On the Internal Fluidity of the Earth ^09 



that Mr. Hopkins might have pressed his argument further, and have 

 concluded that no continous liquid vesicle at all approaching to the 

 dimensions of a spheroid GOOO miles in diameter can possibly exist 

 in the earth's interior without rendering the phenomena of precession 

 and nutation sensibly different from what they are." 



Thus it will be perceived that the objection brought forward by 

 Mr. Hopkins against the ideas generally accepted by geologists as to 

 the interior fluidity of the teiTcstrial globe, has been regarded by 

 many learned Englishmen as perfectly established. 



I am of an opinion diametrically opposed, and I believe that this 

 conclusion of Mr. Hopkins is not based on any true foundation what- 

 ever. This is what I propose to explain to the Academy in all 

 brevity. 



When we apply the theories of rational mechanics to the study of 

 natural phenomena, we immediately find that we have to deal with 

 problems of the greatest complication. If we attempt to consider 

 these questions with full rigour, it is impossible to succeed, for 

 reasons which do not even require enumerating. We are obliged, 

 therefore, to rest contented after resolving, not the problems them- 

 selves at which we aim, but other questions more or less bearing 

 upon them, which in themselves present a degree of simplicity suf- 

 ficient to enable us to arrive at some more or less rigorously correct 

 solution. 



It is thus that we are led to substitute the study of solids of abso- 

 lutely invariable form, for that of those which actually are met with 

 in nature ; thus also we are accustomed to attribute to liquids the 

 property of absolute fluidity, which in nature never exists, etc. 



It becomes necessary, therefore, to place ourselves as it were side 

 by side with the reality, and to remember always, that the results 

 which we may have arrived at, may be vastly modified by circum- 

 stances which we may have neglected to tfike into account. 



In order to concentrate our ideas on the subject, let us take a 

 spherical vessel, a glass globe for example, filled with a liquid, say 

 with water : if now we admit that the liquid is endowed with an 

 absolute fluidity, and we impart to the globe a sudden movement of 

 rotation round its central vertical axis, the globe should alone turn 

 without at all carrying along with it the liquid which it contains, 

 which ought to retain its pristine immobility. 



This is easily verified by imparting a more or less rapid rotary 

 motion to the globe ; light substances floating on or suspended in 

 the water will not appear to change place, notwithstanding the mo- 

 tion given to the globe. But will this always be the case, whatever 

 be the rapidity of the motion given to the globe ? Can we admit 

 that the liquid will remain indifferent to the motion of the envelope 

 which contains it should we revolve the globe very slowly? 



In admitting the absolute fluidity of the liquid we forget to take 

 into account its viscidity. Although this viscidity is extremely feeble 

 in most fluids with which we are acquainted, it is never altogether 

 wanting, and this explains why, provided that the rotary movement 

 communicated to the globe be sufficiently slow, the liquid is carried 



