NUMBER 53,

GENERAL DEFINITIONS.

MATTER exists in three principal forms-solid, liquid, and gaseous or deriform. These forms respectively, and the various modifications of them, are the immediate result of certain principles of attraction and repulsion operating on the atoms or particles of which matter is composed.

The solid, liquid, and aëriform varieties of matter, assume a position on our globe corresponding to their heaviness or density in a given volume. The solid sinks | lowest, and composes the chief mass of the earth; above the solid lies the liquid variety, in the form of the ocean, rivers, and lakes; and above all is the atmosphere, consisting of an expanse of aëriform matter, which wraps the whole earth round to an elevation of from fortyfive to fifty miles above the highest mountains. In this great ocean of air, loaded less or more with particles of moisture from the liquids beneath, we live, breathe, and move, and plants grow and receive an appropriate nourishment.

Though differing both in substance and appearance, the liquid and aëriform varieties of matter resemble each other in many of their properties and tendencies, and constitute the class of bodies termed fluids. Fluids signify bodies which will flow, or whose component particles are easily moved among each other. Some fluids are so thick and viscous, or sticky, that they can scarcely flow, as tar, honey, and some metals in a state of fusion; others flow with ease, as water and distilled spirits; while others are so light and volatile, as to be impalpable to the touch and invisible to the eye, as pure atmospheric air and various gases.

It is common to divide fluids into two kinds-nonelastic fluids and elastic fluids; that is, fluids which cannot be compressed into a smaller bulk, and those which are susceptible of compression. The non-elastic fluids are water and all other varieties of liquid bodies; but recent experiments prove that the term is not strictly applicable to them. It has been found that water may be compressed in a confined vessel, to a small extent, by means of a very great pressure, and it is certain that water at a considerable depth in the ocean is more dense or compressed than at the surface; water, consequently, is an elastic substance; but as it can be compressed only with very great difficulty, the term non-elastic fluid is not altogether inappropriate.

Atmospheric air and all gases are elastic. They can with little difficulty be compressed into a much smaller volume than they ordinarily possess; and when the pressure is removed, they return to their original buik. Some gases may be compressed to such an extent as to assume the form of liquids and solids; in other words, from the condition of being perfectly invisible to the eye, they can be made to appear as a piece of solid matter, which may be touched and handled.

PRICE lad.

In treating the subject of fluids, it is convenient to refer in the first place to those which are of the liquid form, and afterwards to those which are elastic or aëriform. Pure water, at an ordinary temperature, furnishes the most suitable example of liquid bodies. Water also gives the name of the department of science which includes the laws of liquids. Thus, Hydrostatics, from two Greek words signifying water and to stand, treats of the weight, pressure, and equilibrium of liquids in a state of rest; and Hydraulics, from two Greek words signifying water and a pipe, treats of liquids in motion, and the artificial means of conducting liquids in pipes, or raising them by pumps.

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HYDROSTATICS.

In ancient times water was believed to be an element or simple substance in nature. It is now ascertained by experiment that water is not an elementary body, but is a substance composed chiefly of two gases in a state of chemical union, and into these gases it can be resolved by an artificial process. The investigation of this subject belongs to Chemistry.

As a liquid, water consists of exceedingly small particles or atoms of matter in mechanical combination.

The exact nature and form of the atoms composing water are not satisfactorily known, in consequence of their exceeding smallness. They may be compared to very small particles of sand, cohering slightly, and easily slipping or sliding over each other. Whatever may be the nature and form of these exquisitely fine atoms, it is certain that they can adhere firmly together so as to assume the form of a solid, as in the case of ice, and be made to separate from each other, and disperse through the thinner fluid of the atmosphere, in the forms of steam, clouds, or mist.

Thus, imperfect cohesion of atoms or particles is a property common to all fluids. The atoms composing water, being in closer union than those of air, are observable as a mass, and palpable to the touch. When the hand is dipped into them, and then withdrawn, a certain quantity of the atoms is brought away on the surface of the skin; and this adhesion of the particles of water (caused by attraction of cohesion) is what we in ordinary language call wetness. Certain substances, as is well known, absorb water to a great extent; in such cases, the minute particles of the water merely penetrate and fill up the crevices in the substance.

