194 J. A. POLLOCK. 



by holding a flat surface to the ear, no sound due to the 

 ground vibration is heard. This point is of importance as 

 it means that the sound heard when the head rests on the 

 ear is not due to the air associated with the auricle and 

 ear passage being set into vibration by the mere contact 

 of the head with the vibrating surface. 



Such astonishing results are obtained by this method of 

 detecting earth movements with the ear, that clearly a 

 definite physical description is required of the way by 

 which vibrations, of sufficient energy to give rise to the 

 sensation of sound, are created in the air of the ear passage 

 from the minute ground movements. 



In view of the last experience just mentioned, to account 

 for the sounds heard with the ear to the ground, alterna- 

 tives are only possible. Either the auricle acts as an ear 

 trumpet in concentrating disturbances already existing in 

 the air, or there is a local transformation of vibrations 

 from the solid to the air of the ear passage. 



The auricle is not, however, fashioned to concentrate to 

 any extent the energy of air vibrations. Further, the 

 matter may be put to the test of experiment, and it is found 

 that conical ear trumpets with apertures even ten times 

 the area of the auricle will not pick up earth disturbances 

 which are strikingly evident when the ear is laid to the 

 ground. The former alternative may, therefore, be 

 dismissed from further consideration. 



The maximum sound occurs when the auricle is used 

 wholly or in part as a rest for the head. In such a case 

 the air in the channels between the folds of the auricle, 

 still in communication with the ear passage, becomes 

 enclosed. On one side of this confined air is the surface of 

 the ground, on the other the drum-skin attached to the 

 massive head, while the auricle forms an elastic connection 

 between the head and the ground. 



