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 assume 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 dissimilarity 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 absolutely 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 energy, 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, 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 gravitation. 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 regarded 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
- 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 everyone 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.