66 ROTATION OF THE EARTH. SECT. X. 



SECTION X. 



Rotation of the Earth invariable Decrease in the Earth's Mean Tempera- 

 tureEarth originally in a State of Fusion Length of Day constant- 

 Decrease of Temperature ascribed by Sir John Herschel to the Variation 

 in the Eccentricity of the Terrestrial Orbit Difference in the Tempera- 

 ture of the Two Hemispheres, erroneously ascribed to the Excess in the 

 Length of Spring and Summer in the Southern Hemisphere ; attributed 

 by Mr. Lyell to the Operation of existing Causes Three Principal Axes 

 of Rotation Position of the Axis of Rotation on the Surface of the Earth 

 invariable Ocean not sufficient to restore the Equilibrium of the Earth 

 if deranged Its Density and Mean Depth Internal Structure of the 

 Earth. 



THE rotation of the earth, which determines the length 

 of the day, may be regarded as one of the most import- 

 ant elements in the system of the world. It serves as 

 a measure of time, and forms the standard of com- 

 parison for the revolutions of the celestial bodies, which 

 by their proportional increase or decrease would soon 

 disclose any changes it might sustain. Theory and 

 observation concur in proving that -among the innumer- 

 able vicissitudes which prevail throughout creation, the 

 period of the earth's diurnal rotation is immutable. 

 The water of rivers, falling from a higher to a lower 

 level, carries with it the velocity due to its revolution 

 with the earth at a greater distance from the center ; it 

 will therefore accelerate, although to an almost infinites- 

 imal extent, the earth's daily rotation. The sum of all 

 these increments of velocity arising from the descent of 

 all the rivers on the earth's surface would in time be- 

 come perceptible, did not nature by the process of evap- 

 oration raise the waters back to their sources ; and thus, 

 by again removing matter to a greater distance from 

 the center, destroy the velocity generated by its pre- 

 vious approach ; so that the descent of rivers does not 

 affect the earth's rotation. Enormous masses projected 

 by volcanos from the equator to the poles, and the con- 

 trary, would indeed affect it, but there is no evidence of 

 such convulsions. The disturbing action of the moon 

 and planets, which has so powerful an effect on the 

 revolution of the earth, in no way influences its rota- 

 tion. The constant friction of the trade-winds on the 



