RIGIDITY OF THE EARTH 



119 



ocean tides have the effect of diminishing the ratio R by from | to 

 I, as admitted by Schweydar, the rigidity is enormously greater. 



The viscosity is also very great and probably of the same order 

 of magnitude as that of steel. 



The main object of this investigation was to demonstrate the 

 feasibiHty of the method and to determine the order of magnitude 

 of the earth's rigidity and viscosity. Evidently the method is 

 capable of giving results of a high order of accuracy by recording 

 a much longer series of observations. Such a series, in which the 

 microscope will be replaced by the interferometer and in which 

 the record is to be made automatic, is now in preparation. It is 

 expected that the results will furnish a record of the earth tides 

 which will be correct to within a tenth of i per cent. 



Whether it may thereby be possible to obtain a more accurate 

 value of the coefficients of rigidity and of viscosity will depend on 

 the advance which may be made in the theory of the ocean tides 

 and of their perturbing action. Doubtless it would be of impor- 

 tance to repeat the experiments at widely different stations, some in 

 the southern hemisphere, some on islands in mid-ocean, and some 

 on the continent as far as possible from the coast. 



It may also be possible by a comparison of the moon and the 

 sun tides to obtain an independent and perhaps more accurate 

 value of the moon's mass.^ 



The conclusions from these and similar experiments and obser- 

 vations, including precession and variation of latitude, all agree 

 substantially in refuting the old notion that the internal tempera- 

 ture, sufficiently high to melt most of the materials constituting 

 the earth's crust, necessarily involves a fluid or semi-fluid earth 

 supporting a relatively thin solid crust. 



From the definitely ascertained result that the coefiicient of 

 rigidity and the coefficient of viscosity are both very large (of the 

 order of, and perhaps exceeding, those of solid steel, whereas under 

 normal pressure all substances at this temperature would be fluid), 

 it follows that pressure increases the rigidity and the viscosity, 

 at least of the substances which form the body of the earth. 



' It would probably be necessary to make use of a tunnel sufiQciently deep to 

 eliminate the thermal effect, which even in the semi-diurnal period would be appre- 

 ciable. 



