108 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK n. 



towns are telephonic signals which give notice to people at a dis- 

 tance of ceremonies and divine service, and many persons recognize, 

 on hearing the different styles of ringing, what is the nature of the 

 ceremony announced. In case of fire, the tocsin sends forth its 

 sinister sounds, and calls afar for everyone to help. But, in these 

 cases the sound is employed in the open air, without any special 

 process for sending it to a distance by preserving its first intensity. 

 The means invented to conduct sound to much greater distances than 

 its ordinary range, constitutes what is called telephony. 



One method much used for short distances consists in causing 

 the soUnd to be propagated in tubes, in which the mass of air 

 set in motion at one extremity, transmits the full power of the 

 disturbance. 



Speaking-tubes are in the present day very frequently used in 



FIG. (54. Speaking-tube, mouth-piece, and.wlii.stle. 



private houses and commercial establishments, where the employes 

 frequently require to communicate from one distant point to another, 

 or from floor to floor. They are used also in vessels for transmit- 

 ting orders to the men aloft, or to the engineers. These are generally 

 cylindrical and flexible india-rubber tubes, with orifices of bone 

 or ivory in the form of cuplike mouth-pieces ; a whistle is fitted 

 into this mouth-piece. The whistle is sounded first to attract the 

 notice, so that the person thus warned by the sound of the whistle 

 which is put into vibration at the opposite extremity may come to 

 the tube. He then repeats the signal in the same manner to show 

 that he is there, and the conversation goes on in a low or moderate 

 voice, taking care to place alternately first the mouth and then the 

 ear to the opening of the tube. 



