I30 



DEPTHS OF THE OCEAN 



Attraction 

 of the 

 land-masses. 



Measurements 

 of depth. 



Hand line. 



Brooke's 

 sounding 



machine 



than at others ; the gravitational attraction of emerged land 

 causes a heaping-up of the sea around continental and other 

 coasts. The extent of this heaping-up near elevated continents, 

 and consequent lowering of the sea-surface far from land, appear 

 to have been much exaggerated. The difference of level due to 

 this cause has sometimes been estimated at thousands of feet. 

 Recent researches indicate that the differences of level at 

 different points of the sea-surface do not depart more than 300 

 or 400 feet from a true spheroid of revolution. 



The other causes which, in addition to the tides, may affect 

 the level of the ocean are meteorologic, such as barometric 

 pressure, temperature, the action of wind, evaporation, precipita- 

 tion, the inflow of rivers, but in no cases do these affect the 

 level of the ocean more than a few inches or a few feet. 



All depths recorded by the sounding-line in the open sea are 

 referred to the surface of the ocean, and near coasts to mean sea- 

 level. The first method of ascertaining the depth of the ocean 

 was by means of the hand line and lead, armed with tallow, used 

 by ordinary sailors. A great advance was made when Lieutenant 

 Brooke, of the United States Navy, devised the apparatus for 

 detaching the weight or sinker when it struck the bottom, the 

 line bringing up only a small tube with a sample of the bottom- 

 deposit. During the "Challenger" Expedition the line used 

 was a fine hempen rope, and the time when each loo-fathoms 

 mark passed over the ship's side was carefully noted. When 

 a great change of the rate was observed, the lead was known to 

 have reached the bottom. It is believed that even the deepest 

 soundings taken in this way are correct to within 100 feet. 



Another advance was made when fine wire was used for the 

 soundings, and the machine recorded automatically the moment 

 when the sinker struck the bottom. There are many types of 

 wire deep-sea sounding machines now in use, but the most 

 compact and practical of these is the Lucas sounding machine. 

 Sounding instruments are referred to in greater detail in another 

 chapter (see p. 30). 



To give the total number of deep soundings recorded by 

 British and other ships up to the present day, even in depths 

 exceeding 1000 fathoms, would be difficult. An approximation 

 has been made by counting the number of soundings in depths 

 exceeding 1000 fathoms laid down on the latest charts. It 

 must be remembered that not all the recorded soundings can be 

 laid down on small scale charts where they are at all numerous. 



In 1886 Sir John Murray had three hemispheres drawn on 



