I. PHYSICAL GEOGRAPHY. 



I. 



OF THE FIGURE AND DENSITY OF THE EARTH. 



4. The earth is a planet of the solar system, having a 

 globular figure, which differs from a perfect sphere in being 

 irregular at its surface, and flattened at its poles. Abstract- 

 ing from these irregularities, which bear but a small pro- 

 portion to its lineal dimensions, the figure is that of a sphe- 

 roid of revolution. The amount of the flattening at the 

 poles may be determined in several ways : 



(1.) By the actual measure of portions of the meridians; 



(2.) By the variation in the apparent intensity of gravity 

 at the surface, which is ascertained by experiments with the 

 pendulum ; 



(3.) By astronomic phenomena, which are affected by 

 the figure of the earth. 



By these methods the oblateness of the earth is found to be 

 not far from 3^ ; that is to say, the relation of the polar axis 

 to the equatorial diameters is about f 



We learn from mechanical philosophy, that the figure of 

 an oblate spheroid is that which would be assumed by a fluid 

 mass whose particles are mutually attractive of each other, 

 and which revolves upon an axis ; and although we find the 

 external crust of the earth to be at present in a solid form, we 

 know that the figure has exactly that oblateness which is 

 consistent with the state of fluidity. We learn this from the 

 fact, that the cavities in the solid crust are filled with a mass 

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