seen. Granting that the average rigidity of the globe's mass is comparable to that of glass, we see that it must undergo a change of form equal to 0·6 of that which it would experience if it were liquid; and, deducting this elevation from the rise of the oceanic sheet, the height of the tide is not more than 0·4 of what it would be on a perfectly rigid ball.
Assuming the rigidity of the terrestrial mass to be that of steel, Sir W. Thomson estimates that it would still undergo a change from sphericity equal to one third of that of a liquid sphere, and the apparent height of the tides is thus found to be reduced to two thirds of that which would be produced on an absolutely rigid ball. Sir W. Thomson, while fully recognizing the uncertainty in which this question of the height of the tides rests, still deems it inadmissible that the actual height is only 0·4 of the theoretical height on the hypothesis of a globe of absolute rigidity. He accordingly concludes that our globe possesses a rigidity greater than that of glass, and perhaps than that of steel. Regarding the influence on the phenomena of precession and nutation due to the globe's elasticity, the deductions from the hypothesis of absolute rigidity accord with observation, and this would tend to confirm the conclusions drawn from the observations of the tides. Even if the variability of form tends directly to diminish the effect called precession, there still exists an indirect effect of this variation which tends to augment it, so that possibly these two contrary effects may nearly counterbalance each other.
Everything considered, it is not impossible to reconcile these conclusions with the existence of an intense heat in the central portions of the globe. It must not be forgotten that these central beds are subjected to a pressure increasing in intensity toward the center. By M. Roche's law of densities, we find that the pressure at the center exceeds 3,000,000 kilogrammes per square centimetre (3,000,000 atmospheres). We can form no idea of the physical condition of substances exposed to such a pressure. Experiments on the resistance of various substances have shown that small cubes of granite crumble under a weight of 700 atmospheres; basalt and porphyry under 2,000 and 2,500 atmospheres respectively. Under such pressure the rocks disintegrate and are pulverized. Copper, steel, and cast iron resist twice or thrice this pressure, but what will be the state of the metals under a pressure one hundred or one thousand times greater? What is the action of molecular forces, in solids or liquids, subjected to a pressure of several millions of atmospheres, and, at the same time, to a temperature of some thousand degrees? What is the solid or the liquid state under these conditions? Data on this point are absolutely lacking, and anything advanced thereon must be purely hypothetical. "We may compare mathematics," Professor Huxley aptly says, "to a mill of admirable construction, capable of grinding to any degree of fineness, but what comes from it depends upon what has been put into it, and, as the
