CONSTRUCTION OF CHARTS. 9 



depth, then it would rise, then sink lo\\ er again ; and 

 so go on rising and falling at various intervals, until 

 the ship arrived at some island or continent, when 

 the lead would, of course, be once more level with 

 the surface. If we imagine, further, that the com- 

 mander was careful to r; cord his observations from 

 moment to moment, and, finally, to trace on a sheet of 

 paper the secrtion formed by the constantly varying 

 length of the plummet line, we should see at a glance 

 the exact configuration of the sea-bottom throughout 

 the ship's course from one coast to the other. 



Fig. 5 is a vertical section of the Atlantic, in a 

 line from Mexico, across Yucatan, Cuba, San Do- 

 mingo, and the Cape de Verds, to Senegambia, on 

 the African coast ; and it may be legarded as the 

 result of such a voyage as we have imagined. The 

 horizontal line represents the level of the sea. Th(3 

 irn'gular line which cuts it in many points follows 

 the undulati(ms of the sea bottom. Where it rises 

 above the horizontstl line there is land — that is to 

 say, the solid crust of the earth is higher than the 

 watei-8. Where its curve falls below the horizontal 

 line, the land is submarine, or under the water. 

 Thus, supposing that we take our departure from 

 the Mexican coast, the plummet descends at first 

 nearly 2000 feet, and leturns to the surface on the 

 coast of Yucatan. After doubling this peninsula, 



