May, 1922 dynamics of the lithosphere 195 



is caused by the fact that the attractions of the sun and moon 

 are not a single force acting through the earth's center of 

 gravity. Since the variation is due to the attraction of forces 

 which are estimatable, methods of calculation are possible by 

 which the mass of the earth may be revealed. 



Nutation- is a vibratory motion of the earth's axis, pro- 

 ducing a wavy circle of precession, due to the unequal attraction 

 of the moon on the equatorial ring of the earth. This, too, 

 affords another method or means of calculating the earth's 

 mass. 



Variation of latitiide^^'" is caused by a shifting of the axis 

 of rotation within the sphere so that its polar extremities 

 wander in a circle of about fifteen (15) meters in diameter. By 

 computation it should have a period of 305 days; but Chandler, 

 from a great mass of data, discovered an actual period of 427 

 days.* Such a retardation can be due only to a shifting of 

 the equatorial bulge, by the lagging adjustment of which 

 retardation of the period is effected. Now since adjustment 

 in the rocks is slow and incomplete, a tide must be set up in 

 response to the strains developed by the shifting of the axis, 

 because water is very adaptable to external force. Dr. Bak- 

 heyzer and Mr. Christie have independently investigated 

 this problem of tides and both conclude that there is a tidal 

 variation with about a 430-day period. 



The shape of the earth ^~^ has been a subject of long and 

 extensive study into which we do not need to enter. Suffice it 

 to say that the shape has been determined to be very nearly 

 an ellipsoid of revolution, showing that the earth's substance 

 is sufficiently plastic to be nearly perfectly adjustable to 

 rotational forces. The superficial exceptions will be discussed 

 later. 



Schweyder^ has recently determined the nutation period as 

 432.8 days. 



The physical data throwing light on the condition and 

 responsiveness of the earth's substance are varied and extensive. 



-Encyclopoedia Britannica. 

 ^Ibid. 



•'Darwin: Tides, Chap. xv. 



^Chamberlain, Reed, Hayford Schlesinger, Smithsonian Rpt., 1916. 

 ''Encyclopoedia Britannica. 



'Poynting & Thompson: Text Book on Physics. 



^Schweyder W. Naturwissenschaften, 1917, Potsdam, Germany, pt. 38. 

 Quoted from Jour. Geo!. 1921, v. 29, p. 396. 



