466 THE POPULAR SCIENCE MONTHLY. 



seen. Granting that the average rigidity of the globe's mass is com- 

 parable 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 04 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 appar- 

 ent 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 ques- 

 tion 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 hy- 

 pothesis 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 varia- 

 tion 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 con- 

 clusions with the existence of an intense heat in the central portions of 

 the globe. It must not be forgotten that these central beds are sub- 

 jected to a pressure increasing in intensity toward the center. By M. 

 Roche's law of densities, we find that the pressure at the center ex- 

 ceeds 3,000,000 kilogrammes per square centimetre (3,000,000 atmos- 

 pheres). 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 



