98 A. A. MICHELSON 



The first attempt to measure the rigidity of the earth, that is, 

 the resistance which it offers to change of shape, was made by 

 G. H. and Horace Darwin in 1880.^ They employed a horizontal 

 pendulum by the use of which they hoped to measure the change 

 in the direction of the gravitational vertical due to the attractions 

 of the sun and moon; from which, by comparison with the values 

 calculated on the basis of absolute rigidity, the effective rigidity 

 could be determined. The results were so irregular and contradic- 

 tory that no conclusion could be formed, and the Darwins express 

 the belief that such experiments are not likely ever to furnish the 

 desired results. 



In the hands of Rebeur-Paschwitz this method did, however, 

 give positive results confirming the deductions of Kelvin. Since 

 then the method of the horizontal pendulum has been successfully 

 employed by Ehlert, Kortazzi, Schweydar, Hecker, and Orloff, 

 with essentially the same result, namely, that the coefficient of 

 rigidity is found to be of the order 6X10" c.g.s. (about the same 

 as that of steel). The results deduced from Chandler's observa- 

 tions of the variation of the latitude give a value nearly twice as 

 great. 



But, in addition to the elastic yielding of any body ordinarily 

 looked upon as sohd there is a plastic yielding, characterized by a 

 constant termed by Maxwell the "modulus of relaxation," and 

 evidenced by a lag of the distortion of the earth relative to the 

 forces producing it. Such experiments as these should be capable 

 of determining the plasticity as well as the rigidity of the earth. 

 To show what measure of rehability may be accorded to the obser- 

 vations mentioned, the following extract, Table I, is made of a 

 discussion of these by Schweydar.^ 



Here i — K represents the ratio of the observed amplitude of 

 oscillation to that calculated on the assumption of absolute rigidity. 

 K represents the retardation (which should always be negative) 

 of the phase of the observed motion relative to the phase of the 

 disturbing forces. 



' B.A.A.S. Reports, York meeting, 1880. 



^ Dr. Wilhelm Schweydar, Untersuchungen iiber die Gezeiten der Festeit Erde. 

 Potsdam, 191 2; Leipzig: B. G. Teubner. 