Solid bodies, as a stone, or piece of metal, or wood, have a natural tendency to press only in one direction, that is downwards, or in the direction of the earth's centre, in obedience to the law of terrestrial attraction.

Water has a similar natural tendency to press downwards, and from the same cause; as, for example, when

tions.

A.

An instrument called the hydrostatic bellows has been constructed to exemplify the effect produced by the pressure of a small column of water. As represented in fig. 4, it consists of two circular stout boards connected together with leather, in the form of a pair of strong bellows. A tube A communicates with the interior between the boards. Supposing the instrument to be strong enough, a person standing on the upper board may raise himself by pouring water into the tube, and filling it along with the bellows. It is usual to estimate the pressure by means of weights, W. If the tube hold an ounce of water, and has an area equal to a thousandth part of the area of the top of the bellows, one ounce of water in the tube will balance a thousand ounces placed on the bellows.

W

Fig. 4.

B

This remarkable property in liquids, which is called the hydrostatic parador, is analogous in principle to that which in mechanics is called the Law of Virtual Velocities. According to this fundamental rule, a small weight descending a long way, in any given length of time, is equal in effect to a great weight descending a proportionally shorter way in the same space of time. The rule, as applied to liquids, may be stated thus:A small quantity of water descending in a long column is equal in effect to a proportionately great pressure exerted by a large volume of water in a short column. The law of pressure in proportion to height of column is shown in the annexed representation, fig. 5, of a vessel with an uniformly level base, and full of water. Divid 1ing the depth into 10 equal 3sections, to represent feet, as 6 marked from 1 to 10, it is found, that, at the depth of 1, there is 8 a pressure of one foot of water, 10 at 2, two feet, and so on to 10 at the bottom, where there is a pressure of ten vertical feet of water. The average pressure of the whole is at the middle, at 5. These degrees of intensity of pressure have no reference to the horizontal breadth or length of the mass. The same pressure is sustained, whether the vessel be a foot or a mile in breadth.

4

6

9

Fig. 5.

As in this example, whatever deficiency of pressure there is upon the perpendicular sides of a vessel of water above the middle or point of average pressure, is compensated by a corresponding excess of pressure beneath the middle; consequently, the entire pressure diffused over the sides is equal to that at the middle or point of average pressure. A perpendicular side of a cubical vessel, according to this statement, sustains a lateral pressure precisely equal to the half of that which is endured by the bottom.

We may calculate the degree of lateral pressure in vessels having perpendicular sides and flat horizontal bottoms, by first finding the number of square feet in the sides below the surface of the liquid; then multiplying that by the number of feet in half the depth of the liquid; by which calculation, the product will express the number of solid feet of the liquid, whose weight is equal to the lateral pressure. We may find the number of square feet in the sides, by multiplying the number of feet in the circumference of the bottom by the number of feet in the depth of the liquid.

Example.-To find the degree of pressure on the perpendicular sides of a vat 24 feet deep from the surface of the liquid, and 40 feet in circumference.-Multiply the 24 by 40, and the product 960 gives the area of the sides; then multiply the 960 by half the height, that is 12, and the product is 11,520 cubic feet of water, or the volume of liquid whose weight is equal to the

cubic foot, which is reckoned to be 1000 ounces; then 11,520, multiplied by 1000, gives 11,520,000 ounces, which is the pressure of the water on the sides.

In consequence of the pressure of liquids being as the vertical height and area of the base, it may happen that the lateral pressure on the sides of a containing vessel is greater than the whole weight of the liquid; this will be the case when the surface of the sides in contact with the liquid exceeds the ratio of double the magnitude of the bottom-at double the magnitude, both lateral and perpendicular pressures are alike, and each is equal to the weight of the liquid.

