Physical Structure of the Earth. 239 



an intermediate place between solids and gases, in relation 

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

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

 Ciinento, with globes of porous metals, an experiment with 

 equally strong but impervious vessels had been made, the de- 

 formation 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 influenced physical science. 



The Rotation of the Earth considered 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 of Phvsical Geology' in the ' Philosophical 

 Transactions ' for 1839," 1840, and 1842, and the result ob- 

 tained 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. Hopkins'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 a 

 fluid stratum lying between. 



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

 filled with fluid. 



The first and last of those possibilities are not admissible, 

 according to astronomical observations. According to the 

 investigations of Hopkins the action exercised by the Sun and 

 Moon on the position of the Earth's axis in space, by which 

 Precession and Nutation are produced, would be different ac- 

 cording to the structure we attribute to the Earth. The values 

 established by observation compel us to regard the Earth as for 

 the most part in a fluid state, in order that the results may 

 harmonize with calculation (Pfaff, Grundriss der Geologie). 

 This is the reverse of what Hopkins has concluded, and is 

 precisely what I have long since enunciated, which I have 



