192 J. A. POLLOCK. 



present, and the instruments are, in respect to this part 

 of their action, as definitely mechanical as the forms shown 

 in figures 1 and 2. 



A large hollow cone, with its wider opening held against 

 the table, the narrow end being connected by a rubber tube 

 to an ear-piece, is a fairly good detector of small surface 

 vibrations. If sounds are heard with this apparatus they 

 do not cease when the instrument is pressed hard against 

 the surface, and the action is that of an ear trumpet. 



Thus, as a result of the previous discussion, stethoscopes 

 of the forms examined must be considered, from a physical 

 point of view, as instruments which locally transform 

 minute mechanical movements of solids to corresponding 

 vibrations of the air associated with them. Stethoscopes 

 with conical shaped openings have, in addition, the property 

 of concentrating disturbances already existing in the air as 

 a direct result of the vibrations of the surface, but, with 

 the instruments in ordinary use, any effect due to their 

 shape is wholly negligible in comparison with that of the 

 local mechanical transformation. 



The old fashioned stethoscope, consisting of a hollow 

 wooden or ebonite stem with a small conical opening at 

 one end and an ear-plate at the other, still remains to be 

 considered. 



Comparative observations, made by placing the ear 

 against a vibrating surface and then against the ear plate 

 of one of these stethoscopes pressed against the surface, 

 show that very small movements are, in this way, as readily 

 detected without the stethoscope as with it. Also in the 

 case of vibrations of small amplitude, the intensity of the 

 sound heard when using these stethoscopes is the same 

 whether the stem of the apparatus is solid or hollow, so 

 very little experimenting is required to prove that the main 

 function of these instruments is to act as a part of the 



