26 REPORT— 1856. 



On the Physical Structure of the Earth. By Professor Hennesst. 



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 convection 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 enclosed in 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 interior 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 particles of the 

 primitive fluid retained the same positions after the mass had advanced in the pro- 

 cess of solidification as they had before the process commenced, had been tacitly or 

 openly assumed in 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 improved hypothesis has led, show that it 

 fundamentally affects the whole question, not only of the shape and internal struc- 

 ture 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 in 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 in going from the outer to the inner surface. If the fluid contracts 

 in 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 

 law of density of the nucleus will not be the same as that of the primitive fluid, but 

 will vary more slowly, and the mass will thus tend towards a state of homogeneity 

 as the radius of the nucleus diminishes by the gradual thickening of the shell. The 

 surface of the nucleus, and consequently the inner surface of the shell, will thus 

 tend to become more oblate after each successive stratum added to the shell by con- 

 gelation from the nucleus. This result, combined with another obtained by Mr. 

 Hopkins, proves that so great pressure and friction exist at the surface of contact of 

 the shell and nucleus as to cause both to rotate together nearly as one solid mass. 

 Other grounds for believing in the existence of the great pressure exercised by the 

 nucleus at the surface of the shell were adduced. If the density of the fluid strata 

 were due to the pressures they support, and if the earth solidified without any change 

 of state in the solidifying fluid, the pressure against the inner surface of the shell 

 would be that due to the density of the surface stratum of the nucleus, and would 

 therefore rapidly increase with the thickness of the shell. Contraction in volume 

 of the fluid on entering the solid state would diminish this pressure, but yet it may 

 continue to be very considerable, as the coefficient of contraction would always 

 approach towards unity. The phsenomena of the solidification of lava and of volcanic 

 bombs were referred to in illustration of these viev^'s, and their application was then 

 shown to some of the greatest questions of geology- The relations of symmetry 

 which the researches of M. Elie de Beaumont seem to establish between the great 

 lines of elevation which traverse the surface of the earth, appear to Prof. Hennessy 

 far more simply and satisfactorily explained by the expansive tendency of the nucleus 

 which produces the great pressure against the shell than by the collapse and subsi- 

 dences of the latter. The direction- of the forces which would tend to produce a 

 rupture from the purely elevatory action of the pressure referred to would be far 

 more favourable to symmetry than if the shell were undergoing a distortion of shape 



* Philosophical Transactions, 1851, part 2. 



