexplicable, unless Saturn's condition be re- 

 garded as altogether unlike that of the earth. We see in the sun an 

 orb which, notwithstanding its enormous mass, has a mean density 

 much less than the earth's, and little greater than that of water ; but 

 we have no difficulty in understanding this circumstance, because we 



1 It is a misfortune for science that Newton never published the reasoning which led 

 him to the conclusion that the earth's mean density is equal to between five and six 

 times the density of water. This, as every one knows, has been confirmed by several 

 experimental methods ; and, so far as appears, the problem is a purely experimental 

 one. Newton, however, made no experiments ; at least, none have been heard of as 

 effected by him, and it is scarcely probable that he had any instruments of sufficient 

 delicacy for a task so difficult. Prof. Grant ascribes Newton's conclusion to a happy 

 intuition; yet it is very unlike Newton to make a guess on such a matter. It is more 

 probable that he guessed the elements of the problem than the result. He probably 

 assumed that the earth's mass is composed of a substance like granite, and, adopting 

 some law of compression for such a substance (based on experiment, perhaps), calculated 

 thence 'the compression at different depths, and so obtained the mean destiny of the 

 whole mass. 



