132 LAMBERT: CONSTITUTION OF THE EARTH 



generating forces. The assumption is not absolutely free from 

 doubt, but the preponderance both of the argument and the ob- 

 ser\^ational cAddence is in its favor. ^^ 



Let us accept the assumption and consider the consequences. 

 We can predict these long-period tides for the case of a rigid 

 earth; suppose the earth is not rigid at all but plastic, fluid in 

 short, with a crust so thin as to yield to the pressure within and 

 conform to the shape of the fluid within, which itself conforms to 

 the tidal forces just as the water does. We measure the tides by 

 the height of the water on the land. If the land yields as much as 

 the water, no tide at all will be observ^ed. If the earth is rigid, the 

 full vertical tide should be obsers^ed. If the tidal forces make the 

 earth yield, but yield as an elastic body, not as a plastic one, 

 the observed tide will be intermediate between zero and the full 

 theoretical amount for a rigid earth. From the observations of 

 the long-period tides in various parts of the world, Kelvin ^-^ 

 deduced a rigidity probably between that of glass and that of 

 steel. 



Mathematically parallel with the tides is the case of the zero 

 of a pendulum. The tide-generating forces are also the forces 

 that deflect the plumb-line slightly from its mean position. 

 Since the surface of a fluid is perpendicular to the plumb-line, 

 as the plumb-line is deflected, the water tips to follow it so that 

 in theory the water in a wash basin is subject to tides. Instead 

 of using a wash basin, Prof. Michelson,^*^ of Chicago, used a long 

 U-shaped pipe, and measrued the changes in level by a delicate 



1* See Darwin's articles Tides in the nth edition of the Encyclopedia Britannica; 

 also Love, Problems in geodynamics, p. 51 (Cambridge, 191 1). The effect of the 

 continental barriers is an argument in favor of the assumption, as is also the fric- 

 tional effect. This effect is underestimated if the coefficient of viscosity alone be 

 considered. See Harris, Manual of tides, U. S. Coast and Geodetic Survey, Ap- 

 pendix 6 to Report of 1907, Part V, p. 273. 



15 See Thomson and Tait, Natural philosophy (2nd ed.), 2: 422-60. A much 

 larger number of observations is discussed by SchwEydar, Beitrage zur Geophysik, 

 9: 64. 1908. 



18 Journ. Geol. 22: 97. 1914. Astrophys. Joum., March, 1914. An important 

 correction is given in Science, 50: 327. October 3, 1919. See also Astrophys. 

 Journ., Dec, 1919. 



