displace so much liquid; its weight is increased, and it | naturally sinks. Ignorance of these facts in hydrostatics, and want of resolution, cause many deaths by drowning.

There are various kinds of apparatus for preventing drowning, called life-preservers. The most common are those which consist of pieces of cork or other very light material attached to the upper part of the body. But air-tight bags are preferable, as they may be said scarcely to encumber the body when empty, and, as danger approaches, they can be inflated with ease by being blown into. Life-boats have large quantities of cork in their structure, and also air-tight vessels made of thin metallic plates; so that, even when the boat is filled with water, a considerable portion of it still floats above the general surface. The bodies of some animals, as sea-fowl, and many other species of birds, are considerably lighter than water. The feathers with which they are covered add very much to their buoyancy. Quadrupeds swim much easier than men, because the natural motion of their legs in walking or running is that which best fits them for swimming. Fishes are enabled to change their specific gravity by means of an air-bag with which they are provided. When the airbag is distended, they rise to the surface; and when it is contracted, they descend to the bottom.

The buoyant property of liquids is independent of their depth or expanse, for if there be only enough of water to surround an object plunged into it, the object will float as effectually as if it had been immersed in a large mass of water. Thus, a few pounds of water may float an object which is a ton in weight. We account for these phenomena by the law of pressure in liquids being as vertical height, not as width of column, and by a body being buoyed up with a force exactly in proportion to the weight of water which it displaces.

These important truths in hydrostatics teach the practical lesson, that if canals be made only as deep or wide as will afford water to surround the vessels placed upon them, they will be sufficiently large for all purposes of buoyancy and navigation. A ship floats no better on the face of a sheet of water miles in width, than it would do on a mill-pond, provided there be enough of water in the pond to keep it off the bottom. Every solid body possesses a centre of gravity, which is the point upon or about which the body balances itself, and remains in a state of rest, or equilibrium, in any position.

The equilibrium of floating bodies is regulated in the same manner. The floating body has a centre of gravity, about which the whole mass will balance itself in the liquid; the heaviest side will sink lowest, and the more light will be uppermost.

In reference to floating bodies, there is a point called the centre of buoyancy; this is the centre of gravity of the liquid which is displaced. If the floating body be of the same specific gravity as water, then the centre of buoyancy will be at the same point in the floating body as it would have been in the water; but there is seldom this uniformity, at least not in vessels used for purposes of navigation. It is necessary that all such vessels should be of a less specific gravity than water, in order that a part of their weight may be composed of cargo, stores, passengers, &c., and that they may be sufficiently buoyant.

Heavy materials, called ballast, are usually placed in the bottom of the holds of vessels, to ensure a low centre of gravity. A ship of the largest capacity and burden, with its centre of gravity properly regulated, rests in the water with a stateliness and stability which cannot be destroyed except by some extraordinary violence.

HYDROMETERS.

If a substance be weighed in two fluids, the weights which it loses in each are as the specific gravities of those fluids. Thus, a cubic inch of lead loses 253 grains when weighed in water, and only 209 grains when weighed in rectified spirit; therefore, a cubic inch of

rectified spirit weighs 209 grains, an equal bulk of water weighing 253; and so the specific gravity of water is about a fourth greater than that of the spirit. The instrument called a hydrometer is constructed upon this principle. Its name is derived from two Greek words, signifying measure of water; but it is of course used for ascertaining the density of all kinds of liquids. There are various kinds of hydrometers. One of them consists of a glass or copper ball with a stem, on which is marked a scale of equal parts or degrees. When immersed in any fluid, the stem sinks to a certain depth, which is indicated by the graduated scale. The length to which it sinks in the standard of comparison being known, we can thus easily ascertain how much it is specifically heavier or lighter than the fluid.

d

Much in the same manner is constructed another hydrometer of great delicacy and exactness. It consists of a ball of glass about three inches diameter, with another joined to it, and opening into it, of one inch diameter, bc, fig. 15, and a brass neck d, into which is screwed a wire a e, divided into inches and tenths of an inch, about ten inches long and onefortieth of an inch in diameter. The whole weight of the instrument is 4000 grains when loaded with small weights, such as shot, in the lower ball c. When plunged into water in the jar, this instrument is found to sink an inch if a single grain be laid upon the top a; hence a tenth of a grain sinks it a tenth of an inch. So great is the delicacy of this hydrometer, that the difference in specific gravity of one part in 40,000 can be detected. Its total weight of Fig. 15. 4000 grains is convenient for comparing water; but the quantity of shot in the lower ball can be varied, so as to adapt the instrument to measure the specific gravities of fluids lighter or heavier than the standard of comparison.

