THE STETHOSCOPE. 



191 



This is usually tlie case with disturbances caused in wood 

 by the working of a watch. The present discussion is 

 limited to the realm of disturbances of smaller amplitude 

 which do not give rise to sounds if the stethoscopes are 

 pressed hard against the surface. 



That the vibration of the surface is not altered by the 

 pressure can be ascertained by using an ordinary old 

 fashioned wooden stethoscope. The intensity of the sound 

 is not affected in this case by the force with which the 

 instrument is pressed against the table. 



The observation, with stethoscopes like those in Fig. 2, 

 that when held hard against the surface all sounds cease, 

 indicates the solution of the problem of their action. When 

 resting on the table, or held lightly against it, the instru- 

 ments evidently float on the film of air between the surfaces 

 usually considered as in contact. This film of air forms an 

 elastic connection between the mass and the surface, and 

 in explanation of the air pulses which actually reach the 

 ears in the case of these simple forms of binaural stetho- 

 scope there is nothing to add to the description just given 

 in connection with the obviously mechanical type of 

 instrument shown in figure 1. 



The evidence in favour of this explanation is, however, 

 not confined to the fact that the sound ceases when the 

 instruments are pressed against the surface, for before 

 the extinction of the sound occurs, with increasing pressure 

 applied by the hand, the pitch of the sound continuously 

 rises. This effect is a striking one and is to be expected 

 if the mass is supported on a layer of gas. If the gas were 

 enclosed the vibration-frequency would vary as the square 

 root of the gas pressure. 



The recognition of the elastic air film as a definite part 

 of these simple appliances completely solves the puzzle 

 previously mentioned, for the ' steady mass ' is always 



