ACOUSTIC TELEGRAPHY. 
35 
simplicity each subscriber is distinguished by a number ; and 
we will suppose, if you please, that No. 5 wishes to speak. 
He takes his telephone and blows in order to make the signal 
sound, which will of course be heard at the central office. 
The first idea is to provide the wire of each subscriber with 
a telephonic signal at the central office; so that when one of 
Fig. 9. 
them sounds, the clerk at the office may hear it and answer. 
But it is not sufficient for the sound to be heard, the clerk 
must know whence it proceeds. Suppose he is in his office 
among thirty or a hundred telephones : when one of them 
sounds, how is he to know which it is ? Each might have a 
special note, but then a fine ear would be required to recognize 
these notes, and such musical discrimination could not be 
