No, that will not do. We will not confuse our readers by telling them all the theories that have been formed, but will only state what the late Lord Kelvin believed, and most of the present generation of geologists believe. It is that the heat of the earth's crust continues to increase only for a certain distance of the way down, and that owing to pressure the earth is solid (though very hot except towards the surface) for two thousand miles down. There remains a thickness of another four thousand miles on either side of the earth's centre to be considered. That might be molten, but the pressure would be so great that it would behave as if it were a solid. We know the earth cannot be solid all through because it does not weigh enough. The earth cannot, of course, be weighed in any scales, but there are methods of weighing it nevertheless. One of two methods is by seeing how strongly it attracts bodies to itself. But these things belong rather to the romance of astronomy than to that of geology. We need only trouble ourselves at present about the results.
One word more about the deep interior of the earth. Dr. J. J. See, an American astronomer, has found how heavy and how hard the earth is, taken as a whole. He finds that if it were built from surface to surface of hardened steel it would be just about as heavy and as hard--or as rigid. The steel would be like that used for the armour-plate of battleships. Dr. See is not prepared, however, to discard the idea that the earth has a large fluid interior. If it were fluid, yet it would be subjected to such enormous pressure by its own weight, that if there were a moderately thick earth-crust, its tidal surgings would be so "cabin'd, cribbed, confined," that they would be comparatively ineffectual. We must not run away with the idea (against which Dr. See specially warns us), that there is any free circulation of currents within the fluid interior. The rigidity produced by pressure (or weight) is too great for that. Indeed, this pressure is so great that, as another scientific authority, Professor Arrhenius, has pointed out, the matter at the core of the earth might even be gaseous; and yet would be so compressed by pressure that it would possess a rigidity equal to the hardest steel. The earth may be partly solid, partly liquid, partly gaseous, but for all practical purposes Professor See would have us regard it as a solid sphere having an average hardness and weight and "rigidity" greater than that of ordinary steel.