90 PROCEEDINGS OF THE AMERICAN ACADEMY 



is a misnomer, for it does not magnify weak sounds ; it simply 

 reproduces them close to the ear. When it is said that a fly can be 

 heard to walk, the function of distance is left out of the account. 

 One cannot hear a fly walk at the distance of three or four feet ; but 

 if one can get a fly to walk upon his ear, or upon a plate, like a tele- 

 phone plate held at his ear, he will find that he can hear the fly walk ; 

 and the only reason he doesn't hear it walk under ordinary circum- 

 stances is because the fly is too distant. One cannot hear a man walk 

 a mile away, but a telephone will enable him to hear him walk ; but 

 one ought not to say that the sound is magnified. The function of a 

 telescope is to make distant objects appear to be nearer ; the function 

 of a microscope to make minute objects appear larger. Under no 

 circumstances can a microphone make a walking fly to be heard by 

 one holding a telephone at arm's length from his ear ; its function is, 

 therefore, to reproduce distant sounds close to the ear, and it is there- 

 fore simply a telephone. Neither does its action require us to modify 

 in the least the general statements concerning sound-vibrations in a 

 body. Whenever there are two surfaces in contact, and one of them 

 is subject to sound-vibrations, the pressure at the two surfaces must 

 vary with the phase of the vibration, and a current of electricity must 

 vary with such pressure. This is strictly in accordance with Ohms's 

 law, and with experience. 



If one may borrow from Natural History a terminology appli- 

 cable to these cases, it may be said that there are two genera of 

 telephones, — the electro-motive and the variable-resistant. These 

 two differ from each other in every essential j>articular. Of the first 

 there are two species ; the electro-magnetic, and the magneto-electric. 

 Of the second there are very many species already, and it is evident 

 that there may be many more. 



The synoptical table appended shows at a glance the relations 

 specified above : — 



Genera. Species. 



o^-S a 



•2 a 3 * 



8lto& 



■ o 2 



la 



s§ 



' Permanent Magnets, varying a current already 



on the line Dolbear, Aug., 1873 



Electro Magnets, varying a current already 



on the line Bell, Spring, 1876 



Permanent Magnets, originating the only cur- 

 rent on the line . . . . Dolbear, Sept., 1876 



Automatically Page, 1837 



Platinum Contact, varying pressure . . . Keiss, 1861 



Water, „ space .... Gray, Feb., 1876 



Iron Cone in Mercury, „ cross-section . Dolbear, Aug., 1876 



I Carbon, „ pressure . . . Edison, Jan., 1877 



Battery Elements, „ „ ... Dolbear, Dee., 1877 



Carbon-Microphone, „ „ ... Hughes, Spring, 1878 



