ORIGIN OF MOUNTAINS. 809 



thousand miles, this viscid portion would move with the crust, the 

 case would meet the conditions required by Professor Hopkins's cal- 

 culations. 



(2.) As the attraction of the sun and moon causes tides in the 

 oceans, so if the earth has, beneath a thin crust, a perfectly free-mov- 

 ing liquid, this interior liquid, as has been often urged, might have its 

 tides. Sir W. Thomson has investigated mathematically the amount 

 of deformation the earth would thus undergo, and its effect on the 

 height of the oceanic tides, assuming an elastic shell, and an incompres- 

 sible fluid of perfect liquidity within, as the basis of his calculations ; 

 and he reached the conclusion that if the globe were as rigid as glass, 

 it would still yield enough to reduce the tides to three fifths the height 

 they would have if it were absolutely rigid, and to two thirds, if as 

 rigid as steel. Precise observations for a comparison of the results of 

 calculations with facts, he says, cannot be made, because, especially, 

 of the influence on the tides of the distribution of the land. But he 

 concluded that 2,000 to 2,500 miles is the least thickness of crust that 

 would withstand the distorting action. Sir W. Thomson deduced a 

 like conclusion from the amount of precession and nutation. 



This argument, as Professor Hennessy states, is open to the same 

 kind of objection as the former; and he says further, that an elastic 

 shell and an incompressible fluid are wholly imaginary, since the shell is 

 not elastic and a fluid is never incompressible ; and that, as Laplace 

 pointed out, part of the work done in a compressible fluid would be 

 the causing of variations in density. Recently (in an Address at the 

 Glasgow meeting of the British Association in 1876), Sir Wm. Thom- 

 son has acknowledged the invalidity of his arguments from the pre- 

 cession and nutation, and has virtually admitted the force of these 

 objections. 



2. Considerations connected with Solidification. — (1.) Since (a) 

 the pressure increases in a liquid globe from the surface to the centre, 

 and augments more or less the density below, and (b) pressure raises 

 the fusing point of most substances, it has been argued that consolida- 

 tion must have begun at the centre. But the heat may so increase 

 from the surface to the centre as to neutralize any tendency to so- 

 lidification from pressure. And, again, the effect of pressure on the 

 fusing point may have its limit. The fusing point of sulphur, which 

 at the ordinary pressure is 225° F., is raised, according to Hopkins, 

 by 7,790 pounds of pressure to the square inch, to 275^° F., and by 

 11,880 pounds, to 285° F., — showing an increasing rise with the press- 

 ure. But these numbers indicate, as Mr. David Forbes has remarked, 

 that the effect in raising the fusing point diminishes greatly with the 

 pressure, it taking on an average three times more pressure between 



