30 EVOLUTION OF THE EARTH 



is consumed, but a viscous drag will produce retardation. 

 Recent measurements of the rigidity of the earth under tidal 

 stresses, both by the horizontal pendulum and by the water 

 level in a horizontal pipe, show that the earth as a whole is 

 more rigid than steel and that under the exceedingly small tidal 

 stresses the yielding is essentially elastic. The estimates of 

 viscosity are so small that they are within the limits of error 

 of the measurements. The smallness of the tidal strains in 

 the earth may be appreciated by citing Darwin's calculations. 

 According to this investigator, the tides raised by the moon 

 upon the earth generate a stress-difference of 16 grams per 

 square centimeter at the poles, 48 grams at the equator, and 

 128 grams at the center of the earth. Thus the earth is 

 stressed by the lunar tidal forces even at the center to only 

 about one part in fifteen thousand of the strength which 

 granite has at the surface of the earth. 



The tidal force exerted by the earth on the moon is about 

 twenty-two times as great as the lunar tidal force on the earth, 

 and reaches about one part in six hundred or seven hundred of 

 the strength of granite. If the moon were once nearer the 

 earth, the tidal stress-difference was much greater, varying 

 inversely with the cube of the distance. Tidal retardation must 

 have acted efficiently upon the moon, nevertheless, until the 

 moon was at its present distance and the stresses reduced to 

 their present magnitude, in order to have reduced its period 

 of rotation to the same value as its final orbital period about 

 the earth. The action must have been that of a viscous body 

 tide since the moon has never been able to hold to itself an 

 ocean envelope. The tidal force exerted by the earth upon 

 the body of the moon consequently must have produced a 

 notable viscous yielding and continued to do this in spite of 

 increasing distance of the moon and increasing rigidity. 



The far greater mass of the earth prevented such large 

 effects of tidal retardation from being felt, but its period of 



