222 PROCEEDINGS OF THE AMERICAN ACADEMY 



The results of these experiments can be summed up as follows : — 



1. Certain fundamental vibrations of elliptical plates are not changed 



by wide variations in the ellipticity of the plates. 



2. The vibrations of elliptical plates are less varied than those of cir- 



cular plates. A small amount of ellipticity results in a quick and 

 marked limitation in the variety of the vibrations of the plate. 



3. It is to be conjectured, therefore, that an animal, whose ear is pro- 



vided with an elliptical-shaped membrane, if such an animal 

 exists, has less perfect powers of hearing than one provided with 

 a circular membrane, as far as the variety of vibration of the 

 membranes are considered. 



No. XVIII — PERFORATED VIBRATING DISCS. 

 By Francis E. Cabot. 



Presented Dec. 10, 1879. 



Since the membrane of the human ear is often perforated in auricu 

 lar surgery without the destruction of the sense of hearing, at the sug 

 gestion of Professor Trowbridge I tried the effect of the removal of 

 a large portion of the vibrating disc of a telephone. Three discs were 

 made of ordinary tin-type plate ; one with a hole in the centre half an 

 inch in diameter ; one with four holes, each half an inch in diameter, 

 with their centres on two lines at right angles, crossing in the centre 

 of the plate, and half-way from the centre to the edge of the disc ; the 

 third with four holes four fifths of an inch in diameter, and placed 

 similarly to those last mentioned. Substituting these discs for the 

 discs ordinarily used in a Bell telephone, I found that messages could 

 be sent and received, but in a somewhat imperfect manner. On cov- 

 ering the holes with paper, I found that I could use either of the last 

 two plates, in either the sending or receiving telephone, or in both, 

 with nearly, if not quite, as good results as with the unperf orated 

 discs commonly used. 



The disc with the hole in the centre did not give good results, as 

 was to be expected, since the iron was taken away just in front of the 

 magnet ; but the others worked very well indeed. I also found that 

 a disc of mica of the same size as the usual plate, with a piece of the 



