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 depends on the force with which it strikes the ear. It varies very nearly as the square of the amplitude of vibration of the ærial particles. Some of the causes that modify the intensity of sound are noticed in the following article.

157.

Causes that modify the Character of Sound.

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. The amplitude of vibration of the sonorous body determines the length, or amplitude of the vibrations of the ærial 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. 3. 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

BIOT's experiment. Practical applications. (159.) What is a Speaking Trumpet?

distance. It is used by firemen and by mariners, as shown in Fig. 112.

By means of the speaking trumpet, the voice of the captain can be heard above the noise of the winds and waves in a tempest. According to Father KIRCHER, ALEXANDER THE GREAT employed a speaking trumpet in commanding his armies.

The effect of the speaking trumpet has been explained by successive reflections of sound-waves from the schorous material of which the instrument is composed, by virtue of which the voice is transmitted only in the direction of the tube.

But the fact is, that sound is transmitted in all directions, which would indicate that its effect should be attributed to a reinforcement of the voice by the vibration of the column of air contained in the trumpet, according to the principle that sound is reinforced by an auxiliary vibrating body.

The Ear Trumpet.

It is simply the speaking trumpet reversed. It serves to collect and concentrate the sound-waves, which are thus enabled to produce a more powerful impression on the drum of the ear. The shape of the ear in man and in animals is such as to perform the function of the trumpet.

Difference between a Musical Sound and a Noise.

161. A MUSICAL SOUND results from a succession of vibrations of equal duration. Such vibrations are called isochronal.

(160.) What is an Ear Trumpet? How does it differ from the speaking trumpet? What is its use? (161.) What is a Musical Sound?