370

The Perception of Sounds.

The vestibule is adapted to receive sounds conveyed through the external ear; but sonorous vibrations are also transmitted by the bones of the head, and these are better received by the cochlea than by the vestibule, owing to its peculiar form and its direct connection with the solid part of the bone.*

With respect to the semicircular canals, nothing certain is known of their functions. As they are at right angles to each other and, like the cochlea, so situated as to receive sounds directly through the bones of the head, it was supposed by the late Sir C. Wheatstone that they might enable a person to judge of the direction of sounds, as the fluid contained within them would be set in vibration with different degrees of intensity, according to the plane in which the sound was transmitted. Our perception of direction, however, is probably more connected with vibrations through the external ear than with those received through the bones of the head.

Among the common causes of deafness may be mentioned a thickening of the fluid of the labyrinth or of the membrane contained in it, or an obliteration of the canals. In most cases of congenital deafness these canals are found defective. This at any rate shows their importance to the function of hearing.

Perception of Sounds.-It has been stated that the human ear cannot perceive sounds when the number of vibrations is less than 32 or more than 18,000 in a second. Savart placed the limit for the highest sounds at 48,000 half vibrations, or 24,000 impulses in a second, and in reference to low or grave sounds, the ear could perceive those which were produced by 16 half vibrations or 8 impulses in a second. Other authorities have placed the extreme range of hearing of the highest sounds at 73,000 half vibrations the same period of time.

In this estimate an impulse is considered to be equivalent to a complete vibration.†

*The power of the bones of the head to conduct sound may be fully appreciated by placing a musical-box, while playing, on the top of the head, and closing the auditory passages.

+ Physicists do not agree in their conclusions on the range of hearing. 'The differences are partly due, as Professor Tyndall has pointed out, to the different meaning attached to the term "vibration." English and German authorities imply by this a complete vibration, i.e., a motion to and fro. The French, on the other hand, limit this term to a semi- or half vibration, i.e., a motion to or fro. On this subject the same authority states that if the vibrations are less than 16 in a second we are conscious only of the

The Duration and Direction of Sounds.

371

The limits of hearing vary in different persons, and these are probably dependent on the state of the auditory nerve and the size of the membrane of the tympanum. Animals, in which this membrane is large, can hear much graver sounds than man. According to Dr. Wollaston the ordinary range of human hearing is comprised between the lowest notes of the organ and the highest known cry of insects. He found, however, that even in a healthy state of the car, one person could not hear the chirping of a cricket, while another could not hear the chirping of a house-sparrow, and he met with several instances in which persons were unable to hear the piercing squeak of a bat.

Duration of Sounds.-As with vision, so with hearing, the sensation of sound lasts longer than the exciting cause of it. Savart found in his experiments on toothed wheels, that the removal of one tooth did not produce any interruption of the sound. A long-continued noise may be perceived for a short time after the cause of its production has ceased. Subjective sensations of sound, ¿.e., impressions without any external cause for their production, generally indicate disease of the brain. These are equally heard with the auditory passages closed or open. Aural illusions are one of the most common features of incipient insanity.

Direction of Sounds.-The power of judging of the direction from which a sound issues, is not strictly connected with the sense of hearing. It is a mental operation based on experience. In listening, we acquire the habit of turning the head, and with it the ear, until it reaches a point at which the vibrations enter the passage by its axis, and the intensity and distinctness of the sound then produced, lead us to judge of the direction. The ticking of a watch held in a right line with the axis of the auditory passage may be heard at a distance of two feet, but when moved away a short distance on either side, and brought nearer to the head, the ticking will be no longer heard. A person in a thicket listening to the song of various birds, although they may be concealed from his eye by the luxuriance of foliage, can still judge correctly by the ear in

separate shocks, and if they exceed 38,000 in a second the consciousness of sound ceases altogether. The range of the best ear covers II octaves, but the auditory range is sometimes limited to 6 or 7 octaves. The sounds available in music are produced by vibrations comprised between the limits of 40 and 4000 in a second. They embrace seven octaves.-'Synopsis of Lectures.'

372

Distance of Sounds-Ventriloquism.

what tree every little songster is concealed. One ear may receive the vibrations of a sounding body more strongly than the other. This would be in some degree a guide for the direction of the sound, but when the vibrations fall equally on both ears, as when the sound is equidistant in front or behind us, there is nothing by which the direction can be accurately determined.

