FORM OF THE SEA-BOTTOM. \l 



of 16,500 feet or more, and rises as sufldenly to little 

 more than 1J],000 feet. Again, it descends suddenly 

 to 16,000 feet ; and then continut\s to mark a depth, 

 varying by sudden changes, say from 16,000 to 

 10,000 fe( t, until near the Cape de Verd Islands, 

 when the depth, even clo-e inshore, is about 14,700 

 feet. These pinnacled isles rise to the height of 10,000 

 feet above the surlace of the sea. Deep gulfs se- 

 parate the one from the other ; and a still deeper 

 trench or canal, with almost perpendicular sides, 

 brings the navigator to the African coast. 



We have mentioned the uncertainty of the results 

 obtained by the plummet in ordinary circumstances ; 

 and from the description we have given of this im- 

 perfect instrument, it will be obvious that it affords 

 no means for continuous or unbroken observation. 

 It is necessary to make a fresh cast of the lead each 

 time we want to sound the deep. We can only ob- 

 tain, therefore, a series of points separated by inter- 

 vals, whicli must be rendered as short as possible, in 

 order that they may yield an approximately exact 

 representation of the sea-bottom. 



In surveying any portion of land, with a view 

 to its exact delineation, we can generally move 

 freely over the surface itself that we are studying. 

 The operations of geo'esy give, with the utmost 

 accuracy, the positions and the heiglits of as many 



