560 



SCIENCE, 



Science, 

 uiio.iiics 



low cur author through his numerous and interesting 

 details, we shall conclude this abstract with an enu- 

 meration of the leading results which he has obtained. 

 *""" ' 1. That it is net necessary to suppose, as has hither- 

 to been done, the existence of a particular mechanism, 

 Tor continually bringing the tympanum to vibrate in 

 unison with the bodies which act upon it. It is evi- 

 dent, that the tympanum is always in a condition to 

 be influenced by" any number of vibrations. 



2. That its tension does not probably vary, unless to 

 augment or diminish the amplitude of its excursions, 

 as Bichat had imagined. He supposed, however, con- 



trary to the result of experiment, that the tympanum 

 unstretched itself for strong impressions, and stretched 

 itself to receive weak impressions. 



3. That the vibrations of that membrane communi- 

 cate themselves, without any alteration to the labyrinth, 

 by means of the small bones, in the same manner as 

 the vibrations of the upper table of an instrument are 

 communicated to the lower table. 



4. That the small bones modify also the excursions 

 of the vibrating parts of the organs contained in the 

 labyrinth. 



5. That the cavity of the tympanum (Caisse du 

 Tambour) serves probably to keep up near the aper- 

 tures of the labyrinth, and the internal face of the 

 membrane of the tympanum, an aerial medium, whose 

 physical properties are constant. 



3. On Sounds Inaudible lo Certain Ears. 



On sounds In persons labouring under ordinary deafness, it 



inaudible to seems to have been frequently noticed that they hear 



-certain ears, gharp sounds, such as the voices of Women and child- 



ren, better than the deep and grave tones of the male 



voice ; and those accustomed to speak to deaf persons 



acquire a habit of addressing them in a shriller tone 



of voice, which is more efficacious than a louder tone. 



Dr. Wollaston, however, has discovered that per- 

 sons even whose ear is considered as perfect with re- 

 gard to the generally of sounds, may be completely deaf 

 to sounds at one or the other extremity of the scale of mu- 

 sical notes, the hearing or not hearing of which depends 

 wholly on the pitch or frequency of vibration which 

 constitutes the note, and not upon the loudness or in- 

 tensity of the noise. 



In order to illustrate this affection of the ear, Dr. 

 Wollaston proposes the following experiment, the ef- 

 fect of which he considers as resembling the mechani- 

 -cal separation of larger and smaller bodies by a sieve. 



" If, says he, I strike the table before me with the 

 end of my finger, the whole board sounds with a deep 

 full note. If I strike it with my nail, there is also at 

 the same time a sharp sound produced by quicker vi- 

 brations of parts around the point of contact. When 

 the ear is exhausted, it hears only the latter sound, 

 without perceiving in any degree the deeper note of 

 the whole table. Jn the same manner, in listening to 

 the sound of a carriage, the deeper rumbling noise of 

 the body is no longer heard by an exhausted ear ; 

 but the rattle of a .chain or loose screw remains at least 

 as audible as before exhaustion." 



In order to exhaust the ear, the mouth and nose are 

 shut, and by making a forcible attempt to take breath 

 .by the expansion of the chest, the pressure of the air is 

 -strongly felt on the membrane of the tympanum. This 

 state of exhaustion of the ear may, as Dr. Wollaston re- 

 marks, be preserved for a certain time without the con- 

 tinued effort of inspiration, and without even stopping 

 the breath, since, by sudden cessation of the effort, the 

 internal passage to the ear becomes closed by the flex- 

 ibility of the E ustaehian tube, which acts as a valve, 



and prevents the return of the air into the tympanum. , Sci ? nc . e : 



!/-*> / - " " *tllWM+t* 



It is not easy at first to relax the effort of inspira- 

 tion, with sufficient suddenness to close the Eustachian 

 tube, and thus maintain the exhaustion ; neither is it 

 easy to refrain long together from swallowing the sa- 

 liva, which instantly puts an end to the experiment. 

 Dr. Wollaston has given the following scale of sounds, 

 which are scarcely audible by some ears : 



Cry of the Gryllttt campeslris. 

 Piercing squeak of the bat. 

 Chirping of the house cricket. 



Chirping of the house sparrow, or four octaves above 

 F in the middle of the piano forte. 



Dr. Wollaston is of opinion, that human hearing ex- 

 tends but a few notes above the cry of the Gry/lus cain- 

 peslris. He has met with several persons, who never 

 heard it nor the squeak ofthe bat; with some, who never 

 heard the chirping of the house cricket ; and with one 

 gentleman, who never heard the chirping of the common 

 house sparrow. This he considers as the lowest limit of 

 acute hearing, and the cases in which it exists to be very 

 rare. He regards the note ofthe bat as a full octave higher 

 than this, and he believes that some insects may reach, as 

 far as one octave more ; and the range of human hearing 

 he conceives to be comprised between the lowest notes 

 of the organ, and the highest known cry of insects, in- 

 cluding more than nine octaves, the whole of which 

 are distinctly perceptible by most ears, although the 

 vibrations of a note at the higher extreme are 6'00 

 or 700 fold more frequent than those which constitute 

 the gravest audible sound. 



Dr. Wollaston concludes his very important paper 

 by the following curious conjecture. " Since there is 

 nothing," says he, " in the constitution of the atmo- 

 sphere, to prevent the existence of vibrations incompa- 

 rably more frequent than any of which we are con- 

 scious, we may imagine that animals like the grylli, 

 whose powers appear to commence nearly where ours 

 terminate, may have the faculty of hearing still sharp- 

 er sounds which at present we do not know to exist ; 

 and that there may be other insects hearing nothing in 

 common with us, but endowed with a power of excit- 

 ing, and a sense that perceives vibrations of the same 

 nature indeed as those which constitute our ordinary 

 sounds, but so remote, that the animals who perceive 

 them may be said to possess another sense, agreeing 

 with our own solely in the medium by which it is ex- 

 cited, and possibly wholly unaffected by those slower 

 vibrations of which we are sensible." See Phil. Trans. 

 1820, part ii. p. 306314. 



4-. On the Increase in the Intensity of Sound during the 

 Night. 



Every person must have observed, that sounds, such On the in- 

 as that of falling water, &c. which are faintly or not crease in the 

 at all heard during the day time, are distinctly audible intensit y of> 

 at night, even when the direction and force ofthe wind, ? oun< * dui> 

 and every other general circumstance is the same. This 

 curious fact was remarked even by the ancients. In 

 large cities, or in their neighbourhood, this increase in 

 the distinctness of sound has been ascribed to the ces- 

 sation of the powers of animated beings, such as men, 

 insects, and birds, and also to the cessation of the ac- 

 tion of the wind upon the leaves of the trees. When 

 the celebrated traveller, Baron Humboldt, first heard the 

 noise of the great cataracts ofthe Orinoco, in the plain 

 which surrounds the Mission of the Apures, his atten- 

 tion was particularly called to this curious fact ; and he 

 was of opinion that the noise was three times greater in 

 the night than in the day. The usual explanation of 

 the phenomenon was quite insufficient in the present 



