whether strong or feeble, grave or acute, are transmitted with the same velocity. If we listen to a band at a distance, the notes reach us exactly in the same order that they are played, and the sounds of all the instruments are heard together.

Velocity of Sound in Liquids and Solids.

152. Liquids and solids transmit sound more rapidly than air. Experiments made by transmitting sound across the Lake of Geneva, in Switzerland, show that the velocity of sound in water is about 4700 feet per second, which is more than four times its velocity in air.

That sound travels faster in iron than in air, may be shown by placing the ear at one extremity of a long iron bar or tube, whilst it is struck on the other end with a hammer. Two sounds will be heard, the first transmitted through the iron, and the second through the air. It has been shown that sound is transmitted seventeen times faster in iron than in air. The velocity of sound is not so great in the other metals.

Reflection of Sound.

153. Sound is propagated through the air in spherical waves. (Fig. 108.) When these waves meet with an obstacle they are driven back, as an elastic ball is when thrown against a hard wall. The waves driven back take a new direction, or are reflected. The laws of reflection are the same as those which govern the reflection of heat and light, which will be explained hereafter.

Echoes.

154. An ECHO is a repetition of a sound, caused by a reflection of the sound-waves from an obstacle more or less

Do all sounds travel with equal velocity? How shown? (152.) How was it shown that sound travels faster in water than in air? In iron than in air? (153.) Explain the reflection of sound. (154) What is an Echo?

remote. Thus, if we pronounce in a loud voice a sentence at a certain distance from a rock or a building, we often hear, after an instant, the same sentence as if repeated at a distance by another person.

This is due to reflection of the sound-waves from the rock or building. In order that any echo may be clearly distinguished, the reflection must take place from an obstacle which is at least 109 feet distant.

It is not possible to pronounce or to hear distinctly more than five syllables in a second. The velocity of sound being 1090 feet per second, it follows that sound travels 218 feet in one fifth of a second. If, then, an obstacle be placed at the distance of 109 feet, sound will go to it and return in one fifth of a second. At the distance of 109 feet, the last syllable only of the echo will reach the ear after the sentence is pronounced. Such an echo is called monosyllabic. If the echo takes place from an obstacle at a distance of 218 feet, we hear two syllables; that is, the echo is dissyllabic. At distances of 327 feet, the echo is trisyllabic, and so on.

Sound may be reflected from several objects situated in different directions and at different distances. Such echoes are called multiple echoes. It is said that at a place three leagues from Verdun, a multiple echo formed by parallel walls fifty or sixty yards apart, repeats a sound twelve times. At the chateau of Simonnetta, in Italy, there is an echo which repeats the report of a pistol from forty to fifty times.

Echoes modify the tones of sound. Some repeat sounds with a roughened, others with a softened tone; some with a sneering, others with a plaintive accent.

Resonance.

155. When sounds are reflected from obstacles at a less distance than 109 feet, the reflected sound is superposed

Illustrate. What causes the echo? Explain the monosyllabic, dissyllabic, and trisyllabic echoes. What are multiple echoes? Examples. What effect have echoes on the tone of a sound? (155.) What is a Resonance?

upon the direct one, giving rise to a strengthened sound, which is called a Resonance.

In rooms

It is the resonance from the walls of a room that makes it easier to speak in a closed apartment than in the open air. The resonance is more clearly perceived when the walls are elastic. where there are carpets, curtains, stuffed furniture, and the like, the sound-waves are broken up, and the resonance is diminished; but in houses where there is no furniture, the resonance is strengthened. Hence it is, that the sound of voices, footsteps, and the like, is so strongly marked in deserted and unfurnished buildings.

Intensity of Sound.

156. The INTENSITY of sound is its loudness. The intensity of sound depends upon the force with which the vibrating particles of air strike upon the drum of the ear. The original intensity depends upon the power of the exciting cause.

Causes that modify the Intensity of Sound.

157. The following are some of the causes that modify the intensity and rate of propagation of sound:

1. It is shown by theory and confirmed by experiment, that the intensity of sound diminishes as the square of the distance from the sonorous body increases.

This is expressed by saying that, the intensity of sound varies inversely as the square of the distance from the sonorous body.

2. The intensity of sound diminishes with the amplitude of the vibration of the aerial particles.

When a cord vibrates, the sound is observed to diminish as the vibrations become smaller, and when the vibrations cease, the sound

Illustrate by examples. (156.) What is Intensity? On what depend? (157.) What are the laws of intensity? 1. Effect of distance? 2. Amplitude of vibration?

is no longer heard. In this case the length of vibration of the cord determines the length, or amplitude of the vibrations of the aerial particles.

3. The density of the air modifies sound. When the air is rarefied, the intensity is diminished. This fact has been shown by the experiment of a bell in an exhausted receiver.

The presence of watery vapor in the air also modifies sound, that substance being a good conductor of sound. When the air is cooled, it becomes more dense, hence, sounds are louder in cold than in warm weather.

4. The wind modifies sound. The velocity of sound is increased or diminished by the velocity of the wind, according as the direction of the wind conspires with or opposes the propagation.

The effect of the wind is to move the whole mass of air, carrying along the sound-waves unaltered.

5. Sound is increased in intensity when the sonorous body is in contact with, or even in the neighborhood of another body capable of vibrating in unison with it.

Hence, the sound of a vibrating cord is reinforced or strengthened by stretching it over a thin box filled with air, as in the violin. In this case the air in the body of the violin vibrates in unison with the cord. The ancients placed in their theatres vessels of brass, to reinforce and strengthen the voices of the actors.

Intensity of Sounds in Tubes.

158. When a sound is transmitted through a tube, the sound-waves can not diverge laterally, and consequently the

Illustrate. 8. Density of the air? Illustrate. 4. How does wind modify sound? 5. Effect of a neighboring sonorous body? Illustrate. (158.) What effect has a tube on sound?

sound is transmitted to a great distance without much loss of intensity.

M. BIOT was able to carry on a conversation in a low tone through a tube a thousand feet in length. He says that the sound was transmitted so well, that there was but one way to avoid being heard, and that was not to speak at all.

This property of tubes is utilized in hotels and dwelling-houses, for transmitting messages from one story to another. The tubes employed for this purpose are called speaking tubes. The method of employing the speaking tube, is illustrated in Fig. 111.

The Speaking Trumpet.

159. The SPEAKING TRUMPET, as its name implies, is a conical tube employed to transmit the voice to a great