CURVE OF EARTH'S SURFACE 15 



of the barometer and the depth of water in the harbour. He 

 found extremes for the former were 76775 and 750*79 milli- 

 metres, corresponding to the water depths of 6,309 and 6,538 

 millimetres. Here the variation corresponds closely to the 

 difference in specific gravity between mercury and the sea- 

 water that is, according to the above records, = 13' 5. 



Careful modern coastal surveys show no marked differences 

 in sea-level ; those recorded do not exceed half a metre. 

 Surveys connecting up the coast of the Atlantic near New 

 York with that of the Pacific near Seattle show a difference of 

 only 187 millimetres, which lies between the limits of error 

 over such a long distance (7,400 kilometres). 



HYPSOGRAPHICAL CURVE OF THE EARTH'S SURFACE. 



The investigations of modern oceanographers have given 

 us a fairly comprehensive knowledge of the nature and extent 

 of the sea bottom, its depth, physical constituents, and animal 

 life. Generally speaking, the great oceans are far from shallow, 

 and the volume occupied by water below sea-level greatly 

 exceeds the volume of land above sea-level. If the land above 

 sea-level were tipped into the sea it would only partially dis- 

 place the sea-water (see Fig. 2). 



The greatest oceanic depths far exceed the heights of 

 the highest mountains. Nero Deep, off the island of Guam 

 (Marianne or Ladrone Islands, North Pacific) is 5,270 fathoms 

 or 31,620 feet deep. Mount Everest, the highest mountain in 

 the world, is 29,000 feet high. Nero Deep is, however, an 

 extensive depression, and an area equivalent to the islands of 

 Sardinia and Sicily lies below 3,830 fathoms. 



The hypsographical curve gives an idea of the relationships 

 of land to water at the earth's surface. It shows us the rela- 

 tively small area of sea bottom within the 2OO-metre line and 

 below 5,750 metres. The descent from the continental shelf 

 edge in about 200 metres to a depth of 3,000 is extremely 

 sharp, and between 1,000 and 3,000 metres there is only 14-8 



