Why the Earth Is Presumed to Be Solid
Besides the confirmation of some of the most material points of the theory of gravitation which results from the experiment of “Weighing the Earth,”16 it furnishes a presumption of the strongest kind that the earth is solid to the centre, and not, as many have supposed in every age, a hollow shell. The mean density, 5⅔, is very much greater than that of the substances which abound at the surface. All common rocks are under 3, and nothing under the ores of the heaviest metals comes up to 5⅔. The earth is as massive as if it were all composed of silver ore, from the centre to the circumference, so that there must be an increase of density towards the centre. If those who think the earth to be a shell were to presume that its solidity ceased at 500 miles below the surface, they would then be compelled to give to the terrestrial matter, one part with another, a density greater than that of mercury, in order that the whole shell, the hollow part included, might have the mean density which is found by this experiment.—Penny Cyclopædia.