The Hydrosphere 



is the continental land, which has an average elevation of 

 900 feet above sea-level. 



The form of the ocean floor is of some considerable 

 interest, and one of the least anticipated facts is, that 

 the greatest depths are found, not in the middle of the 

 basins, as might be expected, but usually quite near the 

 land. The greatest depth so far recorded in the Atlantic 

 Ocean is only about 80 miles from the Bermudas, 

 and is 3875 fathoms, but greater depths have been 

 recorded in the Pacific, off the coast of Japan, again 

 near the land, where a sounding of 4655 fathoms was 

 taken in what is known as the Tuscarora Deep, from 

 the name of the United States surveying vessel which 

 discovered it. 



The greatest depth yet recorded is near the Kermadec 

 Islands, and about 300 miles north-east of East Cape in 

 New Zealand. Here a depth of 5155 fathoms, or nearly 

 6 miles, was reached. 



It is by no means an easy matter to make soundings 

 in extremely deep water, and much ingenious apparatus 

 has been devised for the purpose. The simplest form 

 consists of a weight attached to a cord. The weight is 

 thrown overboard, and the line paid out, until it is felt 

 to slacken owing to the weight having reached the 

 bottom. The length of line used will then, of course, 

 indicate the depth. When greater depths are to be 

 measured, the weight of the line becomes so great that 

 it is difficult, if not impossible, to tell when the sinker 

 has reached the bottom, and so a heavier weight has to 

 be used. There is, of course, a limit to this increase 

 of weight, as if it is taken beyond a certain point a 

 stronger cord will be required to support it, and this 



