Изображения страниц
PDF
EPUB

single nerve-the chorda tympani ; we cannot doubt that they are capable of being moved in a variety of directions ; that their motions are regular, and that these are productive of effects essential to the development of many phenomena appertaining to hearing. When, therefore, an action is produced in the membrana tympani, a certain effect must follow; the whole of the muscular structures of the tympanum must be immediately called into action, and the ossicula auditus drawn into certain positions, each of which must produce a particular effect on the membrane of the fenestra ovalis. Every one of these actions and effects must harmonize with each other, and of course partake of the nature of that action which is produced in the membrana tympani. It is not by the action of any single part or texture of the apparatus tympani, that the effect required by nature for particular ends can be produced, but by the uniform co-operation of all the textures which enter into the composition of this apparatus. Every part must, whilst it possesses its own percipient principle, perform in its turn, its own particular functions, before the required effect can be produced in the membrane of the fenestra ovalis. That these conclusions are just, is obvious, from the situation of the apparatus of the tympanum ; from the absence of this apparatus being invariably attended with congenital deafness, and from the absence of the power of hearing during sleep*."

The magnitude and figure of the cavity behind the tympanum are so various, that no one can doubt that they have relation to the acuteness of the organ of hearing. In the

Tod's Organ of Hearing, p. 47.

[blocks in formation]

dog, cat, hare, and other animals, the cavity is large; but in what manner the increase of size produces an increased power of hearing, physiologists are unable to determine.

We must now close our account of what has been advanced by others concerning the organ of hearing, by one or two remarks upon the physiology of the labyrinth. The special object of the various parts of the internal ear is but little understood, and particularly of that part to which we now refer. Anatomists do not pretend to assign any particular use to the several parts; and speaking of the labyrinth as a whole, they can only say that it is intended to assist in some way the sense of hearing. Nature is economical in all her arrangements; and it is as impossible that there should be any thing superfluous in her works, as that there should be any thing inappropriate. We are ill acquainted with the offices of the several parts of the human ear, but that they are all necessary for the formation of a perfect organ is proved by experiment, and deduced from our knowledge of the universal operations of the God of nature.

36

CHAPTER II.

VELOCITY OF SOUND.

In the last chapter we attempted to explain the manner in which sound is conducted by fluid media, and the nature of those substances said to be sonorous. Our next object is to determine the velocity of sound.

The velocity with which sounds are propagated must depend upon the nature of the substances conveying them. It may be readily supposed, that the undulations are more rapid in some media than in others. A free and almost instantaneous transit may be given by one substance, and by another the passage may be slow; opposed by its physical constitution at every step. The difficulty of assuming the undulatory form, the extent of surface influenced, and the freedom of motion between the ultimate particles, must have an influence upon the conducting power. The deadening influence of media of unequal densities has been already alluded to. When the vibrations are made to traverse effervescing liquids, the sounds lose all their clearness of tone, and become heavy noises. So when the vibration enters one medium after another, traversing water for instance after its passage through air, the intensity of sound is lost. In speaking, therefore, of the velocity of sound through the media we may select as examples, it must be remembered that they are supposed to be homogeneous.

SOUND PROGRESSIVE.

37

It is scarcely necessary to remark, that sound requires time for its transmission from one place to another. Some ancient philosophers illustrated the motion of light, by comparing it to the progress of a small stick pushed at one end, and moving at the same moment through its whole length. If sound had a direct motion in right lines, this would not be an inappropriate comparison, for the motion throughout is not instantaneous; although the lengths which we are accustomed to experiment on are so small, that the interval between the blow and the motion of the extreme point is insensible. But if the sun and the earth were the extremes of a bar, a motion communicated at one end, would not, it is said, be felt at the other in less than 1074 days. !

Every one knows that the lightning is seen before the thunder is heard; and the report of a gun discharged at a distance, strikes the ear after the flash has ceased to affect the eye. These, and many other instances of the same kind which will be immediately suggested to the mind of the reader, prove that sound has a progressive motion. But it is not so easy to determine its velocity. The experiment is one requiring in all cases extreme accuracy, and the omission of one element of disturbance will affect with essential errors the results that are obtained.

Air being the medium by which sound is commonly conducted to the ear, our first experiments would naturally be directed to ascertain the velocity of transmission by the atmosphere. To solve this problem, we must enter upon a course of experiments, and the manner in which they are performed is of the highest importance. They must evidently be founded on the fact, that the progress of light is so rapid

that at short distances it may be said to be instantaneous. Now if we can accurately determine the instant at which we see the flash of a gun fired by a person standing at a known distance from us, and the instant when we hear the report, the velocity of sound, supposing that of light to be unit, will be at once determined. This plan has been pursued by all who have endeavoured to measure the velocity of sound; but for want of sufficient care, and accurate instruments, the results obtained by the early experimenters were excessively erro

neous.

The first thing will of course be to select some level plain, and, fixing upon two stations, to measure the distance between them with great accuracy. At one station the sound is to be produced, and at the other the observations are to be made. The discharge of some fire-arm is in all probability the best means of producing the sound; for being attended with a flash, the exact instant of explosion can be more accurately ascertained than by any other means.

The difficulty most felt in performing experiments on the velocity of sound, was, in the measurement of that interval which elapsed between seeing the flash and hearing the sound. An error of a small fraction of a second would be sufficient to derange the result considerably. The most accurate experiments ever made were conducted at almost the same time in the years 1822 and 1823, by Moll and Vanbeck in Holland, and Arrago, Matthieu, and others, in France. The Dutch philosophers used a clock most accurately constructed with an index hand, so formed that it could be stopped at any moment without stopping the clock, and registering to the one hundredth part of a second. The French academicians

« ПредыдущаяПродолжить »