The circumstance of pressure increasing in proportion to depth, suggests the valuable practical lesson of greatly increasing the breadth of embankments for dams and canals from the top downwards, so as to give much greater strength to the base than the summit; also of increasing the strength of the lower hoops of large vats, to prevent their bursting. It likewise demonstrates the propriety of making dams, ponds, canals, and vessels for liquids generally, as shallow as is consistent with convenience or their required purpose. In each case, it is important to recollect that the degree of pressure on the sides is irrespective of shape or size of the contents, and depends exclusively on the height of the liquid from its upper surface to its base. That pressure in water is not according to the volume, but the height above the point of pressure, is obvious from many facts both in nature and art. Whether we plunge an object a foot deep in the ocean or in a jar of water, the pressure upon it is the same. The mere extent of the volume of liquid is of no consequence. Therefore, a precipitous shore pressed upon by the sea to the height of any given number of feet, suffers no more pressure (supposing the sea to be at rest) than the side of a canal of the same number of feet in height.

If the law of pressure of fluids were otherwise than that now stated, no species of embankment, no strength of shore, could withstand the pressure of the ocean, particularly in a high state of the tide. In consequence of the law of pressure being simply as the vertical height, we are enabled by artificial means to stem the volume of a far-spreading ocean, and to secure the dry land from its invasion. A knowledge of this important law might induce the attempt to secure many thousand acres of land which are now covered by the tide.

A

B

If a vessel, as for instance a barrel, be filled with water, and three apertures be made in its side at different heights, as in fig. 6, the liquid will pour out with an impetuosity corresponding to the depth of the aperture from the top. The jet A nearest the top of the barrel, having little pressure above it, will be projected but a short way; the jet B, having a greater pressure, will perhaps go to double the distance; and the jet C, having the greatest pressure of all, will go to a greater distance still. Jets of this kind obey the laws which govern solid projectiles in their flight; they describe a curvilinear motion, the width of curve being proportional to the impressed force.

Fig. 6.

C

Practically, the discharge of liquids from apertures is partly affected by the shape and width of the aperture; for water is retarded by friction, and by its own impetuosity or cross currents in a small channel. It is reckoned that the pressure of water on any body plunged into it, or on the bottom or sides of the containing vessel, is about one pound on the square inch for every two feet of the depth.

Pieces of wood sunk to great depths in the ocean become so saturated with water by the pressure of the

superincumbent mass, that they lose their buoyancy, and remain at rest at the bottom. The depth to which divers can descend is limited by the increased pressure they experience in their descent. If a bottle be firmly corked and sealed, and sunk to a great depth in the ocean, the cork will either be forced in or the bottle broken by the pressure. An air-bell rising from a depth, expands as it approaches the surface. At the depth of a thousand fathoms, water is estimated to be about a twentieth part more dense in the bulk than at the surface.

The great effects which may take place by the action of a small but high column of water, are sometimes exemplified in the rending of mountains. In fig. 7, a mountain or high rocky knoll is represented, with a small vertical crevice A reaching from the summit to an internal reservoir of water near the base. If there be no means of outlet to the liquid, and if rain continue to keep the crevice and its terminating reservoir full, the lateral force exerted by the upright column will be very considerable. Supposing the crevice to be an

B

Fig. 7.

water necessarily possesses the same surface level in all its parts; one portion cannot stand higher than another; all portions, great and small, are only distri buted parts of a single mass.

The tendency which water has to stand at the same surface level in all parts of its mass, is usually referred to by the phrase "water finding its level."

It is this inherent tendency in water to find its level that produces the various phenomena of the trickling down of rain and moisture into the ground, the flowing of all kinds of streams, from the small brook to the mighty river, and the shooting of rapids and cataracts over precipices. In each case, the water, in obedience to the natural law or tendency which governs it, is only trying to find its level. In pursuit of this object, the water, by the rubbing force which it exercises, wears down all the solid objects which present an obstacle to it in its course. Thus, the substances of which hills and plains are composed, are carried away by streams into the ocean-the ground of continents and islands diminishes in bulk-new land rises in the sea; and so, by the effects of a simple natural cause, great alterations are produced in the external features of the globe.

There are two kinds of levels the true level and the natural level. The true level is a perfectly horizontal | plane, as for instance an even line, thusor a perfectly even surface of a floor.

The natural level is a surface, every point of which is at the same distance from the centre of the earth. The surface level of water is always the natural level.

