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the rapidity of the descent, makes its escape ; for otherwise oscillations might be produced, which, when communicated to the boat, would be dangerous.

In fig. 134 there is a parachute attached to the network of the balloon by means of a cord, which passes round a pulley, and is fixed at the other end to the boat. When the cord is cut, the parachute sinks, at first very rapidly, but more slowly as it becomes distended, as represented in the figure.

BOOK IV.

ACOUSTICS.

CHAPTER I.
PRODUCTION, PROPAGATION, AND REFLECTION OF SOUND.

153. Object of acoustics.—The study of sounds, and that of the · vibrations of elastic bodies, form the object of acoustics.

Music considers sounds with reference to the pleasurable feelings they are calculated to excite. Acoustics is concerned with the questions of the production, transmission, and comparison of sounds To which may be added the physiological question of the perception of sounds.

Sound is a peculiar sensation excited in the organ of hearing by the vibratory motion of bodies, when this motion is transmitted to the ear through an elastic medium.

Take for instance the string of a musical instrument, when it is pulled or sounded by a bow (fig. 136). When this is pulled aside from the position acb, where it is at rest, to the position adb, all the

Fig. 136.

points being more or less out of their position of equilibrium, when the string is left to itself, owing to its elasticity it tends to revert to its original position acb. In virtue, however, of its acquired velocity, it passes beyond it as far as aeb, all the points being then virtually as far out of their position of rest as they were at adb, But as the elasticity still continues to act, not merely does the string revert to its original position, but it again passes beyond it, and so

-154]
Sound Waves.

149 on, the amplitude of its path becoming smaller and smaller, as represented by the dotted lines in the figure, until it ultimately reverts to its original state of equilibrium. Hence each point of the string makes a backward and forward, or vibratory motion, like that of the pendulum. The passage from the position adb to aeb, and back to adb, is called a complete vibration or oscillation; the passage from adb to aeb, or from aeb to adb, is a semi-vibration or semi-oscillation.

Any body which vibrates and yields a sound, is called a sonorous or sounding body. The vibrations of sounding bodies are generally too rapid to be counted or even distinctly seen. Yet they may be rendered evident in a variety of ways. Thus, if a tolerably large bell jar be made to sound by striking it with the finger, and a small ivory ball suspended by a thread be approached to it, the ball will be observed to receive a series of rapid shocks from the sides of the bell, showing that it is in a state of vibration. Or, if a plate of metal be fixed horizontally at one end, and sand be strewed over it, when the plate is made to vibrate by briskly moving a violin bow against the edge, the sand becomes violently agitated, which is obviously due to the vibrations of the plate.

154. Propagation of sound in the air. Sound waves.After having ascertained that when a body emits a sound, its molecules are in a state of vibration, it remains to explain how these are transmitted to the ear to produce the sensation of sound. Sound always requires for its transmission an elastic medium, which at one end is in contact with the sounding body, and at the other with the organ of hearing Air is the ordinary medium through which sound is transmitted. As air is very mobile, compressible, and elastic, its molecules in contact with different points of the sounding body acquire movements similar to those of these points; they go and come with these points, so that each molecule of air in contact with the body is pushed forward by it in the direction of the sound, and returns, having communicated its motion to the next molecule ; this then acts in the same manner on the next molecule, and so on to the molecules in contact with the tympanum or drum. This is the name given to a membrane placed at the end of the auditory canal of the ear, and which receives the vibrations of the air, which it transmits by a series of small bones and liquids to the acoustic nerve, and thence to the brain, which finally perceives the sensation of sound.

At each impulse imparted by a sounding body to the molecules of air in contact with it, these molecules pressing in turn upon the

succeeding ones, a condensed part is produced in the air to a certain distance which is called the condensed wave; then, when the vibra. ting body reverts to its original position, the molecules nearest to it follow it in its motion, so that there is formed in the air a rarefied part which follows the condensed wave, and which is called the rarefied wave. A condensed and a rarefied wave together form a sound wave. A sounding body is a centre from which these waves are emitted all round it in the form of continually increasing spheres, and thus it is that sound is propagated by a body in all directions. Fig. 137 furnishes a rough illustration of this process. If a stone is thrown into still water, there are found round the point where it falls a series of concentric waves, which continually increase, and

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Fig. 137. which give an idea of the propagation of sound waves in the

air.

In the case of very intense sounds, the disturbance communicated to the air in the form of sound waves may be very considerable. Thus the waves produced by thunder, by the report of cannon, and by gunpowder explosion, are frequently powerful enough to break whole panes of glass.

155. Coexistence of sound waves.It is to be observed that several sounds may be propagated in air without destroying each other. Thus in the most complicated orchestral music, a person with a practised ear can readily follow the sound of each instrument. Yet a loud sound interferes with a weak one; thus the sound of a drum overpowers the human voice. Sounds also which are too weak to be distinctly heard accumulate upon each other, and produce a confused sound, which becomes perceptible to the ear.

-158] Propagation of Sound in Liquids and Solids. 151

Such is the cause of the murmuring of water, the rustling of leaves in woods, and the dashing of waves against the shores.

156. Sound is not propagated in vacuo,—The vibrations of elastic bodies can only produce the sensation of sound in us, by the intervention of a medium interposed between the ear and the sonorous body, and vibrating with it. This medium is usually the air, but all gases, vapours, liquids, and solids also transmit sounds.

The following experiment shows that the presence of a ponderable medium is necessary for the propagation of sound. A tolerably large glass globe (fig. 138), provided with a stopcock, has a small bell suspended in the interior by a thread. A vacuum having been created in the globe by means of the air-pump, no sound is emitted when the globe is shaken, though the clapper may be seen to strike against the bell ; but if air, or any other gas or vapour, be admitted, sound is distinctly heard each time the globe is agitated.

The experiment may also be made by placing a small metallic bell, which is continually struck

Fig. 138. by a small hammer by means of clockwork, or an ordinary musical box, under the receiver of the air-pump. As long as the receiver is full of air at the ordinary pressure, the sound is transmitted ; but, in proportion as the air is exhausted, the sound becomes feebler, and is imperceptible in a vacuum.

To ensure the success of the experiment, the bellwork or musical box must be placed on wadding; for otherwise the vibrations would be transmitted to the air through the plate of the machine.

157. Propagation of sound in liquids and solids.--Sound is also propagated in liquids. When two bodies strike against each other under water, the shock is distinctly heard ; and a diver at the bottom of the water can hear the sound of voices on the bank.

The conductibility of solids is such, that the scratching of a pen at the end of a long wooden rod is heard at the other end. In like manner if a person speaks with a low voice at the end of a pine rod, 25 to 30 yards long, he is heard by a person whose ear is applied against the other end, while a person who is near, hears nothing. The earth conducts sound so well, that at night, when the ear is applied to the ground, the steps of horses or any other noise at great distances is heard.

158. Velocity of sound in air,--Numerous phenomena show

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