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height of a thousand yards above the surface of the earth. He descended in safety.

Remarkable Balloon Ascensions.

144. The first ascension was made in October, 1783, by DE ROZIER. His balloon was filled with heated air, and was confined by a rope, so that he only rose to a height of about a hundred feet. In the following year DE ROZIER and D'ARLANDES ascended in a fire balloon from the Bois de Boulogne, and after a voyage of twenty-five minutes they descended on the other side of Paris. In a subsequent ascent DE ROZIER lost his life in consequence of his balloon taking fire. In 1785, BLANCHARD and JEFFRIES crossed the English Channel from Dover to Calais. During the voyage they had to throw overboard all of their ballast, then their instruments, and finally their clothing, to lighten the balloon. In 1804, GAY LUSSAC ascended to the height of 23,000 feet above the level of the sea. At this height the barometric column fell to 12.6 inches, and the thermometer, which at the surface of the earth was 31°, fell to 94° below 0.

At such heights, substances which absorb moisture, like paper and parchment, become dry and crisp as if hated in an oven, respiration becomes difficult, and the circulation is quickened on account of the rarefaction of the air. GAY LUSSAC relates, that his pulse rose from 66 to 120. The sky becomes almost black, and the silence that prevails is frightful. After a voyage of six hours, GAY LUSSAC descended, having travelled about ninety miles.

On the 1st of July, 1859, Messrs. WISE, LA MOUNTAIN, GAger, and HYDE, ascended from St. Louis, Mo., and descended at Henderson, Jefferson Co., N. Y., having travelled 1150 miles in a little less than twenty hours, or about fifty-seven miles per hour. most celebrated voyage on record.

This is the

During the recent siege of Paris balloons were successfully employed as a means of communication between the forces within the city and those without the lines of the enemy. Balloons have also been used for making observations in the higher regions of the atmosphere.

(144.) Describe some of the most remarkable Balloon Ascensions. That of ROZIER. Of BLANCHARD and JEFFRIES. Of GAY LUSSAC. What effect has the Uses. atmosphere at great elevations? Describe the great American voyage.

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145. ACOUSTICS is that branch of Physics which treats of the laws of generation and propagation of sound.

Definition of Sound.

146. SOUND is a motion of matter capable of affecting the ear with a sensation peculiar to that organ.

Sound is caused by the vibration of some body, and is transmitted by successive vibrations to the ear. The original vibrating body is said to be sonorous. A body which transmits sound is called a medium. The principal medium of sound is the atmosphere; wood, the metals, water, &c., are also media.

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Fig. 107.

Let us take, for illustration, a stretched cord which is made to vibrate by a bow, as in a violin, for example. When the cord is drawn from its position of rest, acb, Fig. 107, to the position adb, every point of the cord is drawn from its position of equilibrium ;

(145.) What is Acoustics? (146.) What is Sound? What is its cause? How is it transmitted? What is a sonorous body? A medium? Examples. Explain the vibrating cord.

when it is abandoned, it tends, by virtue of its elasticity, to return to its primitive state. In returning to this position, it does so with a velocity that carries it past acb to aeb, from which it returns again nearly to adb, and so on vibrating backward and forward, until, after a great number of oscillations, it at length comes to rest.

Sound-waves in Air.-Mode of Propagation.

147. Sound-waves are produced in the air by the vibration of some sonorous body. When the body moves forward it strikes the air in front of it and condenses a stratum whose thickness depends on the rapidity of vibration; the particles of this stratum impart the condensation to those

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of the next, and these in turn to those of the next, and so on; the condensation thus transmitted outward is called the condensed pulse. When the body moves backward, the air in front of it follows and produces rarefaction in a stratum whose thickness depends on the rapidity of vibration; this causes a backward movement and consequent rarefaction in the next stratum, which is transmitted to the next, and so on; the rarefaction thus propagated outward is called the rarefied pulse. Each complete vibration of the sonorous body generates a condensed and a rarefied pulse, and these taken together constitute a sound-wave.

(147.) Describe the mode of sound propagation in the air.

If the vibrations are continuous, a series of sound-waves are generated travelling outward in the form of spherical shells, as shown in Figure 108.

The rate at which the sound-wave travels is the velocity of sound; the distance through which it travels in the time of one vibration of the sonorous body is the wave length; hence the wave length is always equal to the velocity of sound divided by the number of vibrations in one second. The form of the sound-wave is transmitted through the air, but the individual particles of air simply oscillate to and fro in the direction of wave propagation, moving forward on the passage of the condensed and backward on the passage of the rarefied pulse; the distance through which each particle oscillates is called the amplitude of vibration of the particle.

Any two particles situated on a line in the direction of propagation, and a distance from each other equal to a wave length, are always moving in the same direction and with equal velocities; such particles are said to be in the same phase. All the particles of any wave that are in the same phase are on the surface of a sphere, which is called a wave front.

Superposition of Sound-waves.

148. It is to be remarked that many sounds may be transmitted through the air simultaneously. This shows that the sound-waves cross each other without modification. In listening to a concert of instruments, a practiced ear can detect the particular sound of each instrument.

Sometimes an intense sound covers up or drowns a more feeble one; thus, the sound of a drum might drown that of the human voice. Sometimes feeble sounds, which are too faint to be heard separately, by their union produce a sort of murmur. Such is the cause of the murmur of waves, the rustling sound of a breeze playing through the leaves of a forest, and the indistinct hum of a distant city.

It has been shown that two sound-waves may, under certain circumstances, neutralize each other, producing silence.

What is the velocity of sound? The wave length? The amplitude of vibration of a particle? Its direction? A wave front? (148.) Do sound-waves interfere with each other's progress? How shown? Explain the murmur of leaves. Waves.

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