The character of a natural level is understood by a reference to the spherical shape of the earth and the pressure of gravitation. The globe is a ball, and any piece of water which lies upon it, lies in the form of a plaster round the ball. Water, therefore, cannot possibly have a true surface level; its level partakes of the sphericity of the ball. Every piece of water, in a state of entire or partial repose, is in this manner convex in its surface.

Effects of a similar character, but on a less scale, are observable in the bursting of walls behind which earth has been piled, and in which no proper outlets for water have been provided; also in the bursting upwards of drains upon a declivity, when they become choked.

The easy motion of the particles among each other causes them to accommodate themselves to the shape of any vessel. The force of gravity also causes them to seek the lowest level for repose each particle tries to get as low as it can. The result of this general tendency throughout the mass is a perfect levelness of surface the top of the water is smooth.

An uniform levelness of surface takes place in every connected mass of water, whatever be its magnitude or its shape. This forms the third leading feature in the laws of water, and is the cause of many of the phenomena in nature.

B

One of the most familiar examples of the equal height and levelness of surface of water, is that observ. able in a common teapot. In the representation of a teapot, fig. 8, the surface of the liquid in the pot is seen to be at A, and also at the very same height at B in the spout. A straight dotted line is drawn from the one to the other, to show that both surfaces are of the same level. It is customary to say that the small column of water in the spout balances the large mass of water in the pot; but, in reality, there is no balancing in the case. The

Fig. 8.

Fig. 9 represents a segment of the earth's surface, with the appearance of a true and natural level marked

upon it. The curve ES is the earth's surface. PC is a perpendicular line pointing to the centre of the earth. At right angles from this line, a line TL is drawn, representing the true level. Supposing that the line TL is a mile in length, if we draw a line from L to the centre at C, it will cut across the surface of the earth at a point a mile distant from the line at T, which point will be 7 inches and 9-10ths depressed below the part

at L.

The convexity of the earth's surface is not observable in small quantities of water. The surface of a glass of water is not a true level, but the degree of convexity is so small that it cannot be practically estimated or measured. It is only when a sheet of water is * In mathematics, the term apparent level is used instead of true level, and the term dead level instead of natural level.

stretched out to an extent of several miles, that the convexity becomes conspicuous. It is very perceptible on the ocean when a ship is seen approaching on the horizon; first the masts and sails of the ship are seen, and lastly the hull. In order to catch the first glimpse of vessels at sea, the point of outlook for them is placed high above the water. By this means, the person who looks is able to see over a part of the convexity, and give information of the approach of vessels to those placed below.

The convexity of the land is not so conspicuous, in consequence of the many risings and fallings in the surface. It is only in some extensive alluvial plains in different parts of the world that the convexity can be perceived in the same manner as at sea.

In forming roads, railways, and canals, it is necessary to make allowance for the convexity of the earth's surface. The first thing done in such cases is to survey the land by means of an instrument called a theodolite. One of the varieties of the theodolite is a small telescope fixed on a stand, which must, when looked through, be placed perfectly horizontal, or in a true level. To find a true level, an instrument is fixed below it, called a spirit level, and by that it is regulated.

a

A spirit level is in universal request in works of art requiring levelness of foundation or surface. It consists of a cylindrical glass b tube, as in fig. 10, containing a quantity of spirits of wine sufficient to fill it, except a small part, in which

Fig. 10.

the air is left. The tube being completely closed or sealed, the small vacancy where the air is left shows an air-bubble at whatever part of the tube is uppermost. The tube being set in a small wooden case with a level bottom, this case is laid upon the block of stone, wood, or other object to be levelled, and when the air-bubble is seen to rest in the middle of the upper side, it signifies that the object on which the instrument lies is a true level. In the accompanying figure, the air-bubble is seen at the middle at b; the slightest unevenness would cause the bubble to proceed to a at one end, or c at the other.

A true level being found for the theodolite, the surveyor looks through the glass or telescope towards a pole, the lower end of which rests on the ground, and is held in a perpendicular position by a man at (we shall suppose) the distance of a mile, previously measured. The pole having figures marked upon it, a certain figure on a level with the eye is ascertained; 7 inches and 9-10ths are then reckoned down the pole from the figure, and at that depth we have the natural level from which the surveyor makes his subsequent calculations. If a road were to be made on the plan of preserving a true level, it would proceed in its course at a tangent from the earth's convexity, like the line TL in fig. 9, and, consequently, would reach a point above that to which it was destined to go. It would be impossible to make the water in a canal pursue a true level; in the attempt to do so, the water would not remain at rest in the channel prepared for it, but would rush towards the lower end.

