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 heated 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. This is the most celebrated voyage on record.

Balloons have been used with some success in military operations. As means of travelling, they have thus far proved of no value, on account of the difficulty of directing their course.

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

CHAPTER IV.

ACOUSTICS.

1. PRODUCTION AND PROPAGATION OF SOUND.

Definition of Acoustics.

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 origi nal 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.

d

C

e

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. These vibrations are the cause of a sound which may reach the ear through the atmosphere.

The oscillations of sonorous bodies are too rapid to be counted, or even to be seen distinctly. This may, however, be made manifest to the eye in several ways.

For example, if a hollow glass globe be made to vibrate by striking it, and a small ball of ivory be brought near it, a succession of shocks is sufficient to make the vibratory motion manifest.

If a plate of metal be fixed at one of its points, and then made to vibrate by drawing a violin bow over one of its edges, fine particles of sand strewn over it will be seen to dance up and down, and finally to arrange themselves in curious figures. This motion of the particles of sand is due to the vibrations of the plate, and it serves to make them manifest to the eye.

147. The vibrations of a sonorous body give rise to corresponding vibrations in the surrounding air, which are

How may sonorous vibrations be made manifest to the eye? Explain the vibrating globe. The vibrating plate. (147.) How is sound propagated?

transmitted by a succession of condensations and rarefactions, as represented in Fig. 108, until at last they reach the ear and produce the sensation of sound.

The vibrations of the air communicate corresponding vibrations to the tympanum or drum of the ear, whence they are transmitted by a very complex mechanism to the auditory nerve, and so to the sensorium, or seat of sensation.

The aerial vibrations emanating from a sonorous body spread outwards in successive spheres; hence sound is transmitted in all directions.

An idea of the successive spheres or undulations may be had by dropping a stone upon the surface of a pond of still water, and noticing the successive waves as they follow each other to the shores. The central particles continue to oscillate like those of the cord already described, and at each oscillation a new wave or undulation is generated.

Co-existence of Sonorous 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 rumbling 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.

How imparted to the auditory nerve? air? Illustrate by waves on a pond each other's progress? How shown? Of a city.

What is the form of a sound-wave in the

(148.) Do sound-waves interfere with Explain the murmur of leaves. Waves.