522 



PHYSIOGRAPHY, 



[chap. 



the top of St. Paul's, his horizon would then have a radius of 

 more than twenty-four miles. 



Since it is found that the horizon is invariably circular in 

 every part of the world, it is proved that the earth must be 



Fig. :oi. — The earth wiihin the stai-sphere. 



spherical. For a sphere is the only kind of solid which 

 presents a circular contour from whatever point it is viewed. 

 Other means of demonstrating that th^ surface of the 

 earth is rounded, and not flat, may be obtained from obser- 

 vations on some of the heavenly bodies. One interesting 

 mode of proof may be explained by the help of Fig. loi. 