There is another very simple hydrometer, which consists of a number of glass beads of different weights, but whose proportions are known, and the beads marked accordingly. These are dropped into the fluid under examination, until one is found which neither sinks to the bottom nor swims upon the surface, but remains at rest wherever it is placed in the liquid; and this bead being numbered, indicates the specific gravity.

In making calculations of the strength and specific gravity of spirits, by the above or any other means, attention must be paid to the degree of temperature of the liquid. Heat expands the liquor, and renders it specifically lighter; all spirits are therefore more bulky, in proportion to their weight, in summer than in winter, and also apparently stronger, not really so.

HYDRAULICS.

Having detailed the laws and properties of water in a state of rest or equilibrium, we have now to mention some of the more important results of these laws, and also the effects which are produced upon liquids by the application of forces, whether natural or artificial.

WATER A MECHANICAL AGENT.

Water, as already explained in the Laws of Matter and Motion, may be made a useful agent of power, merely by allowing it to act with the force of its own gravity, as in turning a mill; and in this manner it is extensively employed in all civilised countries possessing brooks which are sufficiently rapid in their descent.

But water may be rendered otherwise useful as an agent of force in the arts. Although subtile in substance, and eluding the grasp of those who desire to

handle and hold it, it can, without alteration of temperature, be made to act as a mechanical power, as conveniently and usefully as if it were a solid substance, like iron, stone, or wood. The lever, the screw, the inclined plane, or any of the ordinary mechanical powers, are not more remarkable as instruments of force than water, a single gallon of which may be made to perform what cannot be accomplished (except at enormous cost and labour) by the strongest metal. To render water serviceable as an instrument of force, it must be confined, and an attempt then made to compress it into less than its natural bulk. In making this attempt, the impressed force is freely communicated through the mass, and in the endeavour to avoid compression, the liquid will repel whatever moveable object is presented to it. The force with which water may be squirted from a boy's syringe, gives but a feeble idea of the power of liquids when subjected in a state of confinement to the impression of external force.

The mechanical force of water is exemplified by the hydraulic press. This is an engine employed by papermakers, printers, and manufacturers of various kinds of goods, for the purpose of giving a high degree of pressure or smooth glazed finish to their respective articles. It has generally superseded the screw press, on account of its much greater power, with a less degree of trouble and risk of injury to the mechanism.

Fig. 16 represents the outline of a hydraulic press. AB is the frame, consisting of four upright pillars supporting a cross top of great strength, and against which the pressure takes place in an upward direction. C, the material to be pressed, is forced upward by D, a round iron piston. This piston is very nicely fitted into an iron case E, which has a cavity F for receiving the water: the neck of the case grasps the piston so tightly that no water can escape. A small pipe G conveys water into the hollow cavity from a forcing-pump H, which stands in a trough of water T. All that part of the apparatus below the base of the pillars is sunk out of sight in the ground. The pump apparatus is here represented as exceedingly simple, but in real machines it is very complex and of great power.

The pump, on being wrought, forces the water into the cavity. There the water, in endeavouring to escape, operates upon the moveable piston, which it causes slowly to rise with its burden. The pressure thus exerted by the liquid almost exceeds belief; unless the case for the water be of enormous strength, it will be rent in an instant as if made of the weakest material. When the weight has been raised to the required height, a stopcock is turned upon the pipe, and the apparatus remains at rest. The opening of the cock allows the water to gush out, and the weight accordingly sinks. The mode of calculating the power of the hydraulic

press is analogous to that for calculating lever powers. Thus, the proportion is estimated between the small bore of the pump and the large bore of the cavity or barrel for the piston. Suppose that the pump has only one thousandth of the area of the barrel, and if a man, by means of its lever handle, press its rod down with a force of five hundred pounds, the piston of the barrel will rise with a force of one thousand times five hundred pounds, or more than two hundred tons. A boy working the pump by a long handle, and taking a sufficiency of time, will raise a pressure of thousands of tons. In the hydraulic press, a force-pump is employed for the sake of convenience; the same end could be attained by a small column of water of a great elevation, on the principle of pressure in liquids being as vertical height.

AQUEDUCTS FOUNTAINS.

The tendency in a liquid to find its level, has permitted the construction of apparatus, consisting of pipes and cisterns, for supplying towns with water. No species of hydraulic machine has been of such great use to mankind as this apparatus.

