GEOLOGY. 



OX THE PHYSICAL STRUCTURE OF THE EARTH. 



THE following is an abstract of a paper read before the British Association, 

 by Prof. Hennessy : 



After some preliminary observations as to the impossibility of accounting 

 for the earth's figure, without supposing it to have been once a fused mass, 

 the exterior of which has cooled into a solid crust, the process of solidification 

 of the fluid was described. The influence of the connexion and circulation of 

 the particles in a heterogeneous fluid was shown to be different from what 

 would take place in a homogeneous fluid such as usually comes under our 

 notice. As the primitive fluid mass of the earth would consist of strata 

 increasing in density from the surface towards the centre, its refrigeration 

 would be that of a heterogeneous fluid, and the process of circulation would 

 be less energetic in going from its surface towards its centre. Thus, the earth 

 would ultimately consist of a fluid nucleus inclosed hi a spheroidal shell. The 

 increase in thickness of this shell would take place by the solidification of each 

 of the surface strata of the nucleus in succession. If the matter composing 

 the mterior of the earth is subjected to the same physical laws as the material 

 of the solid crust coming under our notice, the change of state in the fluid 

 must be accompanied by a diminution of its volume. The contrary hypothesis 

 had been hitherto always assumed in mathematical investigations relative to 

 the form and structure of the earth. The erroneous supposition that the par- 

 ticles of the primitive fluid retained the same positions after the mass had 

 advanced hi the process of solidification as they had before the process com- 

 menced, had been tacitly or openly assumed hi all such inquiries until it was 

 formally rejected by the author, who proposed to assume for the fluid similar 

 'properties to those exhibited by the fusion and solidification of such portions 

 of the solidified crust as are accessible to observation. The results to which 

 the unproved hypothesis has led show that it fundamentally affects the whole 

 question, not only of the shape and internal structure of the earth, but also of 

 the various actions and reactions taking place between the fluid nucleus and 

 the solid shell. If the process of solidification took place without change of 

 volume hi the congelation of the fluid, the strata of the shell would possess the 

 same forms as those of the primitive fluid, and their oblateness would diminish 

 hi going from the outer to the inner surface. If the fluid contracts hi volume 

 on passing to the solid state, the remaining fluid will tend to assume a more 

 and more oblate figure after the formation of each stratum of the shell. The 



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