ECHO-SOUNDING 

 AND HYDROGRAPHICAL STUDIES 



By A. KiiLERiCH 



For as long as man has sailed the seas he has naturally acquired know- 

 ledge of depths of water near coasts, where there was reason to be on his 

 guard. But once he was clear of land and sounding ceased to be necessary 

 his interest in exploring the sea diminished, and was never strong enough 

 to make him stop and measure its depths. 



So it remained for many centuries until the first, ineffectual attempt 

 to ascertain ocean depths was made by Magellan, on his voyage round 

 the world. When passing the Tuamotu Islands in the south-east Pacific, 

 in 152 1, he caused his 10 leadhnes to be tied together to form a total 

 length of 700 metres; but failing to reach the bottom with this, he simply 

 concluded that he had found the lowest ocean depths. The first fairly 

 reliable oceanic soundings were made by Sir James Ross in 1840. But 

 though methods steadily improved, every measurement which depended 

 on lead and line was so laborious and took so much time that the charted 

 soundings of the ocean bed remained extremely sparse until the develop- 

 ment of the echo-sounder in the nineteen-twenties. This was the invention 

 of the Frenchman P. Marti and was first used in 19 19. 



The principle of the echo-sounder is based on the simple fact that sound 

 travels through water — as through air — with a definite velocity, in 

 this case about i ,500 metres per second. If it takes one second for a sound 

 impulse emitted from a ship to reach the bottom and return, it follows 

 that the depth is 750 metres. The sound transmitter is either a diaphragm 

 or a column of nickel rings, rapidly vibrated by electricity. A corresponding 

 installation on the opposite side of the ship picks up the echo and trans- 

 mits the impulse to the bridge, where it can be read. In older receivers a 

 pointer moves rapidly round a graduated dial, the moment of reception 

 being indicated by a brief flash. Instead of observing the flash it is pos- 

 sible to hear the echo on earphones, while watching the pointer. This 

 method demands much practice and close attention on the part of the ob- 

 server, and no matter how skilled he is the reading will always be subject to 

 a good deal of uncertainty. Consequently, in newer types the pointer 

 records the depths on a strip of paper, and as the paper moves forward at 

 a constant speed the result is a curve of points showing the profile of the 

 sea bed over which the ship is passing. In this case accuracy depends en- 



