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62

VELOCITY OF SOUND,

exhausting the receiver, you cut off the communication between the air and bell; and the latter, therefore, cannot impart its motion to the air. It has been ascertained that liquids as well as air are capable of conveying the vibratory motion of a sonorous body to the organ of hearing; for sound can be heard under water, Dr. Franklin imagined, that with his ear under water, he heard the collision of stones in that medium, at the distance of a mile.

The vibration of a sonorous body gives a tremulous motion to the air around it, very similar to the motion communicated to smooth water when a stone is thrown into it. This first produces a small circular wave around the spot in which the stone falls; the wave spreads, and gradually communicates its motion to the adjacent waters, producing similar waves to a considerable extent. The same kind of waves are produced in the air by the motion of a sonorous body, but with this difference, that as air is an elastic fluid, the motion does not consist of regularly extending waves, but of vibrations, and are composed of a motion forwards and backwards, similar to those of a sonorous body. They differ also in the one taking place in a plane, the other in all directions: the aerial undulations being spherical, The first sphere of undulations which are produced immediately round the sonorous body, by pressing against the contiguous air, condenses it. The condensed air, though impelled forward by the pressure, reacts on the first set of undulations, driving them back again. The second set of undulations which have been put in motion, in their turn communicate their motion, and are themselves driven back by reaction. Thus there is a succession of waves in the air, corresponding with the succession of waves in the water.

The air is a fluid so much less dense than water, that motion is more easily communicated to it. The firing of a cannon produces vibrations of the air which extend to several miles around, Distant sound, however, takes some time to reach us, and we see the light of the flash long before we hear the report. The velocity of sound is commonly computed at the rate of eleven hundred and forty-two feet in a second. Its velocity varies according to the temperature, density, and humidity of the atmosphere. It is influenced also by the force and direction of the wind. The velocity of sound has been applied to the measurement of

distances.

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If a ship at sea in distress fires a gun, the light of which is seen on shore twenty seconds before the report is heard, it is therefore known to be at the distance of twenty times eleven hundred and forty-two feet, or a little more than four miles and one third. By counting the number of seconds elapsed between the flash of lightning and the clap of thunder, you may ascertain how far distant you are from the cloud.

When the aerial vibrations meet with an obstacle, having a hard and regular surface, such as a wall or rock, they are reflected back to the ear, and produce the same sound a second time; but the sound will then appear to proceed from the object by which it is reflected. If the vibrations fall perpendicularly on the obstacle, they are reflected back in the same line; if obliquely, the sound returns obliquely in the same direction. This reflected sound is called an echo. At Rosneath, near Glasgow, there is an echo that repeats a tune, played with a trumpet, three times, completely and distinctly. At Brussels there is an echo.that answers fifteen times; and in Italy, near Milan, the sound of a pistol is returned fifty-six times. Speaking trumpets, and those made to assist the hearing of deaf persons, depend on the reflection of sound from the sides of the trumpet, and also by its being confined and prevented from spreading in every di

rection.

QUESTIONS.-1. From what does sound arise? 2. Upon what three circumstances does the production of sound depend? 3. What is sound, strictly speaking? 4. How can it be shown that air is necessary to the production of sound? 5. Why cannot a bell be heard in an exhausted receiver? 6. What are conductors of sounds besides the atmosphere? (Ans. water, wood, flannel.) Tie a piece of iron or any metal to the middle of a strip of flannel, 2 or 3 ft. long. Press the ends of the flannel in your ears, and if the metal be struck against iron, you will hear a sound like that of a heavy church bell. 7. How is the tremulous motion of the air as produced by a sonorous body illustrated? 8. What is said of the velocity of sound? 9. Ship at sea? 10. Distance of lightning? 11. How is the sound of an echo produced? 12. Describe the speaking trumpet, fig. 20.

NOTE. The science which treats of sound in general is called acoustics.

64

MUSICAL SOUNDS

LESSON 30.

Nature of Musical Sounds.

Ten'sion, act of stretching, state of being stretched.
Grav'ity, in music, the modification of any sound, by which it be
comes deep or low in respect of some other sound.
Con'cert, many performers playing the same tune.

Line, a small French measure, containing the 12th part of an inch:
geometricians conceive the line subdivided into six points.

Ir a sonorous body be struck in such a manner, that its vibrations are all performed in regular times, the vibrations of the air will correspond with them; and striking in the same regular manner on the drum of the ear, will produce the same uniform sensation on the auditory nerve and excite the same uniform idea in the mind; or, in other words, we shall hear one musical tone. But if the vibrations of the sonorous body are irregular, there will necessarily follow a confusion of aerial vibrations; for a second vibration may commence before the first is finished, meet it half way on its return, intercept it in its course, and produce harsh jarring sounds which are called discords. But each set of these irregular vibrations, if repeated at equal intervals, would produce a musical tone. It is only their irregular succession which makes them interfere, and occasions discord.