As most countries are less or more irregular in surface, canals are usually constructed with different levels, so much of the length being on one level, and so much on another, as the case may be. At every change of level there is a lock, or portion enclosed with gateways, to keep the water at the proper level, and to allow the passage of vessels. The locks of a canal, therefore, are Lke steps of a stair, one at a greater height than another, and by their means vessels may be made to proceed up or down hill.

SPECIFIC GRAVITY.

The more dense in substance that a body is, it is the more heavy or weighty, because it contains the more particles to be operated upon by attraction of gravitation. In reference to the density of bodies, the term specific gravity is employed to denote the comparison

which is made. Thus, the weight of a lump of lead is greater than an equal bulk of cork; therefore its specific gravity is greater; and so on with all other substances, when compared together. For the sake of convenience, pure distilled water, at a temperature of 62 degrees, has been established as a standard by which to compare the specific gravity or relative weight of solid and liquid bodies. Every such body is said to be of either a greater or less specific gravity than water, bulk for bulk.

We have an example of a difference in the specific gravities of liquids, in mercury, water, oil, and spirits. Mercury is considerably more dense or heavy than any of the others; the next in density is water, then oil, and lastly spirits. If we put a quantity of each of these liquids into a glass vessel, one after the other, in the order here mentioned, we shall observe that all keep their respective places, without intermixture, the heaviest at the bottom and the lightest at the top. Should they even be jumbled together in the vessel, it will be noticed that they in time rectify the disturbance, each assuming its own position.

Sea or salt water, in consequence of being loaded with foreign matter, is of greater density or specific gravity than pure fresh water of the same temperature. If we therefore pour a quantity of salt water into a glass vessel, and then gently place some fresh water above it, we shall observe the same phenomenon, of each kind of liquid retaining its position, the heaviest to the bottom, and the lightest to the top. After being jumbled together, the two liquids will, as far as possible, return to their former relative position.

If we fill a bottle with water, and dip it with the open mouth downwards into a jar or barrel of spirits, the water, in virtue of its density, will be emptied and sink into the spirits, and the spirits will immediately rush up into the empty bottle, and supply the place of the water.

A

C

The force which liquids exert in opposing each other in a state of equilibrium, corresponds to their specific gravities; in other words, a small quantity of a heavy liquid will balance a much greater quantity of a lighter liquid. For example, take a bent glass tube, as in fig. 11, and pour as much water into it as will extend from the bottom at E to A. This quantity of water will be balanced or kept to its summit level at A by a quantity of mercury measuring from E to B, or by a quantity of oil from E to C, or by a quantity of spirits from E to D. Each of these experiments may be performed one after the other. The pressure of liquids being as the vertical height, and not as breadth, it would make no difference in the result of the experiments, if the limb of the tube for the mercury, oil, or spirits, were increased to a foot, a mile, or any other diameter.

E

B

Fig. 11.

Water, at its ordinary temperature of 62 degrees, has a specific gravity of 1000 ounces to the cubic foot. Platinum is 224 times heavier, or 224 times the specific gravity of water; gold is 194, mercury 13, copper 83, iron 8, common stone about 24, and brick 2. Alcohol is a little more than 8-10ths of the heaviness or specific gravity of water, or 0-815; and oil of almonds is a little more than 9-10ths, or 0-913. Atmospheric air at the earth's surface is 1-800th part, or 000125; in other words, while a cubic foot of water weighs 1000 ounces, a cubic foot of air weighs one ounce and a quarter.

Sea-water generally possesses a specific gravity of 1-035-that is, to 1000 parts of fresh water there are in addition 35 parts of saline substances.* Sea-water being, therefore, 35 parts for every 1000 of water more dense than fresh water, it possesses a proportionally greater power of buoying up bodies. A vessel which will carry 1000 tons on fresh water, will thus carry 1035 tons on the sea.

* This is given only as a general rule. The sea is not uniformly

salt.