HEARING. 



565 



for, to empower all animals to express their 

 sense and meaning to others; to make known 

 their fears, their wants, their pains and sorrows 

 in melancholick tones; their joys and pleasures 

 in more harmonious notes ; to send their minds 

 at great distances in a short time in loud 

 boations ; or to express their thoughts near at 

 hand with a gentle voice or in secret whispers. 

 And to say no more, who less than the same 

 most wise and indulgent Creator could form 

 such an economy as that of melody and musick 

 is; that the medium should, as I said, so 

 readily receive every impression of sound, and 

 convey the melodious vibration of every musi- 

 cal string, the harmonious pulses of every 

 animal voice, and of every musical pipe; and 

 the ear be as well adapted and ready to receive 

 all these impressions as the medium to convey 

 them. And lastly, that by means of the curious 

 lodgment and inosculation of the auditory 

 nerves, the orgasms of the spirits should be 

 allayed and perturbations of the mind in a great 

 measure quieted and stilled ; or to express it 

 in the words of the last-cited famous author 

 (Willis), * that musick should not only affect 

 the fancy with delight, but also give relief to 

 the grief and sadness of the heart; yea, appease 

 all those turbulent passions which are excited 

 in the breast by an immoderate ferment and 

 fluctuation of the blood.' " 



Preliminary observations on sound.* Sound 

 is the result of an impulse of any kind con- 

 veyed by the air to our ears. The analysis 

 of what takes place on the production of various 

 familiar sounds or noises abundantly explains 

 this. If the ear be applied to one extremity of 

 a long beam of timber and a person tap with 

 his finger on the other, the impulse is distinctly 

 perceived by the impression of sound which is 

 conveyed. A fine probe introduced carefully 

 through the meatus externus, and made to 

 impinge upon the membrana tympani, however 

 gently, will occasion the sensation of sound. 

 To produce the sensation, then, of sound, an im- 

 pulse is necessary either of some solid directly 

 upon the membrane of the tympanum itself, or 

 of the air which is always in contact with that 

 membrane. The body by which the sound is 

 produced, denominated by Professor Wheat- 

 stone f a phonic, occasions in the surrounding 

 air vibrations or oscillations, corresponding in 

 number and extent to those which exist in 

 itself; and these vibrations or oscillations being 

 propagated to the organ of hearing, give rise to 

 the sensation. This agitation of the air sur- 

 rounding the body from whence the sound 

 emanates is manifest in numerous instances ; 

 the report of a cannon, the rushing of waters, the 

 rattling of carriages, which in the crowded tho- 

 roughfares of London communicate their vibra- 

 tions to the walls and floors of the houses and 

 even to the furniture. In the familiar instance 



* It will be perceived that in the ensuing obser- 

 vations the writer has borrowed largely from Sir 

 John HerscheFs admirable essay on sound in the 

 Encyclopaedia Metropolitana. He has also to 

 acknowledge his obligations to the article on Acous- 

 tics in Pouillet's Elemens de Physique. 



t Annals of Philosophy, new series, vol. vi. 



of eliciting sound from a finger-glass partly full 

 of water by rubbing the wetted finger round its 

 brim, the vibrations which this friction excites 

 in the glass are rendered evident by the crispa- 

 tions produced in the water immediately in 

 contact with it. The vibration of the water, as 

 indicated by these crispations, corresponds with 

 that of the glass the greater the intensity of 

 the sound elicited, the more considerable are 

 the vibrations in the glass, and consequently 

 the more manifest are those of the water, and 

 vice versa. " In musical sounds we may also 

 observe an agitation which is often felt commu- 

 nicating itself to the surrounding bodies. If, 

 for example, \*e stand under or near a piano- 

 forte when it is sounding, we feel a sensible 

 tremor in the floor of the apartment. If we 

 lay the finger or hand on the instrument or 

 touch any other, such as a violin when it is 

 sounding, or a bell, we feel the same sort of 

 tremor in every part of them ; and this is well 

 observed in the case of any glass vessel, such 

 as a tumbler or large cup. If we strike it so 

 as to make it sound, and then touch the mouth 

 of it with the finger, we feel a sensible tremor 

 in the glass ; and when this internal agitation 

 is stopped, as it generally is by the contact 

 with the finger, then the sound ceases along 

 with it."* 



The disturbance produced in the air by a 

 sounding body has been from a very early 

 period illustrated by a reference to the waves 

 formed in still water by a stone falling into it. 

 " Voice," says Vitruvius, " is breath flowing 

 and made sensible to the hearing by striking 

 the air. It moves in infinite circumferences of 

 circles, as when, by throwing a stone into still 

 water, you produce innumerable circles of 

 waves, increasing from the centre and spread- 

 ing outwards, till the boundary of the space or 

 some obstacle prevents their outlines from going 

 further. In the same manner the voice makes 

 its motions in circles. But in water the circles 

 move breadthways upon a level plane; the 

 voice proceeds in breadth, and also successively 

 ascends in height."f That the presence of air is 

 necessary for the production of sound is proved 

 by the experiment first tried by HauksbeeJ and 

 repeated by Biot. A bell was made to ring in 

 the receiver of an air-pump, and in proportion 

 as the air was exhausted it was found that the 

 sound died away, and it again returned as the 

 air was re-admitted. On the other hand, the 

 bell sounded more strongly when the air within 

 the receiver was condensed, and the grealer the 

 condensation of the air, the louder was the 

 sound. 



Any irregular impulse communicated to the 

 air produces a noise, in contradistinction to 

 a musical sound. This latter results from a 

 succession of impulses, which occur at ex- 

 actly equal intervals of time, and which are 

 exactly similar in duration and intensity. 

 When these impulses succeed each other with 

 great rapidity, the sound appears continuous, 



* Encycl. Britann. art. Acoustics, 

 t Vitruvi'is de Arch. v. 3, quoted in Whewell's 

 History of the Inductive Sciences. 

 1 Phil. Trans. 1705. 



