SECT. X. ROTATION OF THE EARTH INVARIABLE. 71 



SECTION X. 



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

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



Decrease of Temperature ascribed by Sir John Herschel to the varia- 

 tion in the Excentricity 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 ; attri- 

 buted by Sir Charles 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 elements in 

 the system of the world. It serves as a measure of time, and 

 forms the standard of comparison 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 innumerable vicis- 

 situdes 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 centre ; it will therefore accelerate, although to an almost 

 infinitesimal 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 become percep- 

 tible, 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 velocity gene- 

 rated by its previous approach ; so that the descent of rivers 

 does not affect the earth's rotation. Enormous masses projected 

 by volcanoes from the equator to the poles, and the contrary, 

 would indeed affect it, but there is no evidence of such convul- 

 sions. The disturbing action of the moon and planets, which has 

 so powerful an effect on the revolution of the earth, in no way 



