10 EARTH FEATURES AND THEIR MEANING 



out by Sir Francis Drake, proved that the earth is a closed body 

 bounded by curving surfaces in part enveloped by the oceans and 

 everywhere by the atmosphere. The great discovery of Copernicus 

 in 1530 that the earth, like Venus, Mars, and the other planets, 

 revolves about the sun as a part of a system, left little room for 

 doubt that the figure of the earth was essentially that of a sphere. 



The oblateness of the earth. Every schoolboy is to-day fa- 

 miliar with the fact that the earth departs from a perfect spherical 

 figure by being -flattened at the ends of its axis of rotation. The 

 polar diameter is usually given as ^J^ shorter than the equatorial 

 one. This oblateness of the spheroid was proven by geodesists 

 when they came to compare the lengths of measured degrees of 

 arc upon meridians in high and in low latitudes. 



The oblateness of the geoid is well understood from accepted 

 hypotheses to be the result of the once more rapid rotation of the 

 planet when its materials were more plastic, and hence more re- 

 sponsive to deformation. An elastic hoop rotating rapidly about 

 an axis in its plane appears to the eye as a solid, and becomes 

 flattened at the ends of its axis in proportion as the velocity of 

 rotation is increased. Like the earth, the other planets in the 

 solar system are similarly oblate and by amounts dependent on the 

 relative velocities of rotation. 



The departure of the geoid from the spherical surface, owing to 

 its oblateness, is so small that in the figures which we shall use for 

 illustration it would be less than the thickness of a line. Since it 

 is well recognized and not important in our present consideration, 

 we shall for the time being speak of the figure of the earth in terms 

 of departures from a standard spherical surface. 



The arrangement of oceans and continents. There are other 

 departures from a spherical surface than the oblateness just re- 

 ferred to, and these departures, while not large, are believed to be 

 full of significance. Lest the reader should gain a wrong impres- 

 sion of their magnitude, it may be well to introduce a diagram 

 drawn to scale and representing prominent elevations and depres- 

 sions of the earth (Fig. 1). 



Wrong impressions concerning the figure of the lithosphere are 

 sometimes gained because its depressions are obliterated by the 

 oceans. The oceans are, indeed, useful to us in showing where 

 the depressions are located, but the figure of the earth which we 



