The Wonderful Fathometer 191 



tive and picks up the echoes from the 

 bottom of the sea. These echoes come from 

 blows (which the stoker heard) made by a 

 hammer on the plating of the vessel's 

 bottom. 



When the mechanism is set the hammer 

 strikes about each three seconds and the 

 sound of the blow travels to the bottom 

 of the sea, and then gives back the echo 

 which is transmitted, via the micro- 

 phone, to the bridge. It has been known 

 for some time that sound travels through 

 water at a uniform rate of 4,714 feet per 

 second. The mechanism is arranged to 

 give regular blows at the intervals men- 

 tioned, whilst another very delicate piece 

 of apparatus measures the time taken by 

 the echo to reach the microphone; by that 

 means the depth of the water under the 

 ship is known to a foot. 



The Fathometer is only set to work when 

 the liner is within a reasonable distance of 

 the coast and fog or mist supervenes. In 

 clear weather going into or leaving port the 

 pilot is able to know exactly where he is; 



