The Human Voice. Singing Flames.

365

the mechanical means for enabling the organist to sound any single pipe by opening a valve so as to admit the wind to this pipe alone. In the organs in York Minster and in the Birmingham Town-hall there are 4200 and 3000 pipes respectively; and in some of the Continental organs there are as many as 5000 or even 6000 pipes, The pipes of an organ are divided into sets, each set being of one character or quality, and running through the full musical compass of the instrument; and when the organist draws any stop he merely brings the corresponding set of pipes under the control of his keys. In the York Minster organ there are 56 stops, and in the Birmingham organ, 40 stops. The deepest note is given by a 32-foot pipe, which will consequently send forth the sound-pulses 64 feet long, and, therefore, at the rate of 112064, or about 17 per second ; which, as we have seen, is the inferior limit of the musical scale.

550. The sounds of the human voice are the sweetest of all, and are produced on the reed-principle, by the vibrations of two delicate membranes situated at the top of the windpipe, called the glottis, with a slit or opening left between them, for the passage of the air, called rima glottidis, or chink of the glottis. The tones of the voice are grave or acute, according to the varying tension of these membranes, and to the size of the opening through which the air rushes.

Singing Flames.

551. It has been long known that when a jet of hydrogen is burnt with a low flame, and a long tube from one to two inches wide, and open at both ends, is held over the flame, musical sounds are produced. These vary in intensity according to the length and width of the tube and the thickness of the glass or material, and they are modified by raising or depressing the tube over the flame, or by holding above it other tubes of different dimensions. Narrow tubes produce a shrill, and wide tubes a grave, sound. At the time these sounds are produced, it will be observed that the hydrogen flame is lengthened, and burns with a rapidly vibratory movement. The gas, in fact, burns with a succession of small explosions, which succeed each other at regular intervals.*

* On looking at the reflection of the flame in a mirror waved backwards and forwards by the hand we see not the continuous band of fame which is reflected when the tube is quiescent, but a row of separate luminous tongues. By causing a mirror to revolve with great rapidity before a hydrogen flame, Sir C. Wheatstone was able to demonstrate that the circular band of reflected light was unequal in width, while with an ordinary flame it was of equal width throughout.

The column of air in the tube is set in vibration, and sounds are produced, varying with its length and width. This has been called the Chemical harmonicon. Mr. Herschel has shown that these musical sounds are also produced by the combustion of coal gas in a tube above a layer of iron wire gauze, fixed at about one-third of its length. These flames not only produce sound, but are highly sensitive to sounds produced by other causes.

Thus, if a small gas flame be inclosed in a tube so as to be near the position at which it would make the tube sound, and if, with the mouth or a trumpet, the proper note of the tube be sounded, the inclosed air column will sympathise with the pulse and set the gas flame singing. We have thus seen one tube set off another which was brought near to it. These have been called singing flames. They depend on the vibration of columns of air in tubes (see Art. 546). Musical Glasses.

552. Bells or goblets of glass sound still more perfectly than those of metal, and when, by gentle friction on their edges with the wetted finger, their tones are called forth, nothing can exceed them in softness and purity. These may be continued for any length of time, and may be made to swell and diminish like the human voice or the notes of the Æolian harp. A set of glasses, therefore, attuned to each other according to the harmonic scale, become, for certain kinds of music, a very perfect instrument. They form, in fact, an Æolian harp at command. Dr. Franklin, who first constructed a set, simply doubled the long line of glasses upon itself, making two rows, and placed the half-notes on the outside. The writer, during

Fig. 159.

b

some experiments on sound, found a the zig-zag arrangement here re

presented to possess some advantages. The small open circles in fig. 159 represent the mouths of the glasses standing in a box, abc, and the relation of the glasses to the musical notes, as commonly written, is shewn by the five music lines and spaces which here connect them. The learner discovers immediately that one row of the glasses produces the notes written upon the

lines, and the other row the notes written between the lines; and

The Ear and the Sense of Hearing.

367 we can thus mentally command the instrument after a simple inspection. This arrangement also renders the performance easy, for the notes most commonly sounded in succession, are contiguous; and the usual relations of the notes forming a tune, are so obvious to the eye, that much of the theory of musical combination and accompaniment is easily explained. The set of glasses here represented has two octaves, and with the additional flat seventh and fifteenth, seen at a and c, which, when required, may be substituted for the corresponding glasses in the rows, it is capable of playing the greater part of the simple melodies. Other half-notes, if desired, might be placed in outside rows. The player stands at the side of the box between a and b, and has the notes ascending towards the right hand, as in a pianoforte.

The Animal Ear-The Sense of Hearing.

553. The Ear, which is so admirably adapted to perceive the evanescent tremblings of the air, has of course a structure in most exact relation to their nature, as now explained. The parts of the ear, and the progress of sound to the sentient or auditory nerve, may be simply sketched as follows:

d

Fig. 160.