In ancient times, the fact of water rising to an uniform level in every part of its volume, was either not perfectly understood, or there was a deficiency of ma terials wherewith to construct the apparatus required for carrying water a great distance.

From whatever cause, towns were in these times supplied with water by means of open canals, either cut in the level ground, or supported on the top of arches built for the purpose. These structures, with their elevated channels, were called aqueducts. In Italy, and some other countries in the south of Europe, the remains of stupendous aqueducts, miles in length, still exist.

By a knowledge of the laws of fluids, and by possessing an abundance of lead and iron, we are enabled in the present day to construct apparatus for supplying towns with water in a manner the most effectual and simple; causing a cheap iron or leaden tube, sunk in the ground, to perform the office of the most expensive and magnificent aqueduct.

The method of supplying towns with water consists in leading a pipe of sufficient diameter from a lake, river, or fountain of fresh and pure water, to the place where the supply is required. The iron pipes used for this purpose are composed of a number of short pieces soldered together, and extending to any length, or in any direction. From these main pipes smaller tubes of lead are led into the houses requiring the supply of water; and by means of these minor tubes, the water may be carried to any point which is not of a higher level than the original fountain affording the supply.

Fig. 17 is a representation of the mode of supplying towns with water in this convenient manner. A pipe is observed to proceed from a lake on the top of a hill down into a valley, and thence to supply a house situated on the opposite rising ground. From the pipe, in its passage across the valley, a small tube is carried to supply an ornamental fountain or jet d'eau. The water spouts from this jet d'eau with a force corresponding to the height of the lake above.

In towns not commanding a supply of water from a

Springs in the ground are natural hydraulic operations, and are accounted for on principles connected with the laws of fluids. One kind of springs is caused by capillary attraction, or natural attractive force by which liquids rise in small tubes, porous substances, or between flat bodies closely laid towards each other. This species of power is a remarkable variety of the mutual attraction of matter, and is as unaccountable as the attraction of gravitation, or the attraction exercised by the lodestone.

Springs from capillary attraction are believed to be less common and of smaller importance than springs which originate from the obvious cause of water finding its level. The water which falls in the form of rain sinks into the ground in high situations, and finds an outlet at a lower level, though perhaps at a considerable distance. Some springs are also accounted for by a reference to atmospheric action, but these will form a subject of notice under the head Pneumatics.

FRICTION BETWEEN FLUIDS AND SOLIDS.

The flowing of water through pipes, or in natural channels, is liable to be materially affected by friction. Water flows smoothly, and with least retardation from friction, when the channel is perfectly smooth and straight. Every little inequality which is presented to the liquid, helps to retard it, and so likewise does every bend or angle in its path. A smooth leaden pipe will thus convey more water than a wooden pipe of the same capacity. Practically, an allowance is made in the magnitude of pipes for the loss of speed by friction. Where the length of the tube is considerable, and there are several bendings, it is not unusual to allow a third of the capacity for retardation.

By increasing the capacity of pipes, a prodigious gain is secured in the transmission of water. The loss from friction on a small tube of an inch diameter of bore is so great, that one of twice the capacity will deliver five times as much water.

The rate at which water flows from an orifice in a reservoir, or containing vessel, is affected by the situation and the shape of the orifice.

The most favourable situation for the orifice is at the bottom of the vessel; but the velocity of the emission is not in the ratio of the height of the liquid, or of a perpendicular column of particles; for as the water presses in all directions alike, there is from all parts of the vessel a general rush as it were to the outlet, thus putting the whole mass in motion.

Although the rush of water at the outlet is not as the ratio of the depth, it depends upon the depth. Thus, if a vessel ten feet high be penetrated at the side on a level with the bottom, and the water stand at two feet and a half within, it will issue outwards with a certain degree of velocity. If the height of the water be quadrupled, that is, if the vessel be filled, the velocity will be doubled. In order to obtain a threefold velocity, a ninefold depth is necessary; for a fourfold velocity, sixteen times the depth is required, and so on. In fact, in whatever proportion the velocity of efflux is increased, the quantity of liquid discharged in a given time must be also increased in the same proportion; hence the quantity of water discharged conjointly with its degree of velocity will be increased in proportion to the pressure. There is here a striking coincidence between the descent of water and the relation which exists between the height from which a body falls, and the velocity acquired at the end of the fall.

