HYDROGRAPHY 



147 



tains several of the deepest soundings of over 5,000 fathoms. 

 With the exception of these abyssal " Deeps," the floor 

 of the oceans far from land is a flat or very sHghtly midulating 

 plain, the contom-s being distant and the gradients so slight 

 as to be scarcely noticeable, like those on most good railway 

 tracks on land. On approaching the continents, however, 

 the slope usually becomes steeper to form what Murray called 

 the " Continental Slope." (Fig. 4). Working out from the 

 land, the shore of the continent extends as a shallow " Con- 

 tinental Shelf " to about the 100-fathom line, where, at this 

 " Continental Edge," the steeper gradient (the " Continental 



Slope ") begins and descends, almost abruptly in places, to 

 the great abyssal undulating plain — the floor of the ocean. 



In taking a series of oceanographical observations at sea, 

 the first requisite is to determine the locality and the depth — 

 where you are, and exactly how much water is below you. 

 If you know the exact locality, the depth may perhaps be 

 obtained approximately from the chart, but it is weU to 

 verify it by direct observation with a sounding apparatus, 

 such as the " lead," the Lucas self-recording machine, or 

 the Kelvin sounder, which indicates the distance up a tube 

 that the water is forced by the pressure at that depth. 



There have been many types of sounding machines used 



