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shown, beyond all doubt, that liquids are compressible, though in a less degree than gases. Thus, it is a common experiment to lower a glass bottle, filled with water and carefully stopped with a cork, into the sea. On raising it again the cork is often found forced in, and the water is uniformly brackish. But in a more exact manner the fact can be proved, and even the amount of compressibility measured, by Oersted's machine. consists of a strong glass cylinder, a a, Fig. 48, filled with water, upon which pressure can be exerted by a piston driven by a screw, b. When the screw is turned and pressure on the liquid exerted, it contracts into less dimensions, but at the same time the glass, a a, yielding, distends, and the contraction of the water becomes complicated with the expansion of the glass in which

Fig. 48. it is placed.

Fig. 48.

To enable us to get rid of this difficulty, the instrument, Fig. 48, is immersed in the cylinder of water, as seen at Fig. 49. This consists of a glass reservoir, e, prolonged into a fine tube, e f, with a scale, x, attached to it. The reservoir and part of the tube are filled with water, and a little column of quicksilver, x, is upon the top of the water, serving to show its position. On one side there is a gauge, d, partially filled with air. It serves to measure the pressure.

Now, when the instrument, Fig. 48, is put in the cylinder in the position indicated in Fig. 49, and pressure made by the screw, b, it is clear that the water in the reservoir will be compressed, and the glass which contains it being pressed upon equally, internally and externally, will yield but very little. Making allowance, therefore, for the small amount of compression which the glass thus equally pressed upon undergoes, we may determine the compressibility of the water as the force upon it varies. It thus appears that water diminishes 22000 part of its volume for each atmosphere of pressure upon it. In the same way the compressibility of alcohol has been determined to be 11000

CHAPTER X.

THE PRESSURE OF LIQUIDS.

Division of Hydrodynamics-Liquids seek their own Level -Equality of Pressure-Case of different Liquids pressing against each other-General Law of Hydrostatics-Hydrostatic Paradox-Law for Lateral Pressures-Instantaneous communication of Pressure-Bramah's Hydraulic

Press.

To the science which describes the mechanical properties of liquids the

1

title of HYDRODYNAMICS* is applied. It is divided into two branches, Hydrostaticst and Hydraulics The former considers the weight and pressure of liquids, the latter their motions in canals, pipes, &c.

A liquid mass exposed without any confinement to the action of gravity would spread itself into one continuous superficies, for all its parts gravitate independently of one another, each part pressing equally on all those around it, and being pressed on equally by them.

A liquid confined in a receptacle or vessel of any kind conforms itself to the solid walls by which it is surrounded, and its upper surface is perfectly plane, no part being higher than another. This level of surface takes place even when different vessels communicating with each other are used. Thus, if into a glass of water we dip a tube, the upper orifice of which is temporarily closed by the finger, but little water will enter, owing to the impenetrability of the air; but, as soon as the finger is removed, the liquid instantly rises, and finally settles at the same level inside of the tube that it occupies in the glass on the outside.

H

This result obviously depends on the equality of pressure just referred to, and it is perfectly independent of the form or nature of the vessel. If we take a tube bent in the form of the letter U, and closing one of its branches with the finger, pour water into the other, as soon as the finger is removed the liquid rises in the empty branch, and, after a few oscillatory movements, stands at the same level in both.

B

Fig. 50.

H

If one of the branches of such a tube is much wider than the other, the same result still ensues. Thus, as in Fig. 50, we might C have a reservoir, A F, exposing an area of ten, or a hundred, or ten thousand times that of a tube rising from it, B G C H; but in the latter a liquid would rise no higher than in the former, both being at precisely the same level, A D. We perceive, therefore, from such

C

1

Fig. 51.

an experiment, that the pressure of liquids does not depend on their absolute weight, but on their vertical altitude. The great mass of liquids contained in A exerts no more pressure on C than would a smaller mass contained in a tube of the same dimensions as C itself.

* From the Greek udor ("Yowp) water, and dunamis (Avvaμic) power. [Hydrodyuamics is the science which applies the principles of Dynamics, to determine the con. ditions of motion and rest in fluid bodies, and is divided into four parts, according as fluids are incompressible or elastic, and according as their equilibrium or their motion is considered.]-Playfair's" Natural Philosophy."

From the Greek udor ("Ydwp) water, and stasis (Erάoic) standing. [By hydrosta. tics is commonly understood that part of natural philosophy which considers the equilibrium and pressure of fluids in general, though that word seems to be restrained to water, which is a particular fluid, and the most obvious of all fluids; and by means of which we shall make out most of our following conclusions.]—Cotes's Hydrostatical and Pneumatical Lectures.

From the Greek udor ("Ydwp) water, and aulos (Avλòs) a pipe or tube. [Hydraulics is that branch of natural philosophy which treats of the motions of liquids, the laws by which they are regulated, and the effects they produce.]-Brande's "Dictionary of Science, Literature, and Art."

D

A variation of this experiment will throw much light upon the subject. Instead of using one let there be two liquids, of which the specific gravities are different. Put one in one of the branches of the tube, a b c, Fig. 51, and the other in the other. Let the liquids be quicksilver and water. It will be found, under these circumstances, that the water does not press the quicksilver up to its own level, but that, for every thirteen and a half inches vertical height that it has in one of the branches the quicksilver has one inch in the other. Of course, as they communicate through the horizontal branch, b, the quicksilver must press against the water as strongly as the water presses against it; if it did not, movement would ensue. And such experiments, therefore, prove that it is the principle of equality of pressures which determines liquids to seek their own level.

