Dr. Alison on the Intensification of Sound. 147 



even a small amount of solid material be interposed between the 

 water employed and the mouth of the hearing-tube. A piece of 

 wood, not much thicker than a paper-cutter, materially interferes 

 with the augmenting power of water. 



The augmentation of sound thus obtained by water seems to be 

 due to the complete fitting of the liquid on the solid body and also 

 round the mouth of the hearing-tube, whereby the column of air 

 is thoroughly enclosed ; also to the less impediment to the vibrations 

 of the instrument when held in contact with water, than when held 

 in contact with a solid body, the water yielding in a greater degree 

 than a solid. 



The mode of judging of the augmentation was twofold : 1st, one 

 sensation was compared with another perceived by the same ear, the 

 one sensation following immediately upon the other ; 2nd, the dif- 

 ferential stethophone was employed, by which two impressions are 

 simultaneously made upon the 'two ears; in which case, if one 

 impression be materially greater than the other, sound is per- 

 ceived in that ear only on which the greater impression is made. 

 To obtain the advantage of the differential stethophone,— or " Pho- 

 noscope," as it might here perhaps be more correctly designated— 

 when sounds at some distance from the ear were being examined, its 

 length was increased by the addition of long tubes of india-rubber. 

 Experiments were made upon other Uquids besides water, such as 

 mercury and ether. 



Other materials besides liquids were found to afford a similar in- 

 tensification of sound from solid bodies, such as laminae of gutta- 

 percha, or of india-rubber, and sheets of writing paper, but the 

 amount of augmentation was less. 



The hearing- tubes employed were various. Many of the experi- 

 ments were performed with the author's ordinary differential stetho- 

 phone, an instrument described in the Philosophical Magazine for 

 November 1 858. India-rubber tubes fitted with ivory ear-knobs, and 

 with wooden or glass cups (the size of the cup or object-extreiiiity 

 of ordinary stethoscopes), and having an ear-extremity to pass into 

 the meatus, and brass tubes, were also in turn employed. Tubes 

 closed at their distal extremity with solid material, such as glass, 

 did not answer so well as those closed with membrane. 



The water-bag increases the impression conveyed to the ear by the 

 wooden stethoscope, if it be placed between the flat ear-piece and 

 the external ear. It may be employed alone to reinforce sound. 

 The name of Hydrophone has been given to it. 



A postscript is added, in which the author records an experiment 

 made on the bank of the Serpentine river. A sound produced upon 

 the land was heard at a point in the water when it could not be 

 heard at an equal distance on the ground, if the two limbs of the 

 differential stethophone were employed simultaneously. 



The sensation upon the ear, connected, by means of a hollow tube, 

 witli water in sonorous undulations, was found to be much greater 

 tlian that upon the car coimected with the same water by means of 

 ft solid rod. When both tube and solid rod were cm|)loyc'd simul- 

 taneously, sound was heard in that ear only supplied witli the tube 



L2 



