THE STETHOSCOPE. 193 



vibrating surface to which the ear may be conveniently 

 applied. 



With the experience gained from experimenting with 

 instruments of the types previously described it can hardly 

 be doubted that in connection with the use of the old 

 fashioned form there is some kind of local transformation 

 of vibrations from solid to air. As it is quite certain that 

 the instrument plays no part in any such transformation 

 we are forced to look beyond it. 



A feature of the art of the tracker here comes into view 

 and a problem, appropriate to the point which the discuss- 

 ion has reached, suggests itself: — how is it that minute 

 movements of the ground are appreciated by listening with 

 the ear to the surface ? The experience of the tracker to 

 which reference is here made is known to everyone in a 

 limited way, and certainly merits consideration. It is 

 familiar from the habit of putting a watch under one's 

 pillow at night. 



The matter may be readily investigated. With ground 

 movements only barely detectable the following points are 

 experimentally definite : — 



(1) On resting the head on the ground on the cheek bone 

 no sound is heard until the head is turned so that the air 

 between the folds of the auricle is almost wholly imprisoned 

 by the surface of the ground. One can just appreciate 

 that, in this position, the auricle is slightly pressed 

 against the surface. 



(2) The intensity of the sound notably increases when 

 the head is further turned so that the air associated with 

 the auricle and ear passage becomes completely enclosed, 

 and the weight of the head is definitely borne either wholly 

 or in part by the auricle. 



(3) If while the head rests on the cheek bone, the auricle 

 being free, the air connected with the auricle is imprisoned 



M— December], 1920. 