From this it therefore appears, that a liquid in a vessel not only exerts a pressure upon the bottom in the direction in which gravity acts, but also laterally and upward.

From what was proved by the experiment represented in Fig. 50, it follows that these pressures are by no means necessarily as the mass, but in proportion to the vertical height. If one hundred drops of water be arranged in a vertical line, the lower one will exert on the surface on which it rests a pressure equal to the weight of the whole. And from such considerations we deduce the general rule for estimating the pressure a liquid exerts upon the base of a vessel. "Multiply the height of the fluid by the area of the base on which it rests, and the product gives a mass which presses with the same weight."

Fig. 52.

to the pressure on

Fig. 54.

Thus, in a conical vessel,
ECDF, Fig. 52, the base,
C D, sustains a pressure
measured by the column
A B C D. For all the rest

[graphic]

Fig. 53.

of the liquid only presses on A B C D laterally, and, resting on the sides E C and FD, cannot contribute anything the base, C D. But in a conical vessel, E C D F, Fig. 53, the pressure on A B is measured by A B C D, as before; but the other portions of the liquid, not resting upon the sides, press also upon the bottom, E F, and the result, therefore, is the same as if the vessel were filled throughout to the height C D.

This law is nothing more than an expression of the fact that the actual pressure of a liquid is dependent on its vertical height and the area of its base. Its applications give rise to some singular results. Thus, the hydrostatic bellows consists of a pair of boards, A, Fig. 54, united together by leather, and from the lower one there rises a tube e B e, ending in a funnelshaped termination, e. If heavy weights are put upon the upper board, or a man stands upon it, by pouring water down the tube the weight can be raised. It is immaterial how slender the tube, and, therefore, how small the quantity of water it contains, the total pres

[graphic]

sure resulting depends on the area of the bellows-boards, multiplied by the vertical height of the tube.

Theoretically, therefore, it appears that a quantity of water, however small, can be made to lift a weight however great-a principle sometimes spoken of as the HYDROSTATIC PARADOX.

But liquids exert a pressure against the sides, as well as upon the bases of the containing vessel the force of that pressure depending on the height. The law for estimating such pressure is, "the horizontal force exerted against all the sides of a vessel is found by multiplying the sum of the areas of all the sides into a height equal to half that at which the liquid stands.'

When bodies are sunk in a liquid, the liquid exerts a pressure which depends conjointly on the surface of the solid and the depth to which its centre is sunk. Thus, if into a deep vessel of water we plunge a bladder, to

Fig. 55.

the neck of which a tube is tied, the bladder and part of the tube being filled with coloured water, it will be seen, as the bladder is sunk, that the coloured water rises in the tube.

A pressure exerted against one portion of a liquid is instantly communicated throughout the whole mass, each particle transmitting the same pressure to those around. A striking illustration of this is seen when a Prince Rupert's drop is broken in a glass of water, the glass being instantly burst to pieces. Bramah's press, or the hydrostatic press, is an illustration of the principle developed in this lecture-that every particle of a fluid transmits the pressure it receives, in all directions, to those around. It consists of a small metallic forcing-pump, a, Fig. 55, in which a piston, s, is worked by a lever, c b d. This little pump communicates with a strong cylindrical reservoir, A, in which a water-tight piston, S, moves, having a stout flat head, P, between which and a similar plate, R, supported in a frame, the substance to be compressed, W, is placed. The cylinder, A, and the forcing-pump, with the tube communicating between them, are filled with water, the quantity of which can be increased by working the lever, d. Now it is obvious that any force, impressed upon the surface of the water in the small tube, a, will, upon the principles just described, be transmitted to that in A, and the piston, S, will be pushed up with a force which is proportional to its area, compared with that of the piston of the little cylinder, a. If its area is one thousand times that of the little one, it will rise with a force one thousand times as great as that with which the little one descends-the motive force applied at d, moreover, has the advantage of the leverage, in proportion as c d is greater than c b. On these principles it may be shown that a man can, without difficulty, exert a compressing force of a million of pounds by the aid of such a machine of comparatively small dimensions.

[graphic]

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CHAPTER XI.

SPECIFIC GRAVITY.

Definition of the term-The Standards of Comparison-Method for Solids -Case when the Body is Lighter than Water-Method for Liquids by the Thousand-Grain Bottic-Effects of Temperature-Standards of Temperature-Other Methods for Liquids-Method for Gases-Effects of Temperature and Pressure-The Hydrometer or Arëometer.

BY the specific gravity of bodies we mean the proportion subsisting between absolute weights of the same volume. Thus, if we take the same volume of water and copper, one cubic inch of each, for example, we shall find that the copper weighs 8.6 times as much as the water: and the same holds good for any other quantity, as ten cubic inches or one cubic foot. When of the same volume the copper is always 8.6 times the weight of the water.

Specific gravity is, therefore; a relative affair. We must have some substance with which others may be compared. The standard which has been selected for solids and liquids is water; that for gases and vapours, atmospheric air.

When we speak of the specific gravity of a substance which is of the liquid or solid kind, we mean to express its weight compared with the weight of an equal volume of water. Thus, the specific gravity of mercury is 13.5; that is to say, a given volume of it would weigh 13.5 times as much as an equal volume of water.*

A TABLE OF THE MEAN SPECIFIC GRAVITIES OF VARIOUS BODIES, AT A TEMPERATURE OF 60° FAHRENHEIT.

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