THE OCEAN 325 



Another area of almost equal depth (30,930 feet) is the Aldrich Deep 

 northeast of New Zealand. The Tuscarora Deep, nearly 28,000 

 feet, is east of Japan. 



The greatest depth of water in the Atlantic is north of Porto 

 Rico in the Blake Deep (27,366 feet). This deep, like those of the 

 Pacific, is long and narrow, has steep slopes, and is parallel to the 

 great ridge of which Porto Rico is a part. In few other places in 

 the Atlantic does the depth reach 20,000 feet. The Indian Ocean 

 is not known to have depths much exceeding 20,000 feet, and the 

 deepest known place in the Southern Ocean is still less. 



The depth of the ocean is known by soundings. Soundings are 

 made from ships, by reeling out a heavy metallic weight held by a 

 fine steel wire. The weight is so fastened to the line as to be set 

 free when it reaches the bottom, for it is much simpler to leave it 

 at the bottom than to draw it up again. A sounding of 3,000 

 fathoms may be made in about an hour. (Why not use a rope, 

 instead of a wire, in sounding?) 



Volume. The volume of water in the oceans is nearly fifteen 

 times the volume of land above sea. If all the material of the land 

 were carried to the sea and deposited in its basin, it would raise 

 the level of the water about 650 feet. If the surface of the litho- 

 sphere w r ere brought to a common level by planing down all ele- 

 vations and building up the deep parts of the ocean basins, the 

 ocean water would cover the whole of the earth to a depth of about 

 9,000 feet, or nearly two miles. 



Topography of the bottom. The larger part of the sea's bottom 

 is nearly flat, and is therefore very unlike the land. The surface 

 of the land is made rough in various ways, but especially by run- 

 ning water; but rivers do not flow on the bottom of the sea, and 

 the difference between the topography of the sea bottom and that 

 of the land is due largely to their absence from the sea bottom. 



In spite of the general flatness of the sea bottom, its relief is 

 not less than that of the land. The irregularities of its bottom 

 are of several types. These are (1) volcanic cones, often built up 

 from the bottom of the deep sea to the surface of the water, and 

 even far above it (p. 167); (2) steep slopes or scarps, such as those 

 where the continental platforms slope down to the deep sea basins, 