It has been ascertained that water rushes with most advantage from an orifice, when the orifice is in the form of a short round tube inserted into the vessel, and of a length equal to twice its diameter.

issued through the simple hole without any pipe. The singular fact of a pipe and hole of the same diameter discharging different quantities of water under different circumstances, whilst the head or pressure remains the same, must be accounted for by cross or opposing currents being created by the rush which all fluids make to the orifice. Currents will thus form from the top and sides of the containing vessel, and by their inertia they will cross each other, and thus impede the descent of the perpendicular column, causing the water which issues to run in a screw-like form; this, however, is in a great measure obviated by the application of a short tube from the aperture. That the projection of the tube too far into the interior of the vessel should make the flow less than if there were no pipe at all, may be thus explained :-The columns whichi descend from near the outside of the vessel, by turning up again to reach the discharging orifice, come into more direct opposition to the motion of the central descending columns, whilst they are at the same time themselves compelled to turn suddenly in opposition to their own inertia, before they can enter the pipe. Thus, the discharge is more effectually impeded than if it were proceeding from a mere opening in the bottom of the vessel.

The tube for the discharge of water should not only be short and round, but also trumpet-mouthed or funnel-shaped, both internally and externally, that being the form which admits the flow of liquid with the least possible retardation.

The effects of friction between liquids and solids are nowhere so conspicuous as in the flowing of rivers. The natural tendency in the water to descend at a certain speed, is limited by the roughness of the bottom, bends in the course of the stream, and small projections on the banks. From these causes, the water in a river flows with different velocities at different parts in any vertical section across the current. It flows at a slower rate of speed at and near the bottom than at the surface, and also slower at the sides than at the middle.

The resistance which a body moving in liquid meets with, when it comes in contact with a solid, is as the square of the velocity of the moving body; in other words, the resistance is not twice but four times with a double rate of speed. This is easily explained :—

A vessel moving at the rate of one mile per hour displaces a certain quantity of water, and with a certain velocity; if it move twice as fast, it of course displaces twice as many particles in the same time, and requires to be moved by twice the force on that account; but it also displaces every particle with a double velocity, and requires another doubling of the power on this account; the power thus twice doubled, becomes a power of four. When the body is moved with a speed of three or four, a force of nine or sixteen is wanted, and so on. Thus, the resistance increases as the square of the speed.

This important law suggests practical hints of considerable importance. For instance, in steam naviga tion, if an engine of fifty horse power impel a vessel at the rate of seven miles an hour, it would require two of the same power to drive her ten miles an hour, and three such to drive her twelve miles an hour. Hence the enormous expense of fuel attending the gaining of a high degree of velocity.

ACTION OF WATER IN RIVERS.

In cases where it is desirable to preserve the banks of rivers from injury, either from the regular action of the current or from floods, the water ought to be allowed a free open channel, with banks of a very gradual descent. The utmost violence of water in a state of motion may be rendered comparatively harmless, by allowing the flood or torrent to expend itself on a sloping or shelving shore. Inattention to this simple fact in hydraulics frequently causes much destruction to pro. perty on the banks of rivers.

A very small fixed obstacle, such as a stone or pebble, may partially impede and turn aside a brook of a slow current. The water, by striking on a stone at one side, is bent aside to the opposite bank, a little farther down; there it strikes upon the bank, and is returned to the side it formerly struck. Thus, proceeding in currents from side to side, the banks become worn down at particular places, and in time a new and serpentine course is given to the stream. In the case of rivers flowing with considerable velocity, impediments of this kind are usually overcome, and the stream pursues its straight onward course, dashing down all obstacles to its progress. Thus, rivers are generally winding in their course in flat countries, and straight in mountainous regions.

It sometimes happens that the water at the surface of a river may be moving in one direction, while the water at the bottom is flowing in an opposite direction. This is an exceedingly interesting phenomenon, which is observed to occur in certain rivers communicating with the sea, and is caused by the action of the tides and the difference of specific gravity in salt and fresh water. When the tide is flowing inwards, the salt water rushes up the channel of the river, but not in such a manner as to stem the current of fresh water, which, being lighter, floats on the top of the salt water, and pursues its downward course to the ocean. In those instances in which there is no great disturbance of the two liquids, the fresh water, by its specific lightness, floats on the surface of the sea to a distance of many miles from the land.

WAVES.

Waves are the risings and fallings of the water, caused by some power, such as the blowing of the wind. The power, whatever it happen to be, communicates a force to the mass of liquid, and a series of undulations is the consequence.

