H

Fig. 47.

gas descending and the heavy one rising, until both are equally commixed. We see, therefore, that this property of gases is intimately concerned in determining the constitution of the atmosphere, which is made up of different substances, some of which are light and some heavy-the heavy ones not sinking nor the light ones ascending, but both kept equally commixed by diffusion into each other.

SECTION II.-HYDROSTATICS AND HYDRAULICS.

CHAPTER IX.

PROPERTIES OF LIQUIDS.

Extent and Depth of the Sea-Its Influence on the Land-Production of Fresh Waters-Relation of Liquids and Gases-Physical Condition of Liquids-Different Degrees of Liquidity-Florentine Experiment on the Compression of Water-Oersted's Experiments-Compressibility of other Liquids.

HAVING disposed of the mechanical properties of atmospheric air, which is the type of gaseous bodies, in the next place we pass to the properties of water, which is the representative of the class of liquids.

About two-thirds of the surface of the earth are covered with a sheet of water, constituting the sea, the average depth of which is commonly estimated at about two miles. This, referred to our usual standards of comparison, impresses us at once with an idea of the great amount of water investing the globe; and, accordingly, imaginative writers continually refer to the ocean as an emblem of immensity.

But, referred to its own proper standard of comparison-the mass of the earth-it is presented to us under a very different aspect. The distance from the surface to the centre of the earth is nearly four thousand miles. The depth of the ocean does not, therefore, exceed th part of this extent: and astronomers have justly stated, that were we on an ordinary artificial globe to place a representation of the ocean, it would scarcely exceed in thickness the film of varnish already placed there by the manufacturer.

In this respect the sea constitutes a mere aqueous film on the face of the globe. Yet, insignificant as it is in reality, it has been one of the chief causes engaged in shaping the external surface, and also of modelling the interior to a certain depth-for geological investigations have proved the former action of the ocean on regions now far removed from its influence, in the interior of continents; and also its mechanical agency in the formation of the sedimentary or stratified rocks which are of enormous superficial extents and often situated at great depths.

Besides the salt waters of the sea, there are collections of fresh waters, irregularly disposed, constituting the different lakes, rivers, &c. The direct sources of these are springs, which break forth from the ground, the little streams from which coalesce into larger ones. But the true source of all our

terrestrial waters is the sea itself. By the shining of the sun upon it a portion is evaporated into the air, and this, carried away by winds and condensed again by cold, descends from the atmosphere as showers of rain, which, being received upon the ground, percolates until it is stopped by some less pervious stratum, and flowing along this at last breaks out wherever there is opportunity in the low grounds-thus constituting a spring. Such streamlets coalesce into rivers, which find their way back again to the sea, the point from which they originally came-an eternal round, which is repeated for centuries in succession.

From these more obvious phenomena of Nature, we discover a relationship between aërial and liquid bodies-the one passing without difficulty into the other form-and, indeed, many of the most important events around us depending on that fact. Experiment also shows that, in many instances, substances which under all common circumstances exist in the gaseous condition, can be made to assume the liquid. Thus carbonic acid, which is one of the constituents of the atmosphere, can by pressure be reduced to the liquid form, and can even be made to assume that of a solid. The main agents by which such transmutations are effected are cold and pressure.

The parts of liquids seem to have little cohesion. Viewing the forms of matter as being determined by the relation of those attractive and repulsive forces which are known to exist among particles, it is believed in that now under consideration-the liquid-that these forces are in equilibrio. For this reason, therefore, the particles of such bodies move freely among one another; and liquids, of themselves, cannot assume any determinate shape, but conform their figure to the vessels in which they are placed. Portions of the same liquid added to one another readily unite.

Among liquids we meet with what may be termed different degrees of liquidity. Thus the liquidity of molasses, oil, and water, is of different degrees. It seems as though there was a gradual passage from the solid to this state, a passage often exhibited by some of the most limpid substances. Thus alcohol, when submitted to an extreme degree of cold, assumes that. partial consistency which is seen in melting beeswax, yet at common temperatures it is one of the most mobile bodies known. So, too, that compound of tin and lead, which is used by plumbers as a solder, though perfectly fluid at a certain heat, passes, in the act of cooling, through various successive stages, and at a particular point becomes plastic and may be moulded with a cloth.

If a quantity of atmospheric air is pressed upon by any suitable contrivance, it shrinks at once in volume. We have already proved this phenomenon and determined its laws. If water is submitted to the same trial, the result is very different-it refuses to yield for this reason, inasmuch as the same fact applies to the whole class, liquids are spoken of as incompressible bodies.

It was at one time thought that the experiment of the Florentine academicians, who filled a gold globe with water, and on compressing it with a screw found the water ooze through the pores of the gold, proved completely the incompressibility of that liquid. But more recent experiments have

*

The experiment alluded to by Professor Draper was performed at the Academy Del Cimento, in Florence, more than two centuries ago, and is now generally adduced to prove the porosity of gold in common with all other bodies.

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. This 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, ef, 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

title of HYDRODYNAMICS* is applied. It is divided into two branches, Hydrostatics 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, Dand, after a few oscillatory movements, stands at the same level in both.

B

Fig. 50.

H

C

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 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 an experiment, that the pressure of liquids does not depend on their absolute weight, but on their vertical altitude. 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.

Fig. 51.

The

* From the Greek udor ("Yowp) water, and dunamis (Avvaμic) power. [Hydrodynamics 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 (Eráoic) standing. [By hydrostatics 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 (Avλòg) 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 of the liquid only presses on A B C D laterally, and, resting on the sides E C and FD, cannot contribute anything Fig. 53. 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 Be, 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