matter composing the Interior of the Earth. 265 



attributes this conclusion to Hopkins, whereas it is precisely 

 that which I had long since enunciated, and is entirely opposed 

 to the views of Mr. Hopkins. More recently Sir William 

 Thomson and Mr. Darwin have investigated the tidal action of 

 an internal fluid nucleus upon its containing solid shell. They 

 have both supposed the liquid to be totally incompressible, and 

 the containing vessel to be elastic and therefore compressible. 

 They have thus given the liquid a property which no liquid in 

 existence possesses, and the solid a property which solids pos- 

 sess in a much less degree than liquids. Their hypothesis is thus 

 totally inadmissible as a part of the problem of inquiry into the 

 earth's structure. I at once admit that a thin elastic spheroidal 

 envelope filled with incompressible liquid and subjected to the 

 attractions of exterior bodies would present periodical defor- 

 mations, owing to tidal action far surpassing the tides of the 

 ocean. But I do not admit that such impossible substances 

 can represent the materials of the earth. My hypothesis is 

 that the liquid interior matter, instead of being incompressible, 

 is, like all liquids we observe, relatively far more compressible 

 than its solid envelope. A highly compressible liquid con- 

 tained in a very much less-compressible shell would be a hy- 

 pothesis more in harmony with physical observation. The 

 tidal phenomena of a compressible fluid, it is easy to see, 

 would be very different from those of an incompressible fluid. 

 The work done by the action of certain disturbing bodies in 

 the strata of compressible fluid would partly result in causing 

 variations of density, instead of producing tidal waves of great 

 magnitude. This has been already shown in the Mdeanique 

 Celeste by Laplace, in discussing the tides of the atmosphere. 

 Theory shows that the atmospheric tides should be nearly in- 

 sensible, notwithstanding the great depth of the atmospheric 

 column, because the work done in the atmosphere is very dif- 

 ferent from what is peformed in the less-compressible water of 

 the ocean. Observation has fully verified this result. 



3. It is admitted that the earth's density increases from its 

 surface towards its centre. If its interior is occupied by a 

 compressible fluid, the law of density of this fluid would result 

 from the compression of its own strata; just as the law of density 

 of the atmosphere is produced by the pressure of the upper 

 atmospheric layers upon those below. But instead of suppo- 

 sing the interior of the earth to be filled by a fluid thus con- 

 forming to the observed properties of fluids, both Sir William 

 Thomson and Mr. Darwin have applied their great powers as 

 accomplished mathematicians to the tides of an incompressible 

 and homogeneous spheroid, such as I admit to have no real 

 existence whatsoever. 



