/■'. // /•< /■ — Proof of thi Ea /7//\ Rigidity, 



The average rigidity of the rocks exposed at the surface of 

 tin' earth is certainly much less than that which a continuous 

 mass of glass <>r braes would exhibit and there is thus a distinct 

 difficulty In accounting \\>v so high a rigidity as the earth 

 evinces, even on the theory that the globe is solid. <>u this 

 point, however, certain experimental results of Dr. William 

 Hallock are very instructive. He subjected wax and paraffin 

 to a pressure of 96,000 pounds per square inch in a horizontal 

 steel cylinder; and on the top of these substances he placed 

 small silver coin-. The coins were forced against the tube 

 with such violence as to leave in the steel impressions which 

 Could be t'elr as well as seen.* Thus substances which at one 

 atmosphere show trifling rigidity, may develop a rigidity com- 

 parable with that of steel at pressures such as are to he found 

 at about 15 miles below the surface of the earth. 



On the other hand it is at first surprising to learn that a 

 globe of the size of the earth and as rigid as the best drawn 

 brass, would yield to the attraction of a distant body half as 

 freely as if it were fluid. But it is a matter of common ob- 

 lation that bodies of large dimensions are not so strong as 

 small ones in proportion to their size. Thus a bar of steel of 

 the size of a needle held horizontally by one end does not bend 

 even sensibly and the strain is far within the elastic limit ; 

 while a bar of the same relative dimensions, but 1000 feet 

 long, evidently could not be supported in a similar manner. 

 In fact, when two bodies are geometrically similar, the strength 

 varies as the squares of the corresponding dimensions, while 

 the mass varies as the cubes of these dimensions. When we 

 come to deal with a mass like the earth, containing about 

 11x10'° cubic metres each weighing 5,500 kilograms, it there- 

 fore is not strange after all that it presents but a feeble resi>t- 

 ance to external forces. 



The opposition which some geologists have made to the 

 theory of the solidity of the earth is of course based upon the 

 hypothesis that geological phenomena cannot be accounted for 

 on the theory of solidity. So far as the contortion of strata 

 is concerned, there is no conflict between geology and physics. 

 Time enters into the expression of viscosity ; and the fact 

 that the earth behaves as a rigid ma>s to a force which changes 

 it- direction by 360° in '2± hours is not inconsistent with great 

 plasticity under the action of small forces which maintain 

 their direction forage-. For a considerable number of years 

 I have constantly had the theory of the earth's Bolidity in 

 mind while making field observations on upheaval and sub- 

 sidence, with the result that to my thinking, the phenomena 



* This Journal, vdL \x.\iv, 1887, p. '-'77. 



