THE STETHOSCOPE. 189 



seismographs in books on seismology. Here an elementary 

 description is all that is needed. 



Referring to figure 1, if the surface on which such a 

 stethoscope rests suffers a sudden upward displacement, 

 the plate a moves with it and the case of the instrument 

 thus receives an impulse. Owing to the comparatively 

 large mass of the apparatus, the displacement may be 

 completed before the case appreciably alters its position. 

 In virtue, then, of the elasticity of the connecting ring e 

 and the inertia of the case, the initial result of a sudden 

 displacement of the surface is a movement of the lower 

 plate relative to the upper one. 



Such movements of the lower plate relative to the rest 

 of the instrument create pressure pulses in the air con- 

 tained in the apparatus which are transmitted through the 

 air enclosed in the rubber tubes to the ears, to be there 

 perceived as sounds. 



A similar description is clearly applicable to instruments 

 like that shown in figure 2. To the uninitiated, stetho- 

 scopes of this latter class must appear unnecessarily 

 massive. 



Simpler forms of instrument have now to be considered. 

 The immediate discussion deals with those types of which 

 the common trumpet or bell shaped binaural stethoscope, 

 not provided with a diaphragm, may be taken as an example. 



As I am concerned here merely with the physical action 

 and not at all with the interpretation of the sounds heard, 

 my experiments have been made in connection with the 

 vibrations of the surface of a slate bench, or wooden table, 

 caused by very gently tapping or stroking the surface with 

 the tips of the fingers or, in some cases, by the working of 

 a watch with a smooth movement. With such surfaces, 

 as contrasted with that of the human body, purely local 



