206 ON THE PHYSICAL STRUCTURE OF THE EARTH. 



cooling are precisely those which contract least under pressure. Gases 

 which contract most bj" pressure are also the most dilatable by heat. 

 Liquids occupy an intermediate place between solids and gases in rela- 

 tion both to the dynamical effect of pressure and the action of loss of 

 heat. If, instead of the experiment of the Academia del Cimento with 

 globes of porous metal, an experiment with equally strong but impervi- 

 ous vessels had been made, the deibrmation of each globe would have 

 been unaccompanied by the exudation of the liquid, and the totally 

 false statement that solids are more compressible than liquids would 

 not have so long injuriously intlueuced [)hysical science. 



THE ROTATION OF THE EARTH CONSIDERFD AS PARTLY FLUID AND 



PARTLY SOLID. 



(1) The problem of the precessional motion of the Earth considered 

 as a solid shell filled with liquid devoid of viscidity and friction has 

 been elaborately investigated by Mr. Hopkins, in his "Researches ot 

 Physical Geology," in the Fhilosophical Transactions for 1839, 1840, and 

 1842, and the result obtained by him has been often quoted as extremely 

 remarkable. Before treating the same question, it may be necessary 

 to state that on the continent of Europe the application made by Mr. 

 Hopkins of his result to geology is not generally admitted, and views 

 such as I have always firmly upheld seem to be more generally adopted; 

 but some confusion appears to exist as to Mr. Ho])kins's results and 

 those to which I have been led. Thus in a recent treatise on systematic 

 geology the author says, with reference to the thickness of the solid 

 crust of the earth, there are plainly only four possibilities to be thought 

 of: 



1. The Earth is through and through solid. 



2. The Earth is through and through fluid, with a solid crust. 



3. The Earth has a solid nucleus and a solid crust, with fluid stratum 

 lying between. 



4. The Earth is solid, but furnished with cavities which are filled 

 with fluid. 



The first and last of these possibilities are not admissible, according 

 to astronomical observations. According to the investigatiors of 

 Hopkins the action exercised by the sun and moon on the positive of 

 the Earth's axis in space, by which precession and nutation are i)ro- 

 duced, would be different according to the structure we attribute to the 

 earth. The values established by observation compel ns to regard the earth 

 as for the most part in a fluid state, in order that the results may har- 

 monize with calculation (Pfatt", Grundriss der Geologic). This is the 

 reverse of what Hopkins has concluded, and is precisely what I have 

 long since enunciated, which I have always cnnlinued to maintain, and 

 which forms the cumulative result of the investigations in the text of 

 this paper. In a report to the Koyal Irish Academy on " Experiments 

 on the Influence of the Molecular Influence of Fluids on their Motion 