The intensity of sound is to the ear in some degree a measure of distance. On a windy night the sound of a distant bell may be brought so quickly, that it has not yet had time to spread and be weakened; and a person is often roused from a reverie by its unusual loudness and apparent nearness. When a stormy wind blows directly upon a coast, and the swollen waves roll furiously upon the sandy beach or among the rocks, the countryman living many miles inland hears the uproar, almost as if the ocean had burst its barriers, and were pouring in upon the land. The scenecontrivers at our theatres heighten the illusion of an approaching procession by letting the accompanying music be first heard from a closed chamber or with very feeble tones, and afterwards with gradually increasing loudness. To the imagination, already excited by the suitable drama, the advancing host is thus most vividly portrayed; and when at last, with thunders of drums and trumpets from the front of the stage, the crowd also appears, the desired effect is complete.

In ventriloquism we have a remarkable illustration of the ease with which the ear is deceived not only in the direction, but in the distance of sound. A man by great skill may so imitate sounds as to make it appear that they issue from a box or a closet, or from behind a door, at different distances in an apartment. The sound, of course, is always seated in the ear, but it is inferred that it proceeds from a distinct body set in vibration.

PART IV.

GENERAL REMARKS ON HEAT, LIGHT, ELECTRICITY, AND MAGNETISM. THE MATERIAL AND DYNAMIC THEORIES.

554. The four subjects which will now require special notice, HEAT, Light, ELECTRICITY, and MAGNETISM, were formerly classed under the head of Imponderable Substances. This name was assigned to them because there was nothing to show that they had weight or even a material existence. Recent researches have led to the hypothesis that these physical agents are but variouslymasked forms of vibrations or undulations of an infinitely elastic ether, which pervades all space and penetrates even into the intimate molecular structure of all substances, solid, liquid, or gaseous. Undulatory or vortical movements of the particles of matter, varying in form and velocity, are transmitted to the ether, and through this to other particles, so that the atoms of matter and the ethereal medium are successively the recipients and the sources of motion. On this theory one definite kind of ethereal motion constitutes radiant HEAT; another, more rapid in its progress, LIGHT; and a third, differing from the preceding in form and character, produces the effects of ELECTRICITY and MAGNETISM. All physical phenomena produced by these agents are thus referred to one single mechanical cause, viz., the transference of motion. This exposition of what has been called the Dynamic theory, is adopted by the most recent writers on physics.

555. Philosophers now incline to the opinion that there is at least one subtle fluid or medium occupying the wide space of the universe, and tending to equable diffusion, which fluid pervades denser substances somewhat as water pervades a sponge or loose sand, and that it has peculiar relations to each chemical element. They believe farther that physical phenomena, exhibiting sometimes the highest beauty, sometimes awful intensity and power, are

374

Preliminary Remarks.

dependent more or less on the motions of such fluid or fluids, some what as the sensation of sound in all its varieties, is produced in the delicate structure of the car by modifications of motion in the air. Many philosophers until lately held the causes both of light and of heat to be material particles projected through space, somewhat as sand might be scattered by an explosion, such peculiar particles being present only when the effects were perceived; but now they hold the phenomena to be connected with vibratory motions in an elastic medium such as that above described.

We here refer to these hypotheses, not with the view of entering upon a detailed examination of their respective merits, or of asserting that either of them furnishes a complete explanation of all the facts, but merely to make the reader aware of the direction which inquirers' minds have taken in pursuing the investigation. The ascertained facts and laws of change important to be known to the general student, can be described and studied independently of such hypotheses.

556. The successive steps by which men have approached their present knowledge of the nature of heat have been nearly as follows:

1st. Of old it was thought that sound was a subtle something which shot and spread around from sounding bodies and entered the ears of persons within a certain distance, producing the sensation called sound. It was in the course of time observed that bodies whilst sounding were generally in a state of visible tremor or vibration, like the lip of a bell or the string of a musical instrument; but until the time of Galileo it was not known that the air surrounding all things on earth is a material elastic fluid, having weight, inertia, and bulk like other kinds of ponderable matter, being capable, therefore, of receiving and conveying to a distance the tremors or minute vibrations of a sounding body, nearly as the surface of a pond into which a pebble is dropped, exhibits a succession of circular waves spreading from the spot where the stone falls to considerable distances around. The air might thus act as by a gentle touch on the delicate structure of the internal ear. A proof that such a supposition was well founded was afforded after the invention of the air-pump by experiments which have been elsewhere described (Art. 472). Other facts were soon observed, all proving the same truth-that the sound was conveyed by undulation of the air. Thus if two musical strings similarly stretched were placed not far from each other, the sounding vibration produced by the movement of one of them, was quickly