some of the water to enter. The air in D is at first condensed by the pressure of the water thus admitted; but, immediately expanding by reason of its elasticity, it drives the water into F, for the closing of the valve prevents it from returning to B. By this time the water in B is again at rest, the valve E opens, and the whole process is repeated.

By successive impulses the water may be raised in F to a great height. A descent of four or five feet from the reservoir is sufficient. Care must be taken to have the valve E just heavy enough to fall when B is at rest, and not so heavy as to prevent it from readily rising as the momentum of the stream increases. The pipe B must also be of such length that the water, when arrested in its course, may not be thrown back on the reservoir.

387. Hydraulic Rams afford a cheap and convenient means of raising water in small quantities to great heights, wherever there is a spring or brook having a slight elevation. They are used for a variety of purposes, and particularly when a supply of water is needed for agricultural operations.

EXAMPLES FOR PRACTICE.

Friction is left out of account in these examples.

1. (See § 356, rule in italics.) Two streams issue from different orifices in the same vessel with velocities that are to each other as 1 to 6. How many times farther from the surface is the one than the other?

2. The stream A runs from an orifice in a vessel three times as fast as the stream B. How do their distances below the surface of the liquid compare?

3. In a vat full of beer there are two orifices of equal size; one 9 inches below the surface, and the other 25. How does the velocity of the latter compare with that of the former?

4. There are three apertures in a reservoir of water, 1, 4, and 16 feet below the surface. With what comparative velocity will their streams flow? 5. A stream flows from an aperture in a vessel at the rate of 4 feet in a second. I wish to have another stream from the same vessel with a velocity of 16 feet per second. How much farther below the surface than the first must it be?

6. (See § 359.) A vat full of ale, 3 feet high, has four apertures in it, 3, 12, 18, and 24 inches respectively from the top. Through which will the liquid spout to the greatest horizontal distance? Which next? Which next? 7. (See $360.) How much water will be discharged every minute from an orifice of 3 square inches, the stream flowing at the rate of 5 feet in a second, and the vessel being kept replenished?

its mode of operating. How great a descent is required? What precautions are necessary? 387. In what case may hydraulic rams be used with advantage?

EXAMPLES FOR PRACTICE.

165

How much will be discharged every minute from another orifice in the same vessel, equally large, but situated four times as far below the surface of the liquid?

8. A stream flows from a hole in the bottom of a vessel with a velocity of 6 feet in a second. The hole has an area of 5 square inches, and the vessel is emptied in 15 seconds. How much water does the vessel hold? 9. (See § 376.) A stream having a momentum equivalent to 100 units of work is applied to an Undershot Wheel; how many units of work will it perform? Ans. 25.

(See § 377.) How many units of work will it perform, if applied to an Overshot Wheel?

(See § 378.) How many, if applied to a Breast-wheel?

(See § 379.) How many, if applied to a Turbine?

CHAPTER XII.

PNEUMATICS.

388. PNEUMATICS is the science that treats of air and the other elastic fluids, their properties, and the machines in which they are applied.

389. DIVISION OF ELASTIC FLUIDS.-The elastic fluids are divided into two classes:

I. GASES, or such as retain their elastic form under ordinary circumstances. Some of the gases, under a high degree of pressure, assume a liquid form; as, carbonic acid and chlorine; others, such as oxygen and nitrogen, can not be converted into liquids by any known process.

II. VAPORS, or elastic fluids produced by heat from liquids and solids. When cooled down, they re

sume the liquid or solid form. Steam, the vapor of water, is an example.

390. All gases and vapors have the same properties.

388. What is Pneumatics? 389. Into what two classes are elastic fluids divided? What are gases? What difference is there in the gases? What are vapors? 390. In

The principles of Pneumatics, therefore, relate to all alike, though they are most frequently exhibited and applied in the case of air, with which we have far more to do than with any other elastic fluid.

Air.

391. Air is the elastic fluid that we breathe. It surrounds the earth to a distance of about fifty miles from its surface, and forms what is called the Atmosphere. It exists in every substance, entering the minutest pores.

392. VACUUMS.-Air may be removed from a vessel with an instrument called the Air-pump. A Vacuum is then said to be produced. Vacuums sometimes result from natural causes; but they last only for an instant, as the surrounding air at once rushes in to fill them. Hence the old philosophers used to say, Nature abhors a vacuum.

393. PROPERTIES OF AIR.-Air can not be seen, but it

can be felt by moving the hand rapidly through it. It is therefore material, and has all the essential properties of matter.

