THE SEA 7 



As yet, comparatively little of the floor of the 

 ocean has been sounded. Instruments have recently 

 been perfected which enable navigators to make 

 soundings while their ships are proceeding at full 

 speed. These instruments, variously called "sonic 

 depth finders" and "fathometers," measure the time 

 elapsing between the emission of a sound and the 

 return of the echo from the bottom of the sea. Sound 

 travels in water at the rate of 4800 feet a second. 

 Thus, if two seconds elapse between the emission 

 of the sound and the return of the echo, a depth 

 of 4800 feet is indicated. Of course the instruments 

 required to produce the sound, to hear the echo, and 

 to measure the time interval are rather complicated, 

 but they have been designed to work continuously 

 and automatically, and indicate depths without ad- 

 justment or attention as long as they are allowed to 

 operate. 



Before many years topographic maps of the 

 floor of all the important parts of the ocean will be 

 prepared by the use of this instrument. Such maps 

 will be a great boon to navigation, as they will enable 

 navigators to find their exact position at any time 

 by noting the character of the bottom over which 

 their ships are passing. Navigators will be warned 

 by the depth finder as soon as their boats reach 

 shallow water ; thus few ships will run aground, and 

 many wrecks will be prevented. 



The exact volume of the ocean cannot be estimated 

 until a topographic map of its floor is prepared. 

 Scientists have variously estimated the volume to be 

 between 300 and 320 millions of cubic miles, whereas 



