THE EARTH 17 



Cause of the Shape of the Earth. Gravitation tends 

 to make a mass spherical in shape. This result is ac- 

 complished more readily if the mass is plastic and elastic, 

 which the earth is to a slight extent, although it seems 

 very firm and rigid to us. In addition to the effect 

 which gravitation has, there 

 is also the effect of rotation, 

 which causes the slight ob- 

 lateness or flatness at the 

 poles. The particles of a 

 rotating body tend to fly off 

 in straight lines tangent to FIG 15 



the direction in which the 



body is rotating, and this tendency would cause an 

 accumulation of the earth's material at the equator and 

 the loss of it at the poles. 



If a metal hoop which is free to move along its axis 

 is rotated on a whirling table, this effect of centrifugal 

 force will be well illustrated (Figure 15). 



Size of the Earth. - - The earliest recorded estimate 

 of the size of the earth was made by a Greek philosopher 

 in the third century B.C. He reckoned it to be about 

 8111 miles in diameter, arriving at this conclusion through 

 measurements taken on the surface of the earth. The 

 actual polar diameter as discovered later is 7899 miles, 

 and the equatorial diameter is 7926 miles. 



Consequences of the Shape and Size of the Earth. 

 The great size of the earth and the irregularities of its 

 surface have affected the distribution and growth of 

 peoples, animals, and plants to a remarkable degree. 

 The great distances between tribes, and the almost in- 

 surmountable barriers in the form of mountains, oceans, 

 and deserts^ for centuries hindered the mingling of tribes 



