Bodies which are neither rigid in themselves, and which are Dot rendered rigid by tension, such as the skin, a silken fabric, paper, &c. are, even when they are not stretched, susceptible of being thrown into vi brations by the influence of a body vibrating at a dis tance; and it appears that, under some circumstances, they are even more susceptible of this kind of action than most elastic membranes. This may be proved by covering a horizontal portion of any or these substances with sand, and sounding the pipe of an organ at the distance of a foot or so. The sand will be violenly agitated, and will form figures composed of numerous curved and bending lines interlaced with one another.
In the second part of his able memoir, M. Savart applies these experiments to the illustration of the uses of the membrane of the tympanum, and of those of the external ear, both of which, as he shows by direct ex periments on the ears of animals, are susceptible of be ing thrown into a state of vibration, by bodies vibrat ing at a distance. As our limits will not permit us to follow our author through his numerous and inter esting details, we shall conclude this abstract with an enumeration of the leading results which he has ob tained.
1. That it is not necessary to suppose, as has hith erto been done, the existence of a particular mechan ism, for 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 excur sions, as Bichat had imagined. Ile supposed, how ever, contrary 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 hones, 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 (Cctisse du Tambour) serves probably to keep up near the aper tures of the labyrinth, :aid the internal face of the membrane of the tympanum, an aerial medium, whose physical properties are constant.
3. On Sounds Inaudible to Certain Ears.
In persons labouring under ordinary deafness, it seems to have been frequently noticed that they hear sharp 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 car is considered as perfect with re gard to the generality of sounds, may be completely (leaf to sounds at one or the other extremity of the scale of musical notes, the hearing or not hearing of which de pends wholly on the pitch or frequency of vibration which constitutes the note, and not upon the loudness or intensity 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.
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. In 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 car; 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 or the ear may, as Dr. Wol laston remarks, be preserved for a certain time with out the continued 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 flexibility of the Eustachian tube, which acts as a valve, and prevents the return of the air into the tympanum. It is not easy at first to relax the ef fort of inspiration, with sufficient suddenness to close the Eustachian tube, and thus maintain the exhaustion; neither is it easy to refrain long together from swal lowing the saliva, which instantly puts an end to the experiment.