THE EARTH'S ROTATION 23 



a glass tube a large drop of oil below the surface of the solution. 

 The oil will float in the solution in the shape of a sphere. This illus- 

 trates the fact that if a liquid is relieved from the action of outside 

 forces, it will take the form of a perfect sphere. 



A spherical surface is the smallest surface by which a 

 solid can be bounded, and so the maximum distance which 

 can separate places located on a given solid will be least 

 when its surface is spherical. Thus the inhabitants of 

 the earth, considering the surface over which they may 

 scatter themselves, are brought into the closest possible 

 relation to one another. 



The Size of the Earth. It is easy to say that the polar 

 diameter of the earth is 7900 miles, its equatorial diameter 



BOSTC.fr ?TO CHICAGO lOOO MILES 



DIAMETER OF EARTH 8OOO MILES 



CIRCUMFERENCE OF EARTH 25OOO MILES 



FIGURE 4. LINES TO INDICATE COMPARATIVE DISTANCES 



7927 miles, and its equatorial circumference 24,902 miles, 

 but a true conception of these distances is not so easy. 



Using as our standard any distance with which we are 

 really acquainted, we shall find that the lines representing the 

 different dimensions of the earth are very long. (Figure 4.) 

 How vastly greater, then, must be the distances which were 

 mentioned when treating of the sun and the stars ! 



The Earth's Rotation. As has already been stated, the 

 ancients considered the earth as the center of the universe 

 and thought that the sun and stars revolved around it. 

 We of the present day, however, know that it is the rotation 

 of the earth from west to east that causes the appearance of 

 the rising and setting sun and thus makes day and night. 



