114 ' THE APPLICATIONS OF PHYSICAL FORCES. [BOOK n. 



the speaker. The reinforcing effect of the ear- trumpet has been 

 attributed to the successive reflections of the sound-waves, which 

 multiplies their action on reaching the tympanum. But, as in the 

 speaking-trumpet, experiment has shown that the influence of the 

 walls, and consequently the reflection of their inner surface, is very 

 feeble, if any at all. The effect produced is in reality owing to the 

 progressive diminution of the sections of the air-surface which 

 transmit the sound, and which then transmit it with increasing 

 energy towards the organ. This effect may be compared with that of 

 a jet of water which issues from the orifice of a hose with a much 

 greater force than that of a body of water of equal diameter in the 

 interior of a pump-barrel. 



The stethoscope is a kind of ear-trumpet invented by Laennec, 

 and used by physicians to study chiefly the sounds produced in the 

 interior of the chest by the action of the heart. This is a wooden 

 cylinder, widened out at the end applied to the body, and pierced with 

 an opening some millimetres in diameter, at the extremity of which 

 the ear is applied. M. Kcenig has invented a new stethoscope based 

 on the refraction of sound-waves. " It is composed of a small hemi- 

 spherical capsule, in which a ring is placed covered with two india- 

 rubber membranes. An opening made through the ring allows the 

 inflation of these two membranes, in order to give them the form of 

 a lens. The small capsule has at top a small tube made to receive 

 an indiarubber pipe which puts the interior mass of air in direct com- 

 munication with the ear. The outer membrane, thus inflated, is 

 applied to the sounding body which is to be examined. It then 

 takes the form of this body, receives the vibrations and communi- 

 cates them to the opposite membrane by the intervention of the 

 inclosed air ; the second membrane afterwards communicates them 

 to the tympanum by means of the air contained in the capsule and 

 tube. Five tubes may be fixed to the capsule without interfering 

 with the clearness with which the sounds reach the ear, and then five 

 persons are able to study the sounds at the same time." 



