192 SEE— FURTHER RESEARCHES ON 



[April 24, 



the outer layers of our globe to have a rigidity about half that of 

 glass, and assume that at a depth of o.i of the radius it becomes 

 nearly 2.5 times as great as it is at the surface. 



Whether it becomes at a depth of twenty miles less than it is at 

 the surface we cannot tell, but such a decrease is not impossible, 

 perhaps not improbable; because at this depth the molten rock 

 moves in earthquakes, and yet in confinement it must have a very 

 sensible rigidity, though probably not more than half that of granite. 



Accordingly, it looks as if the rigidity at the surface is about 

 half that of glass, at a depth of 20 miles about one half that at the 

 surface, and at the depth of 40 miles nearly the same, but increas- 

 ing below that depth and at 160 miles again equal to that at the 

 surface, and at a depth of 400 miles considerably larger yet, or about 

 1.4 times that of glass. Increasing below this depth according to 

 the pressure, it becomes at the center over 3 times that of nickel 

 steel used in armor plate. The rigidity of steel is attained at a little 

 over 0.3 of the depth to the center of the earth. If this be the dis- 

 tribution of rigidity in the earth, the curve of rigidity is as follows : 



This postulated fall in the rigidity just beneath the crust is 

 probable for several reasons : 



1. The temperature increases quite rapidly as we go downward, 

 while the pressure increases proportionately more slowly, so that 

 a depth would be reached at which the matter would become a 

 plastic if not a viscous fluid. 



2. The eruption of volcanoes and lava flows on a vaster scale 

 show that a molten layer underlies the crust, and occasionally is 

 forced to the surface. 



3. This underlying molten rock moves in world-shaking earth- 

 quakes, and freciuently is expelled from beneath the sea under the 

 land to form mountain ranges along the coast. 



4. We may prove this expulsion of lava by the observed seismic 

 sea waves which indicate a sinking of the sea bottom, and by the 

 simultaneous uplift of mountains and coasts. 



From these considerations it follows that the earth is most nearly 

 liquid just beneath the crust, and has the greatest rigidity at the 

 center. As the plastic or quasi-viscous layer beneath the crust is 

 thin, and possessed of considerable rigidity, it too remains quiescent 



