THE INTERIOR OF THE EARTH. 465 



If it is admitted that this liquid is endoAved with an absolute fluidity, 

 it is evident that, by giving the globe a brisk rotary movement on a 

 vertical axis, it will turn without carrying the liquid around with it. 

 This may be readily verified by giving it a more or less rapid move- 

 ment of rotation. Light substances suspended in the water will appear 

 to remain still, despite the ball's rotation. But will this always be the 

 case, whatever the speed of rotation ? If the globe were very slowly 

 revolved, would the liquid still be unaffected by the movement of its 

 envelope ? In conceding the perfect fluidity of the liquid, its viscosity 

 has been lost sight of. Now, this viscosity, though slight, is not nil ; 

 hence, if the rotation be sufficiently slow, the liquid will be carried 

 around with the glass globe, the whole revolving as one piece, or solid 

 ball." This rotation of the liquid under the conditions described has 

 been demonstrated by M. Champagneur in a series of experiments 

 undertaken, by request of M. Delaunay, in the laboratory of the Sor- 

 bonne. 



Applying this reasoning to the earth, we will assume that it is 

 composed of a liquid mass covered by a solid pellicle. It is first of all 

 evident that, if we set aside the perturbations caused by the equatorial 

 enlargement, the entire mass will turn as one piece on its axis ; if any 

 difference whatever could exist between the rate of the envelope's 

 rotation and that of the nucleus, friction will speedily annul it. The 

 perturbing influence of precession and nutation imparts to the solid 

 envelope's proper movement of rotation an extremely slight accelera- 

 tion. The question is, Does the internal liquid participate in this addi- 

 tional movement, or is the crust only affected by it ? " As for me," 

 says M. Delaunay, " there is no room fcr the slightest doubt. The 

 acceleration due to the causes indicated is so slight that the fluid of 

 the interior must follow the inclosing shell exactly as though the whole 

 were a solid mass. So enormous is the pressure to which the various 

 parts of the liquid mass are subjected that we can form no idea of its 

 effect on the viscosity of the fluid in question. But if that fluid is in 

 the condition of those we are familiar with, what we have described 

 would occur." M. Delaunay concludes by stating that, in his opinion, 

 the phenomena of precession and nutation can furnish no data con- 

 cerning the greater or less thickness of the earth's crust. 



Sir William Thomson again considers the question, and from a new 

 point of view. In theoretically determining the height of the tides, 

 the water only is supposed to yield to the luni-solar attraction the 

 solid shell of the earth being unaffected by it. Now, it is evident that 

 even an entirely solid sphere will be slightly changed in form by these 

 forces, and that the change will be still greater in a partially liquid 

 sphere. "We will first suppose that the entire mass of the globe yields 

 to the attracting forces as readily as if it were liquid. In this case 

 sea and solid land will be raised alike, and, the surface of the sea 

 always being at the same distance from the bottom, no tides will be 



VOL. XVII. 30 



