ON THE PHYSICAL STRUCTURE OF THE EARTH. 211 



shows that this can not proceed otherwise than slowly. The changes 

 in shape of the surface of the nuclens would be correspondingly slow 

 and gradual. When once a comparatively rigid outer crust had been 

 formed, the process of molding additional strata of solidified matter 

 against the inner surface of the crust from the nucleus would proceed 

 in a slow and gradual order, so that the resulting solid strata would 

 conform to the shape impressed upon them by the molding forces. A 

 remarkable illustration of the way in which fused matter ejected from 

 the Earth's interior may, while turning on its center and at the same 

 time cooling, mold itself against a solid crust formed upon it has been 

 adduced by Charles Darwin, and has already been quoted by me on a 

 former occasion. From these considerations I have been led to conclude, 

 that the cUipficity of the shelVs inner surface may exceed hut can not he less 

 than the elliptieity of its outer surface;* and referring to the same ques- 

 tion, Plana used the words, "La loi des ellipticites a subi dans le passage 

 de I'etat liquide a I'etat solide une alteration sensible parlaquelle toutes 

 les couches se sont constituees de maniere a avoir un nuhne applatisse- 

 ment et plus grand que la precedent." M. Plana has further stated his 

 views in the same volume of the Asironomische N'achrichfen for 1852, 

 thus: " II est permis de penser que ces couches (de lafluide interieuse) 

 en se consolidant, ont subi des modifications a la verite fort petites, mais 

 assez grandes pour nous empecher de pouvoir deriver, avec tout I'exacti- 

 tude que I'on pourrait souhaiter, I'etat de la Terre solid de son etat 

 anterieure de fluidite." 



This paragraph gives a distinct adhesion to the improved form of the 

 hyi)Othesis of the original fluidity of the Earth; and this concurrence 

 on the part of M. Plana is the more important, as it is possible that he 

 had formed his conclusions independently. He refers to a letter written 

 by him on the subject to Humboldt; and it is remarkable that, in the 

 fifth and last volume of "Cosmos," published not long before the author's 

 death, some adjacent notes allude to Plana's views, and contain refer- 

 ences to the investigations of Mr. Hopkins and to my early researches. 



At this period Humboldt could scarcely have had time to examine 

 the mechanical and physical reasonings, and he merely quoted the 

 papers in the PhiJosopMcal Transactions as if he had seen them for 

 the first time. I am not aware of any evidence as to whether Plana had 

 known their contents; and it is possible that his conclusions as to the 

 forms of the strata of the shell and nucleus had been formed independ- 

 ently, though published a short time after my investigations. 



The annexe<l figure may assist in making clear the results of the pre- 

 ceding paragraph. The outer ellipse represents the outline of the exte- 

 rior surface of the Earth's crust, which is shaded and bounded inwardly 

 by a surface slightly more elliptical. The lluid nucleus included within 

 the shell is represented with strata decreasing in elliptieity towards the 



' See the subjoined ropresentatioo of a section of the shell and nucleus. 



