CHAPTER IV 



THE EARTH A SPINNING BALL 



8 1 . The Earth a Sphere. The field of view at sea or 

 on a level plain is always bounded by an unbroken circle 

 called the horizon ; and in all parts of the Earth when one 

 watches a receding object at sea or on a level plain the 

 horizon appears slowly to swallow it up, and it disappears 

 like a traveller over a hill. In all parts of the Earth if the 

 eye is placed 5 feet above sea -level the lower 5 feet of 

 any object are concealed when 4 miles away. Across a 

 lake 4 miles wide, two men of ordinary height standing 

 erect and looking at each other with telescopes can see only 

 the head and hands of the other apparently floating on the 

 water, their bodies being entirely concealed from view (Fig. 

 9). So from the sea-shore the hull of a ship 10 feet above 

 the water vanishes at 5 miles' distance, and its masthead 



FIG. 9. Curvature of the Earth, exaggerated 400 times. 



100 feet high sinks out of sight at 12 miles. Since the 

 same length of an object is concealed by the horizon at the 

 same distance from the observer in all parts of the Earth, it 

 is evident that the dip of the horizon, as it is termed, is 

 practically the same everywhere, and that the surface of the 

 Earth is uniformly curved in a convex form. The only 

 figure which has uniform convex curvature is a sphere, and 



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