The quicker a sonorous body vibrates, the more acute, or sharp is the sound produced; and the vibrations of the same string, at the same degree of tension, are always of a similar duration. Striking the note in quick succession, produces a more frequent repetition of the tone, but does not increase the velocity of the vibrations of the string. The duration of the vibrations of the strings or chords depends upon their length, their thickness, or weight, and their degree of tension. The different length and size of the strings of musical instruments, therefore, serve to vary the duration of the vibrations, and consequently the acuteness or gravity of the

notes.

Among the variety of tones, there are some which, sounded together, please the ear, producing what we call harmony or concord. This arises from the agreement of the vibrations of the two sonorous bodies; so that some of the vibrations of each strike upon the ear at the same time. If the vibra

MUSICAL BAROMETER

65

tions of two strings, for instance, are performed in equal times, the same tone is produced by both, and they are said to be in unison. But concord is not confined to unison; for two different tones harmonize in a variety of cases. If one string or sonorous body vibrates in double the time of another, the second vibration of the latter will strike upon the ear at the same instant as the first vibration of the former; and this is the concord of an eighth or octave. If the vibrations of two strings are as two to three, the second vibration of the first corresponds with the third vibration of the latter, producing the harmony called a fifth. There are other tones which, though they cannot be struck together without producing discord, yet if struck successively, give us the pleasure which is called melody.

A sort of musical barometer has been invented in Switzerland, called the weather harp, which possesses the singuiar property of indicating changes of the weather by musical tones. In the year 1787, one was constructed in the following manner. Thirteen pieces of iron wire, each three hundred and twenty feet long, were extended across a garden, in a direction parallel to the meridian. They were placed about two inches apart; the largest were two lines in diameter, the smallest only one, and the others one and a half; they were on the side of the house, and made an angle of twenty or thirty degrees with the horizon; they were stretched and kept tight by wheels made for that purpose. Every time the weather changes, these wires make so much noise that it is impossible to continue concerts in the parlour, and the sound resembles that of a tea-urn when boiling, and sometimes that of a distant bell, or an organ.

QUESTIONS.-1. When do the vibrations of a sonorous body produce the same musical tone? 2. How are discords produced? 3. On what does the sharpness or acuteness of a musical sound depend? 4. On what does the duration of the vibrations of strings or chords depend? 5. How is harmony or concord produced? 6. How is an octave concord produced? 7. The harmony called a fifth? 8. Describe the musical barometer or weather harp. [NOTE. In the opinion of a celebrated chemist, this is an electro-magnetical phenomenon.] 9. Illustrate the vibrations of a musical string by figures 17, 18, and ‍19.

6*

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Lu'minous, shining by its own light.

Transpa'rent, admitting rays of light to pass through.
Opaque', stopping the rays of light.

Zenith, a point in the heavens directly over our heads, the pole of
the horizon. Na'dir is a point diametrically opposite to the
zenith, constituting the other pole of the horizon.

OPTICS is the science which treats of light, and of the instruments by which it is applied to useful purposes. It is one of the most interesting branches of natural philosophy, but not one of the easiest to understand; it will be necessary, therefore, that you give to it the whole of your attention.

Light, when emanated from the sun, or any other luminous body, is projected forwards in straight lines in every possible direction; so that the luminous body is not only the centre from whence all the rays proceed, but every point of it may be considered as a centre which radiates light in every direction. The particles of light are so extremely minute, that although they are projected in different directions, and cross each other, yet they are never known to interfere, and impede each other's course. It is still a disputed point, however, whether light be a substance composed of particles like other bodies. In some respects it is obedient to the laws which govern bodies; in others, it appears to be independent of them: thus, though its course is guided by the laws of motion, it does not seem to be influenced by the laws of gravity. It has never been discovered to have weight, though a variety of interesting experiments have been made with a view of ascertaining that point. Some suppose that the rays of light, instead of being particles, consist of the undulations of an elastic medium, which fills all space, and which produces the sensation of light to the eye, just as the vibrations of the air produce the sensation of sound to the ear. Most of the phenomena may be accounted for by either hypothesis, but that of their being particles applies more happily to some of the facts respecting the modifications of light by refraction and reflection.

When rays of light encounter an opaque body, part of them are absorbed, and part are reflected, and rebound just

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