igoS] THE PHYSICS OF THE EARTH. 181 



hundreds of kilometers must be capable of resisting a tangential 

 stress of nearl}- o. i of a gramme weight per square centimeter; this 

 would rapidly draw out of shape any plastic substance which could 

 be properly called a viscous fluid. " An angular distortion of 8" 

 is produced in a cube of glass by a distorting stress of about ten 

 grammes weight per square centimeter. We may therefore safely 

 conclude that the rigidity of the earth's interior or substance could 

 not be less than a millionth of the rigidity of glass without very 

 sensibly augmenting the lunar nineteen yearly nutation." {Xatnre, 

 February i, 1872, p. 258.) 



Notwithstanding these early criticisms of Delaunay's paper, Lord 

 Kelvin subsequently concluded that the phenomena of precession 

 and nutation do not decisively settle the question of the earth's in- 

 ternal fluidity. Yet the semiannual and lunar fortnightly nutations 

 may be considered to disprove absolutely the existence of a thin rigid 

 shell full of liquid. If the fluid were arranged in successive layers 

 of equal density, the only nutational or precessional influence exerted 

 upon it would depend on the non-sphericity of the shells. " A very 

 sHght deviation of the inner surface of the shell from perfect spher- 

 icity would suffice," according to Lord Kelvin, " in virtue of the 

 quasi-rigidity due to vortex motion, to hold back the shell from 

 taking sensibly more precession than it would give to the liquid, and 

 to cause the liquid (homogeneous or heterogeneous) and the shell 

 to have sensibly the same precessional motion as if the whole 

 constituted one rigid body." (Sir W. Thomson, British Assoc. 

 Report, 1876, Sections, p. 5.) 



It will be seen from this discussion that the argument from 

 precession and nutation is only in part conclusive. If the fluid had 

 a viscosity approaching high rigidity for rapidly acting forces, or it 

 were subjected to such pressure that the particles in confinement 

 acquired the properties of a solid, there would evidently be no 

 sensible deviation from the precession and nutation appropriate to a 

 cold solid globe. 



§ 14. On Lord Kelvin's Researches on the Earth's Rigidity 

 Based on the Analysis of the Tides. — The state of the earth's in- 

 terior had early engaged the attention of Lord Kelvin, for the 

 propagation of heat through the crust was before him as early as 



