190 



J. A. POLLOCK. 



disturbances, occurring, perhaps, within the area of the 

 aperture of the instrument, are wholly avoided. 



Types of the forms tested, drawn about one half natural 

 size, are shown in section in figure 3. 



mm 



y A 



Fig. 3. 



The instruments either rested on the table or were held, 

 against the surface with the hand, the connection to the 

 ears being through a rubber tube fixed to the projecting 

 pipe. No. 3 gives the loudest sounds, but with respect to 

 performance there is not much to choose between the forms 

 shown; they are all efficient detectors of the surface 

 movements, even No. 4 with a plane undersurface, only 

 broken by the central aperture, gives quite good results. 

 Here, indeed, one meets with a puzzle of long standing. It 

 really seems impossible to make anything with an aperture 

 connected to an ear piece which will not act, in association 

 with the ears, as a detector of small surface vibrations. 



As ordinarily used the stethoscopes are lightly held 

 against the surface. The intensity of the sound is a 

 maximum for a certain pressure of contact, but under the 

 conditions of the experiments, all sounds cease if the 

 instruments are pressed hard down on the table. 



The air in a stethoscope can be set into appreciable 

 vibration by mere contact of the appliance with a vibrating 

 body if the amplitude of the movement is sufficiently great. 



