448 



THE EARTH. 



every side limited, and one which niust therefore have such a surface that a 

 traveller or navigator can completely surround it in one continuous course. 



Let us see, however, whether we may not obtain evidence more distinct as 

 to its form. If we stand on the deck of a ship at sea, and out of sight of land, 

 the view being bounded only by sea and sky, and look at the horizon when a 

 ship approaches, we shall at first see its topmast rising out of the water like a 

 pole. As it gradually comes nearer to us, more of the mast will become visi- 

 ble, and the sails will be seen cut off, however, horizontally, by the line at 

 which the water and sky unite. Upon the nearer approach of the ship, the 

 hull will at length become visible. Now, since this takes place on all sides 

 around us, it will follow that when the ship is at a distance, there must be 

 something interposed between the eye and it which intercepts the view of it ; 

 but as the surface of the water is generally uniform, and not subject to sudden 

 and occasional inequalities like that of the land, we can only imagine its gen- 

 eral form to be convex, and that its convexity is interposed between the eye 

 and the object so as to intercept the view. 



Since the same effects are observed from whatever direction the ship may 

 approach, it will follow that the same convexity must prevail on every side. 



If we admit the earth to be globular, or nearly so, and the surface of the 

 water to partake of this figure, 1, the manner in which a ship becomes visible 

 on approaching the eye will be easily and simply explained. 



Fig. i. 



In the position c, in the annexed figure, the convexity of the globe being 

 between the ship and the eye, the view of it is intercepted ; but as the ship 

 approaches toward b, the masts first and then the sails and rigging rise above 

 the line of sight and come into view, and lastly the hull will be seen. 



If, on the contrary, the surface extending from the eye to the ship were a 

 plane, the ship would be rendered invisible only by reason of its distance ; 

 whereas it is ascertained that a ship frequently is invisible at a distance at 

 which it must be seen but for the interposition of some other object ; this may 

 be tested, and in fact is frequently tested at sea by mounting to the masthead, 

 whence the seaman being enabled to overlook the convexity, sees vessels which 

 are invisible from the deck, athough, strictly speaking, he is nearrr to those 

 vessels on the deck than at the masthead. 



When the mariner, after completing a long voyage, discovers by his obser- 

 vations and reckonings that he is approaching the desired coast, be ascends to 

 the topmast and looks out for the appearance of mountains or other elevated 

 land, and he invariably sees them from that point long before they are visible 

 from the deck. He afterward sees them from the deck long before the gen- 

 eral level of the country will be observed by him. All these are natural and 

 necessary consequences of the convexity of the surface of the ocean. The 

 same effects would be seen ia any part of a continent which is sufficiently free 

 from mountains and otler inequalities. 



But we have a still more conclusive and convincing proof of the general 

 form of the earth even than those which have been explained. "When the 



