THE STETHOSCOPE. 



195 



The features so familiar in connection with seismoscopes 

 are here again prominent, and in view of the whole evidence 

 it may be confidently concluded that the head acts as a 

 4 steady mass,' the auricle as the elastic support, and that 

 the mode of transformation of vibrations from solid to air 

 is exactly similar to that involved in the use of the stetho- 

 scopes described in the earlier part of the paper. On this 

 view, as the ground rises and falls, the auricle yields before 

 the head, on account of its considerable mass, appreciably 

 moves. Relative to the head the ground acts like the end 

 of a concertina, the expansible sides being represented by 

 the elastic auricle. The movement of the ground, though 

 on a microscopic scale, thus gives rise to condensations 

 and rarifications of the air in the ear passage which, if of 

 sufficient intensity, are perceived as sounds. 



It appears, then, as a general result of the discussion, 

 that the acoustic determination of surface vibrations has, 

 in the last resort when the disturbances are very small, a 

 definite dynamical aspect, the detection, in all the instances 

 described, depending on the movements of the surface 

 relative to a 'steady mass' elastically supported. In 

 detecting small movements with the old fashioned 

 stethoscope, or after the manner of the tracker, the 

 mechanism is supplied by the head and ear, the auricle 

 having the very definite function of acting as the elastic 

 connection between the mass and the surface. In other 

 cases, where the air disturbances are led by tubes directly 

 into the ear passages, the mechanical action is recognisable 

 associated with the instruments. 



The minuteness of the movements which may be appre- 

 ciated by this local transformation of vibrations from solids 

 to air, either by instrumental means or by head and ear, is 

 only another tribute to the. well known extraordinary 

 sensitiveness of the ear to slight periodic fluctuations of 

 pressure in the air. 