394. Air is impenetrable.

395. The Diving-bell.-The impenetrability of air is shown by the Divingbell, represented in Fig. 175. AC is a large iron vessel, shaped somewhat like an inverted tumbler, and attached to a chain, by which it is let down in the water. As the vessel descends, the air in it is condensed by the upward pressure of the liquid, and water enters. The lower it gets, the more the air is compressed, and the greater the amount of water admitted. The impenetrability of the air, however, keeps the greater part of the bell

what are the principles of Pneumatics most frequently exhibited, and why? 391. What is Air? How far does it extend from the earth's surface? What does it constitute? 392. What is a Vacuum? What did the old philosophers say, and why? 893. What proves the air to be material? 394. What apparatus shows the impenetrability of air? 895. Describe the Diving-bell. Explain how descents are made with

PROPERTIES OF AIR.

167 clear of water, so that several persons may descend in it to the bottom of

the sea.

As fast as the air is vitiated by the breath, it is let off by a stop-cock, while fresh air is supplied from above by a condensing syringe, through the pipe B. Air may be thus forced down in sufficient quantities to expel the water altogether from the bell, so that the divers can move about without difficulty on the bottom of the sea. If air were not impenetrable, the bell would be filled with water, and the divers drowned.

When the diving-bell was invented, is not known. History makes no mention of it before the sixteenth century. At that time, we are told, two Greeks, in the presence of the emperor Charles V. and several thousand spectators, let themselves down under water, at Toledo in Spain, in a large inverted kettle, and rose again without being wet. In 1665, a kind of bell was employed off the Hebrides, for the purpose of recovering the treasure lost in several ships belonging to the Invincible Armada. From that time to the present, various improvements have been made in the diving-bell; and it is now extensively used for clearing out harbors, laying the foundation of submarine walls, and recovering articles lost by shipwreck.

396. Air is compressible.

This is proved with the diving-bell. If the air were not compressible, no water would enter the bell as it descended.

397. Air is elastic.

This also may be shown with the divingbell. When, on its descent, water has entered, on account of the air's being compressed, let the bell be raised, and the air will resume its original bulk, expelling the water.

Fig. 176.

Bottle Imps.-The compressibility and elasticity of air may be exhibited in an amusing way with the apparatus represented in Fig. 176. In a vessel nearly full of water are placed several small balloons, or hollow figures of men, &c., made of colored glass, and called Bottle Imps. Each figure has a little hole in the bottom, and is of such specific gravity that it will just float in water. A piece of thin india rubber is tied over the mouth of the vessel, so as to cut off communication with the external air. Now press on the india rubber cover. water at once transmits the pressure to the air in the hollow figures. This air is condensed, water enters, the specific gravity of the figures is increased,

The

BOTTLE IMPS.

it. What is the first mention made of the diving-bell in history? In 1665, for what purpose was it used? For what is it now extensively used? 396. How does the diving-bell prove air to be compressible? 397. How does it prove air to be elastic? What properties in air do the Bottle Imps illustrate? Describe the bottle imps, and

and they descend. On removing the fingers from the cover, the air, by reason of its elasticity, resumes its original bulk, and the figures rise. By thus playing on the india rubber, the figures may be made to dance up and down.

398. Mariotte's Law.-The elastic fluids are the most easily compressed of all substances. The greater the pressure to which they are subjected, the less space they occupy, and the greater their density. A body of air which under a certain pressure occupies a cubic foot, under twice that pressure will be condensed into half a cubic foot; under three times that pressure, into one-third of a cubic foot, &c. This principle, variously stated, is called, from its discoverer, Mariotte's Law.

The more the elastic fluids are compressed, the greater is their resistance to the pressure. Hence, their elastic force increases with their density.

399. The Air-gun.-By subjecting a body of air to a great pressure, we may increase its elastic force sufficiently to produce wonderful effects. The Air-gun is an example. It consists of a strong metallic vessel, into which air is forced till it is in a state of high condensation. The vessel is then attached to a barrel like that of an ordinary gun, to the bottom of which a bullet is fitted. Pulling a trigger opens a valve, the condensed air rushes forth, and drives the bullet out with great force.

One supply of condensed air is sufficient for several discharges, though each is weaker than the preceding one. The labor required for condensing the air prevents this instrument from being much used; but as it makes less noise, when discharged, than the ordinary gun, it is sometimes employed by assassins.

400. Air has weight.

Weigh a flask full of air, and then weigh the same flask with the air exhausted. The difference indicates the weight of the air contained.

401. Experiments show the weight of 100 cubic inches of air to be about 30 grains. This makes it 828 times lighter than water. It has been computed that the weight of the whole atmosphere surrounding the earth is equal to that of a globe of lead 60 miles in diameter.

explain the principle on which they dance up and down. 398. What substances are the most easily compressed? What is Mariotte's Law? To what is the elastic force of gases and vapors proportioned? 399. How may a body of air be made to produce wonderful effects? What instrument proves this? Describe the Air-gun, and its operation. Why is not the air-gun used more? By whom is it sometimes employed? 400. Prove that air has weight. 401. What is the weight of 100 cubic inches of air?