56 PROCEEDINGS OF THE AMERICAN ACADEMY 



D^y — after the first — were found to be almost the same. To obtain 

 an absolutely reliable equation, it is necessary to take a great many 

 points very near together; and as each point raises the equation one 

 degree, and adds one more equation to those from which the coefficients 

 are found, the mere mechanical work of finding the equation of tlie 

 curve becomes very great. 



No. XXV. — EFFECT OF THE TENSION OF MEMBRANES 



ON SOUND. 



By G. F. Hartshorn. 



Presented June 9, 1880. 



Se\'t:ral experiments were tried bearing upon this subject by speak- 

 ing alternately through two telephones, the mouth of one being covered 

 with a membrane and the mouth of the other being open, the difference 

 in sound between the two being observed by means of a telei)hone in 

 the circuit. These experiments were not thought trustworthy, as it 

 was difficult to speak with equal intensity and at precisely the same 

 distances from the two instruments ; to obviate these difficulties the 

 following apparatus was devised. 



An ordinary Bell telephone was provided with two tubes, which 

 were placed opposite to each other on opposite sides of the vibrating 

 plate. Rubber tubes were slipped upon these two tubes and led away 

 to a mouth-piece which consisted of a chamber of clay, so moulded as 

 to allow the extremities of the rubber tubes to terminate in broad glass 

 funnels which were imbedded at a certain angle with each other in the 

 clay. Another telephone was included in the circuit to enable one to 

 hear what was transmitted. 



Sounds sent to the telephones by both the mouth-pieces in the clay 

 chamber would not be heard at the receiving telephone, because the 

 equal vibrations striking opposite sides of the sending telephone plate 

 would neutralize each other, but by closing alternately the connecting 

 tubes the sound can be passed at will through either of the mouth- 

 pieces. 



In experimenting with the voice it was found that, with both a loose 

 and a tight membrane, the articulation was plainer, and the sounds 

 somewhat louder, than when passed through the open mouth-piece. 



