53.

bags. In like manner, in descending, if the velocity is too great, o if the balloon tends to fall in a dangerous place, the weight of the balloon is diminished by emptying some of the sand bags.

To render the descent less difficult, the aeronaut is provided with an anchor or grapple, suspended from a cord, by means of which he can seize upon some terrestrial object when he comes near the earth. When the anchor is made fast, the aeronaut draws down the balloon by pulling upon the cord. The anchor, the sand bags, and the wicker car, are represented on the ground in Fig. 105.

18-31 153/5-8.

POPULAR PHYSICS.

At the top of the balloon is a valve kept closed by a spring; it can be opened by means of a string descending through the balloon to the car of the aeronaut. When he wishes to descend, he opens the valve, and allows a portion of the gas to escape. To ascertain whether he is ascending or descending, the aeronaut is provided with a barometer; when ascending, the barometric column falls, and when descending, it rises. By means of the barometer the height at any time may be determined.

143.

The Parachute.

A PARACHUTE is an apparatus by means of which an aeronaut may abandon his balloon, and descend slowly to the earth.

The form and construction of a parachute is shown in Fig. 106. It consists of circular piece of cloth, 15 or 16 feet in diameter, presenting, when spread, the form of a huge umbrella. The ribs are made of cords, which, being continued, are attached to a wicker car, as shown in the figure.

When the aeronaut wishes to descend in the parachute, he enters the car and detaches the parachute from the balloon. At first he descends with immense rapidity, but the air soon spreads the cloth, and then acting by its resistance, the velocity is diminished, and the aeronaut reaches the earth without injury. A hole is made at the centre of the parachute, which, by allowing a part of the compressed air to escape, directs the descent and prevents violent oscillations that might prove dangerous.

The parachute was first tried by BLANCHARD, who placed a dog in the car, and detached it from the balloon. A whirlwind arrested its descent and carried it up above the clouds, where BLANCHARD soon after fell in with it, to the great joy of the poor animal. A current again separated the two voyageurs, but both reached the earth in safety, the dog being the last to descend.

J. GARNERIN was the first man who ventured to descend in a parachute, which he did by detaching himself from a balloon at the

What is the use of the valve at the top? (143.) What is a Parachute? Describe it.

What is the use of the barometer!
Explain its use and action.

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.

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

C

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.