Ist. There is external to the head a wide-mouthed tube or eartrumpet, a (fig. 160), for catching and concentrating the waves of sound. This organ varies greatly in shape in man and animals, but in all cases it is so constructed as to collect and transmit to the auditory passage, h, the aërial undulations which produce sound. Those which reach the concha, i, or hollow part of the ear, just in front of the entrance to the auditory passage, are so collected and reflected as to enter it in larger numbers, and thus intensify the sound. In many animals the external car is movable, so that they can direct it to the place from which the sound comes. This is remarkably seen in the mobility of the ears of the horse, which are tubular ear-trumpets. These can be moved at the will of the animal in various directions, the head remaining fixed. In man the ears are not movable, but by the voluntary movements of the head, either ear can be placed in a position to receive favourably the sound-waves. When these are in the direction of the axis of the auditory passage, they strike the membrane of the tympanum, g (fig. 160), directly, and with the greatest

368

The Structure of the Ear.

intensity. The tubular ear of the horse, which resembles a portable ear-trumpet, enables the animal to receive the vibrations in the most favourable manner for hearing, and he not only hears slight sounds, but can determine the direction in which they come. It is stated that, when horses or mules march in company at night, those in front direct their ears forwards; those in the rear, backwards; and those in the centre, laterally or across;—the whole troop seeming to be actuated by one feeling, which watches over the common safety.

The dog, the cat, the hare, rabbit, and squirrel, as well as many other animals, are provided with movable ears, more or less of a tubular shape, which enable them to hear and trace the direction of the slightest sounds.

2nd. The sound concentrated at the bottom of the ear-tube falls upon a membrane stretched obliquely across the channel, like the parchment of an ordinary drum, over the hollow space called the tympanum or drum of the ear, b, and causes this membrane to vibrate. This membrane is called the membrana tympani or membrane of the drum. Its situation is indicated by a dark line across the passage indicated by the letters, g g, in the figure. It completely closes the drum of the ear and cuts off all direct communication between the external atmosphere and the inner ear. It is placed in a slanting direction, forming an angle of 45°, with the lower surface or floor of the auditory passage, h, which is in consequence longer than the upper part or roof.

In order that this membrane may move freely in receiving the vibrations of air through the auditory passage, h, it is necessary that the air contained within the drum should have a direct communication with the external atmosphere. This is effected by the open passage, ƒ, called the Eustachian tube, leading to the back of the mouth and opening into the throat. This tube serves to keep the air in the cavity of the tympanum of a uniform temperature and pressure, conditions necessary for the proper vibration of the membrane and the perception of sounds by the auditory nerve. The aperture of the tube in the throat is liable to be closed by a collection of the mucous secretion, or by a swelling of the membranes of the throat arising from an attack of cold. The temporary deafness which thence results, arises from the fact that the membrane of the tympanum can no longer freely vibrate. Sometimes the opening of the tube itself is plugged with mucus; and a crack or sudden noise, with an immediate return of the power of hearing, is generally

The Compartments of the Ear.

369

experienced when, in the effort of sneezing, or otherwise, the obstruction is removed. If, when the tube is closed from any cause, the pressure of the external air is increased, the air in the tympanum cannot exert a proper degree of counter-pressure; the membrane will then be forced inwards and deafness will result. In descending in a diving-bell, the membrane of the tympanum is rendered very tense owing to the condensation of the external air, there is tenporary deafness, and voices sound faintly. An undue tension of this membrane, which results from inequalities of pressure, is a frequent cause of deafness.

3rd. The vibrations of the membrane of the drum are conveyed farther inwards, across the cavity of the drum itself, by a chain of four small bones (not here represented on account of their minuteness), reaching from the centre of the membrane to the oval door or window (fenestra ovalis) leading into the labyrinth, e.

4th. The labyrinth, or inner compartment of the ear, over which the nerve of hearing is ramified, is full of an albuminous liquid; and therefore by the law of fluid pressure, when the force of the moving membrane of the drum, acting through the chain of bones, is made to compress this liquid, the pressure is felt instantly over the whole cavity, as in a hydrostatic press. The labyrinth itself is a cavity in the bone, completely shut off from the cavity of the tympanum, b, and it contains within it a membranous layer. Thus, what is called the membranous labyrinth supports all the minute ramifications of the auditory nerve, and being bathed on both sides with the liquid, it receives and transmits to the extremities of these nerves, the most delicate sonorous vibrations. It is this mechanical effect on the nervous fibres that produces, through the connection of the auditory nerve with the brain, the sensation of sound. The labyrinth consists of three distinct parts, the vestibule, e, the three semicircular canals, c, and a winding cavity resembling that of a snail-shell, thence called the cochlea, d.

The three canals and the cochlea also contain a membrane bathed with a fluid similar to that above described. The terminations of the auditory nerve are spread over this membrane, and equally receive and transmit the vibrations produced in the fluid. As the intensity with which sonorous undulations are communicated to a body, is proportionate to the extent of, surface over which they can act upon it, it will be perceived that under this arrangement the auditory nerve is most favourably placed for receiving impressions, since it is spread over a large surface contained within a small space.