80 ROTATION OF THE EARTH. [SECT. x. 



SECTION X. 



Rotation of the Earth invariable Decrease in the Earth's mean Temperature- 

 Earth originally in a state of Fusion Length of Day constant Decrease 

 of Temperature ascribed by Sir John Herschel to the Variation in the Ex- 

 centricity of the Terrestrial Orbit Difference in the Temperature 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 important ele- 

 ments in the system of the world. It serves as a measure of 

 time, and forms the standard of comparison for the revolu- 

 tions of the celestial bodies, which, by their proportional in- 

 crease or decrease, would soon disclose any changes it might 

 sustain. Theory and observation concur in proving that, 

 among the innumerable vicissitudes which prevail through- 

 out 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 centre ; it will 

 therefore accelerate, although to an almost infinitesimal ex- 

 tent, the earth's daily rotation. The sum of all these incre- 

 ments of velocity, arising from the descent of all the rivers 

 on the earth's surface, would in time become perceptible, 

 did not nature, by the process of evaporation, raise the 

 waters back to their sources, and thus, by again removing 

 matter to a greater distance from the centre, destroy the ve- 

 locity generated by its previous approach ; so that the descent 

 of rivers does not affect the earth's rotation. Enormous 



