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Scientific Intelligence. 



lseaster is proposed for this group, which is represented by four or five 

 species. The genera are ; — Palceaster (Salter), without disc, avenues deep, 

 Upper and Lower Silurian, 4 species ; Palasterina (ib.), pentagonal, disc 

 moderate, Up. Sil., 1 species ; Palceocoma (ib.), no disc, avenues very 

 shallow, Up. Sil., 4 species; Protaster (Forbes), disc small, arms long, ex- 

 tended, Upper (and Lower) Sil., 4 species. There appear to be no other 

 forms yet described. 



3. On the Physical Structure of the Earth ; by Prof. Hennessy, (Proc. 

 Brit. Assoc. ; Ath. No. 1504.) — 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 heteroge- 

 neous fluid was shown to be different from what would take place in a 

 homogeneous fluid such as usually come under our notice. As the prim- 

 itive 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 ener- 

 getic in going from its surface towards its centre. Thus, the earth would 

 ultimately consist of a fluid nucleus inclosed 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 has been hitherto always assumed in mathemati- 

 cal 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 process 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 hypoth- 

 esis 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 in the congelation of the fluid, the strata of the shell would 

 possess the same forms as those of the primitive fluid, and their oblate- 

 ness 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 sur- 

 face 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 congelation from the nucleus. 