These undulations, or waves, exhibit the transmission of the communicated force. The force does not advance or alter the lateral position of the water at any given point; it only alters the water in its vertical position, or in relation to its depth. When, therefore, waves advance, the water does not advance with them: the water but rises and falls, and assumes the figure of undulations on its surface. When the undulations approach the shore, the water then acquires a progressive motion, where it is shallow, and by friction on the bottom or impulsion against the shore, the communicated force is exhausted. The shaking of a carpet affords an exact representation of the action of waves or undulations.

Waves are comparatively superficial; they seldom, even in the greatest storms, rise to a height of more than twelve feet above the level of calm water, and make an equal descent beneath, making altogether an appearance of twenty-four feet; at eight or ten feet below the hollow or trough of the waves the water is tranquil. Waves "mountains high" is only a figure of speech.

ALTERATION OF TEMPERATURE.

By altering the temperature of liquid bodies, they become liable to peculiar laws, and exhibit peculiar phe

nomena.

At a temperature of 40 degrees of Fahrenheit's thermometer, water is at the point of greatest density. When the temperature is reduced below this point, the liquid gradually increases in volume till it reaches 32, when it freezes. When the temperature is raised above 40, the volume increases till it reaches the boiling-point, at which it has expanded to the extent of 1-22d additional to its bulk.

In consequence of this expansibility in heating, hot or warm water is specifically lighter than cold water; therefore, in heating any mass of water in a vessel over a fire, the lighter or warmed particles rise to the top, while the cold and heavy particles sink to the bottom, he heated and to rise in their turn. In this manner

the process of heating proceeds, until all the particles are of an uniform temperature, which is at the boilingpoint, when the liquid gradually flies off in steam. If water be heated by the action of fire, or the sun's rays on its upper surface, the mass is longer in attaining the vaporific point than when heated below, because water is a bad conductor of heat, and therefore the heat penetrates with difficulty through the upper stratum of warmed liquid to reach that which is beneath; and if the mass be very large, as, for instance, the ocean, no intensity of heat applied above can warm it throughout, or to any considerable depth.

Certain currents or sets of the ocean are known to be produced by the effort to attain an equability of temperature throughout. The power of the sun's rays at and near the equator heats the sea in that part of its volume, to the depth of two or three hundred feet. This upper stratum of heated water flows in currents towards the north and south poles, and there to a certain extent tempers the severity of the cold. The waters of the northern and southern tracts of ocean, displaced by these currents, necessarily sink below them, and push on towards the equator, to supply the deficiency caused by the departure of the waters above. Thus, in the economy of nature we see a process in constant action precisely the same in principle as that upon which the artificial hot-water apparatus has been established.

Having now discussed Hydrostatics and Hydraulics, we come to the kindred subject of Pneumatics, for which, as will be observed, we have reserved a notice of certain hydraulic machines involving pneumatical agency.

PNEUMATICS.

GENERAL DEFINITIONS.

Pneumatics, from the Greek word pneuma, breath or air, is the name of the department of science which relates to the weight, pressure, or motion of air, or of any aëriform or gaseous fluids.

It was anciently supposed that the air of the atmosphere was an element or simple substance in nature. It is now satisfactorily established that air is not an elementary body, but is composed of certain gases in intimate union, and these gases can be separated from each other by a process in art.

Air, in its common condition, is a thin transparent fluid, so subtile that it cannot be handled, and when at rest it cannot be felt.

That it is a body, however, is quite obvious, because we feel its impression or force when agitated as wind, or when we wave our hand quickly through it. In the quick motion of the hand, we feel that it is partially opposed by something; and in inhaling breath into the lungs, we feel that we are drawing something through the mouth-that something is air.

Air, like every other substance, whether solid or fluid, possesses a certain gravity or weight. The weight of air certainly, bulk for bulk, is much less than that of water; still the weight may be accurately computed. A bottle full of air weighs heavier in a balance than a bottle of the same capacity from which the air has been extracted.

A cubic foot of water, as has been mentioned, weighs 1000 ounces. A cubic foot of air weighs only 523 grains, being a little more than one ounce; water, therefore, is about 840 times heavier than the air of our atmosphere. Inasmuch as water is a standard for comparing the gravities of liquids, air is a standard in the same respect for all aërial substances.

The specific gravity of air being denominated 1000, oxygen gas is 1111; nitrogen gas 972; hydrogen gas 69; and carbonic acid gas 1529. The lightest of these kinds of gas, therefore, is hydrogen, and the heaviest carbonic acid. Hence, if indefinite quantities of these aëriform bodies were placed in a vessel, or in an apart