Geology 



air, but when it reaches the surface we can tell how 

 far up the tube it has been by seeing how far up the 

 red lining of the tube has been turned white. From 

 the height to which the water has been forced, the 

 pressure at the bottom of the ocean can be ascertained, 

 and from this it is easy to calculate the depth. 



The modern forms of instrument consist of a metal 

 pressure gauge, which registers automatically the highest 

 pressure to which it has been subjected, and thus gives 

 us a measure of the depth, without our having recourse 

 to the length of wire paid out. 



Round the edges of the continents there is usually 

 a submerged plateau, sometimes called the " continental 

 shelf," and this varies very much in width. The water 

 upon it is usually shallow, and its slope is very gradual. 

 At the edge of the " continental shelf" the slope 

 becomes much more steep, and the transition to deep 

 water is consequently rapid. 



If we go westward from the coast of Ireland we find 

 that the ocean floor falls about 6 feet per mile for the 

 first 230 miles, then, the slope suddenly changing to 

 450 feet per mile, it drops 9000 feet in the next 20 

 miles, beyond which the bottom is fairly level but 

 gently undulating, until a similar shelf is met with 

 near the American continent. 



Some idea of the total bulk of the oceanic waters 

 may be gained by comparing it with that of the land 

 above sea-level. If we could shave off the continents 

 and islands of the world down to sea-level, and trans- 

 port the materials of which they are built into the 

 Atlantic Ocean, they would scarcely suffice to fill up 

 one-third of it, leaving the great Pacific, Indian, and 



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