800 



SOUND. 



12th. Fluids are more powerful conductors than 

 productors of sound, but conduct less rapidly than 

 solids. Their atoms or component parts pass each 

 other, and do not return to their original places as 

 do those of solids ; this accounts for sound passing 

 in all directions in the atmosphere, but most in the 

 direction in which most impulse is given; a/so why 

 the same degree of percussion produces more sound 

 in hard solid* than in the atmosphere ; and why in 

 transit there is less change in its original charac- 

 ter. 



13th. Sound is much influenced by moisture in 

 the atmosphere. Intensity and distance of transit 

 are regulated more by the adjustment of par- 

 ticles than the proportion of moisture. For in- 

 stance it is loud and passes furthest during frost, 

 and at all times when objects are seen to a great 

 distance. This is peculiarly perceptible within the 

 tropics, and in this country in summer just as the 

 sun sinks under the horizon ; but when cold 

 increases and the particles of moisture become 

 larger, these effects are diminished. It follows 

 as a matter of course, that its transit must be 

 more or less rapid under such varying circumstan- 

 ces. 



14th. Water conducts more powerfully and 

 rapidly than the surrounding atmosphere, and, so far 

 as I have been enabled to ascertain, with increased 

 effect as it approaches the temperature of the hu- 

 man body. This is exemplified in tropical rivers 

 and in the human ear, where this fluid is the only 

 body in contact with the nerve of hearing, to which 

 it must communicate sound consistently with its 

 original character. 



Sound is not produced by the atmosphere alone, 

 without violent concussion, or being in contact with 

 a more dense medium. Being the offspring of 

 atomical action in bodies, and in degree of intensity 

 in the ratio of their adhesive and repulsive princi- 

 ples, it cannot be produced in a vacuum, nor in a 

 still body; but the latter may, by concentrating or 

 confining a fluid, increase action and sound for a 

 limited time after a certain impetus has been given 

 to the fluid, in like manner as a stream of water is 

 increased in velocity by being confined. As it not 

 only operates differently in every different substance, 

 but undergoes certain change in intensity or the 

 nature of sound by every change in the molecules 

 or surface of any mass of matter, there are no li- 

 mits to the various effects that are produced by 

 such changes. 



In all cases it is difficult to reason upon the ope- 

 rations of nature and the motions and influence of 

 matter not perceptible to the eye. In the present 

 case, however, we are enabled to judge partly by 

 our ocular faculty, and in part from our sense of 

 hearing. 



That the vibratory and undulatory or oscillatory 

 actions are not only prevalent in the musical string, 

 but in all matter in a state of agitation, is indicat- 

 ed by the following facts. In a musical string of 

 given diameter and tension, when set in motion, 

 the extent of the undulations are in the ratio of 

 the length of the string; each undulation gives 

 out a distinct sound, conformable in duration to 

 the extent of the undulation. The cause of the 

 separation in the sounds, and that each continues 

 just so long as the motion continues, is demonstrat- 

 ed by the simple fact, that if any light small mate- 

 rial be placed on the nodal point between the un- 

 dulations, it remains on the string; but if placed on 

 the wave, it is thrown from the string. In the 



flat brasses in a musical box it is regulated in 

 like manner, but differing somewhat in the un- 

 dulations; because in the former case the string 

 is fixed at both ends from which there is a more 

 determined reaction, while the vibrating brasses 

 are fixed at one end only. Similar action is ap- 

 parent in a common rope or cord, when stretched 

 on the ground, in order to define a straight line for 

 any purpose ; the line is stretched by a vertical 

 motion of the hand, a single motion of which pro- 

 duces many undulations in the cord, in number in 

 conformity to its length. The same principles of 

 action are exhibited in aqueous fluids, in the atmo- 

 sphere, and in solids of great expanse. 



In the ocean, sound continues just so long as a 

 wave is in motion, and the greatest run of sea, and 

 elevation and extent of wave, is exhibited where 

 there is a deep indent in the land. Were the mo- 

 tion of the water in this case ruled alone by the 

 current of air, on its surface, the waves would 

 move uniformly in the same direction ; but this is 

 not the case. 



These circumstances are, however, most per- 

 fectly defined by the atmosphere, because sounds 

 are by it transmitted in all directions with the 

 least change in their original character, and if ana- 

 logical reasoning is to be applied in this case, and 

 it be admitted that sound is only produced by the 

 action of bodies on each other, and ceases the in- 

 stant these become still, in it there must be spheri- 

 cal intervals at rest during vibratory motion, in or- 

 der to keep apart sounds in conformity to their ori- 

 ginal formation. 



Thus the Creator has adjusted and regulated the 

 atmosphere for these purposes, but in all artificial 

 materials it is left for man to economize his arrange- 

 ments.* 



In order to demonstrate that predominant sounds 

 are conducted by the fibre of the wood of which 

 the stethoscope is composed, I have made various 

 trials with this instrument on the human ear, and 

 advanced many experimental facts, to prove that 

 the tremulous and sonorous pulsations of the at- 

 mosphere are not, as has been universally believed, 

 collected on the superior surface of the auricle or 

 pinna of the ear, and conducted by what is termed 

 the auditory canal to the membrana tympani or 

 pelt of the drum, and thence through the central 

 cavity of the ear to the labyrinth, but must neces- 

 sarily operate by the vibratory pulsations of the 

 atmosphere impinging upon and producing vibration 

 in the pinna, which is a fibro-cartilagenous lever and 

 fulcrum, and that it is conducted by the solids and 

 denser fluids that surround the auditory passage 

 and tympanic cavity to the labyrinth. To confirm 

 this theory, the following facts are important. 



1st. It appears to merit particular notice, that 

 notwithstanding sounds reflected on a speaker oc- 

 casion great fatigue and exhaustion, arising, I sup- 

 pose, from their influence on the mouth and parts 

 adjacent, this should take place while the external 

 ear is supposed to be without vibratory action, 



* In reasoning- on this abstruse subject, I have been averse 

 to oppose whatever has been uniformly propagated and re- 

 ceived as truth to demonstration; but after much consideration, 

 I believe that vibration and sound operate most influentially 

 in the atmosphere, in a vertical direction, and least towards 

 the earth's surface, mainly from the facts, that sound from un- 

 derneath, striking within a vertical arch, produces louder and 

 more prolonged reflections than when it operates within a 

 horizontal arch. This is peculiarly perceptible in speech, in 

 which the vertical curvature prolongs sound so as to mar ar- 

 ticulation more than a horizontal curvature. This I shall il- 

 lustrate more fully in an Essay, now in the course of progress 

 for the Medical Gazette. 





