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is heard in both ears ; but it will be found, if the intensity in respect 

 to one ear be moderately yet decidedly increased, by bringing the 

 sounding body nearer that ear than the other, or otherwise, as by the 

 employment, in respect to one ear, of a damper or obstructor of 

 sound, or in respect of the other ear, by the employment of some 

 intensifier, or good collector or conductor of sound, the sound is heard 

 in that ear only which is favoured and has the advantage of greater 

 intensity. 



There is little doubt that this law holds with regard to sounds 

 passing through the air, and carried to the ear in the ordinary 

 manner, without the aid of any mechanical contrivance, as for in- 

 stance those of a watch placed in front of the face ; but as the re- 

 striction of hearing to one ear, and its suppression in the other, 

 admit of being rendered more obvious by an apparatus that shall 

 collect sound, prevent its diffusion through the air, and carry it 

 direct to the ear, I propose to give the results of experiments made 

 with an instrument which I have invented for hearing with both 

 ears respectively, and which, as it is specially adapted for the auscul- 

 tation of differences in the sounds of different parts of the chest, I 

 have named the Differential Stethoscope, or Stethophone. 



The results thus procured will be more satisfactory than those ob- 

 tained by ordinary audition ; a sound will be increased as a visual 

 object is magnified by the microscope, and as both ears are similarly 

 dealt with, a perfect parity of conditions will hold in respect of 

 both ears. 



The differential stethophone (see figure) is simply an instrument con- 

 sisting of two hearing-tubes, or trumpets, or stethoscopes, provided 

 with collecting-cups and ear-knobs, one for each ear respectively. The 

 two tubes are, for convenience, mechanically combined, but may 

 be said to be acoustically separate, as care is taken that the sound, 

 once admitted into one tube, is not communicated to the other. The 

 tubes are composed of two parts nearly equal in length, one near the 

 ear-knob, made of metal (C) ; while the other part, near the collecting- 

 cup, is made of metal wire (B), to impart flexibility. The ear-end is 

 curved, so as to approach the ear, and is supplied with an ivory knob (D) 

 for insertion into the meat us externus. The other end of the tube, 

 being intended to collect sound, is supplied with a hollow cup, or 



