184 ON PROFESSOR BELl"s ARTICULATING TELEPHONE. 



to each was attached a rod of wood about one inch square, which, 

 passing through the intermediate rooms, terminated just above the 

 floor of the lecture-room in the topmost story. When the 

 instruments were played, an auditor placing his ear on tlie upper 

 ends of the rods successively, heard each instrument, and when 

 sounding-boards were placed in contact with the rod-ends, the 

 vibrations coming through the rods were transferred to the air, 

 and fell upon the ears of the audience in the form of musical 

 sounds. The common toy telephone, now so largely sold, is 

 merely another illustration of this. It consists of two boxes united by 

 a string which passes through the bottom of each. When the 

 string is stretched tightly, and a person speaks in one box, what 

 is said can be heard by an ear applied at the other, the sound 

 vibrations being mechanically conveyed along the string. In this 

 way conversation has been carried on between points one thousand 

 feet apart. 



It can very easily be shown that a mechanical movement, or series 

 of vibrations, is set up in any substance whenever sound is trans- 

 mitted through it. As one experimental proof may be quoted the 

 fact that if a glass tumbler or bell which gives out a particular note 

 when sounded, be taken, and that same note is produced by other 

 means — the human voice, for instance — in close proximity to it, 

 the bell can be broken by the violent vibrations set up in it, which 

 have travelled through the air. No effect is produced upon it if a 

 note is sounded, however loud, other than that to which it answers. 

 This fact is susceptible of many curious applications ; and as 

 another instance of sympathetic vibrations may be mentioned the 

 case of two clocks, which, although going at slightly different 

 rates, will adjust themselves to go precisely synchronously, if they 

 are within reach of each others pendulum vibrations. 



The loudness of a sound depends upon the amplitude or size of 

 these vibrations. The pitch of a sound on the other hand, depends 

 wholly upon the number of vibrations per second which produce it ; 

 and if one of two sounds consists of twice as many vibrations per 



