162 



COSMOS. 



the most recent explanations of earthquakes in trachytic re- 

 gions is the result of geognostic suppositions regarding the 

 want of cohesion in rocky masses raised by volcanic action. 

 The following summary furnishes a more exact but very 

 brief indication of the variety of views as to the nature of 

 the first impulse to the commotion : 



The nucleus of the earth is supposed to be in a state of 

 igneous fluidity, as the consequence of every planetary 

 process of formation from a gaseous material, by evolu- 

 tion of heat during the transition from fluidity to solidity. 

 The external strata were first cooled by radiation, and 

 were the first to become consolidated. The commotion is 

 occasioned by an unequal ascent of elastic vapors, formed 

 (at the limit between the fluid and solid parts) either from 

 the fused terrestrial mass alone or from the penetration 

 of sea- water into higher strata of rock, nearer to the sur- 

 face of the earth, the sudden opening of fissures, and by 

 the sudden ascent of vapors produced in the hotter and 

 consequently more elastic depths. The attraction of the 

 moon and sun* on the fluid, fused surface of the nucleus 



* Hopkins has expressed doubts as to the action upon the fused 

 il subjacent fluid confined into internal lakes" at the Meeting of the 

 British Association for 1847 (p. 57), as Mallet has also done with re- 

 gard to " the subterraneous lava tidal wave, moving the solid crust 

 above it," at the British Association Meeting for 1850 (p. 20). Poisson 

 also, with whom I have often spoken regarding the hypothesis of the 

 subterranean ebb and flow caused by the sun and moon, considers the 

 impulse, which he does not deny, to be inconsiderable, "as in the open 

 sea the effect scarcely amounts to 11 inches." Ampere, on the other 

 hand, says: "Those who admit the fluidity of the internal nucleus of 

 the earth do not appear to have sufficiently considered the action which 

 would be exercised by the moon upon this enormous liquid mass — an 

 action from which would result tides analogous to those of our seas, 

 but far more terrible, both from their extent and from the density of 

 the liquid. It is difficult to conceive how the envelope of the earth 

 should be able to resist the incessant action of a sort of hydraulic 

 ram(?) of 1400 leagues in length" (Ampere, Theorie de la Terre, in 

 Revue des deux Mondes, July, 1833, p. 148). If the interior of the 

 earth be fluid, which in general can not be doubted, as, notwithstand- 

 ing the enormous pressure, the particles are still displaceable, then the 

 same conditions are fulfilled in the interior of the earth that give rise 

 on the surface to the ocean tides ; and the tide-producing force will 

 constantly become weaker in approaching the centre, as the difference 

 of the distances of every two opposite points, considered in their rela- 

 tion to the attracting bodies, constantly becomes less in receding from 

 the surface, and the force depends exclusively upon the difference of 

 the distances. If the solid crust of the earth opposes a resistance to 

 this effort, the interior of the earth will only exert a pressure against 

 its crust at these points j as my astronomical friend, Dr. Brunnow, 



