20 A. A. MICHELSON 



assumption. But the appearance of ^ = ^/t is a more serious matter, 

 making the use of the formula much more difficult. 

 The operator which should replace n is therefore 



,-ds 

 fit 



I-\ A2 



But the application of this formula to such a problem as the 

 earth's viscosity is still further complicated by the fact that all 

 the constants are functions of the pressure and of the temperature 

 in the earth's interior. Even though more or less probable assump- 

 tions may be made regarding the value of temperature and pressure 

 as functions of the distance from the center, we know but little 

 regarding the effect of these factors on either rigidity or viscosity. 

 It was found that the temperature effect may be represented with 

 considerable accuracy by the expression 



^=Ee'^+bP)e 



in which P is the applied stress, 6 the temperature, and E, K, and b 

 constants. 



For room temperature the values of bd = h are given in 

 Table IV. 



If we take /?2 = o . 2 as fairly representative 



The unit P = lop gm., so that G the couple = Pr gm. cm. Thus 

 we get for the displacement, after a sufficiently long time, 



S=Pee -^^ and-=n- 

 W2 P 



