10 Prof. J. Joly on Scientific 



The earlier attempts were directed mainly to develop- 

 listening devices which could be towed astern of the ship or 

 could be attached without to her sides. It was expected that 

 the noises on the ship would render any other mode of 

 listening ineffective. Later results showed that listening to 

 such acoustic vibrations as the Avails of the ship pick up 

 from the sea is the most effective method. This method, 

 too, permits of determining the direction whence the sounds 

 proceed. It was ascertained that the deeper the listening 

 device was located in the ship the better. A small vessel is 

 thus at a disadvantage in hearing the bell, and over-board 

 receivers will not do. 



As finally worked out the listening arrangements are 

 simple. A small cast-iron tank is screwed on to the inner 

 wall of the ship, being open against the ship's plates. This 

 tank is filled with water. In it two microphones are 

 immersed near each other, but one forward, the other more 

 aft. One such tank holding two microphones is fixed to 

 starboard, another to port. The sound gathered by the iron 

 walls of the vessel passes directly to the water in these tanks, 

 and this in tarn conveys it to the microphones. The best 

 position for the tanks is well forward, nearly in the bow,, 

 this being the most frequent presentation to the source of 

 sound. The best position of the tank is found by direct 

 trial and varies with various peculiarities of the particular 

 vessel. 



Leads from the microphones pass upwards to the bridge. 

 There two telephone receivers are used for listening : one 

 being applied to each ear. One of these telephone receivers 

 goes to the forward, the other to the after microphone in the 

 one tank. A switch enables either the port or starboard 

 tank to be put on to the telephones. A semaphore tells the 

 sailor to which side he is listening. The operator listens 

 alternately to the sound received on port and starboard. If 

 the signal station lies to port the telephones when switched 

 on to the microphones on that side are loud while the star- 

 board microphones are mute. If the bell is right ahead both 

 microphones speak equally loudly. For obtaining an accurate 

 bearing of the bell it is usual to swing the ship till she is 

 bow on to the bell as judged by the equality of the sound in 

 the microphones. The course of the vessel is then the 

 bearing of the bell. 



The conditions which are most favourable to the receipt 

 of the sounds involve the presentation of the surface 

 of the ship where the tanks are placed towards the source of 

 the sound. It follows that the loudness of the sound and 



