fr. /•; Becker— Proof of thi Earth's Rigidity, 



A />/»'/■> nt <»■> 'inn- tiil> .v. — It' the earth were a fluid of density 

 and the ocean rested upon the heavier Liquid, both would 

 yield to the attraction of the moon almost equally and there 

 would be no considerable apparent tide. It' the earth were a 

 perfectly rigid mass and always turned the same exposure 

 toward the moon, the water directly under the moou would 



rise feet or 4J feet higher than it would Btand at an 



angular distance of 90° from the moon. The solar tides are 



about 1 2 <>\ : the lunar tides. The actual total amplitude of the 

 tides at oceanic ports is usually from 2 to 3 feet. Were the 

 actual titles equal to the equilibrium tides, it would he per- 

 fectly easy to state the approximate rigidity of the earth. The 

 real tides are greatly affected by the rotation of the earth, the 

 obstruction of the continents, and the inequalities of the 

 bottom. They are not static but dynamic. Nevertheless the 

 fact that these are apparent tides shows that the earth posset 

 some rigidity, and the fact their amplitude is so considerable 

 compared with the equilibrium amplitude shows that this 

 rigidity is great. Thus if the globe were as rigid as glass, the 

 apparent tides would he only 2/.") of the real tides or in other 

 words perfect rigidity would then increase the apparent tides 

 to 5 2 of their present amplitude, so that the amplitude of the 

 tides at oceanic stations would increase to from 5 to 8£ feet 

 and would be much greater than the lunar equilibrium tides. 

 If the rigidity is really that of brass, infinite rigidity would 

 raise the oceanic tides to from 4 to 6 feet. If the earth were 



strong as steel these tides would be raised by perfect rigidity 

 to from ~2J to 4^ feet. 



Messrs. Thomson and Darwin haye examined the tidal 

 theories and observations with especial reference to the ques- 

 tion of rigidity. In 1888, the latter wrote, " it cannot be ad- 

 mitted that perfect rigidity of the earth would augment the 

 tides in the proportion of 5 to 2, although they might perhaps 

 be augmented in the proportion of 4 to 3."* The context 

 shows that Darwin includes tides of short period as well as 

 those of long period in this statement. 



Rigidity of the earth. — The conditions affecting the tides 

 of short period are so complex that no more information can 

 be derived from them than is contained in the above quotation, 

 which amounts to the statement that the earth's modulus of 

 rigidity must exceed 24,400 kilograms per square centimeter 

 by some indefinite amount. But the tides of long period must 

 conform much more nearly to the equilibrium values than do 

 those of short period. Were the period BO long that the 

 kinetic energy of the ebb and flow currents would be sensibly 

 dissipated, or were the waters practically motionless at high 



* Encyc. Brit., Article Ti«le-. 



