neight a. which it stands at A. It is on this Drinciple that water is now conveyed under ground, through conduit pipes, and made to rise to the level of the fountain whence it is drawn. The city of Edinburgh, a considerable part of which is elevated above the level of the surrounding country, is supplied with water from a reservoir on the Pentland hills, several miles distant. The water is conveyed in leaden pipes down the declivity of the hill, along the interjacent plain, and up to the entrance of the castle, whence it is distributed to all parts of the city. If the point A represent the level of the reservoir, C D will represent the plain along which the water is conveyed, and B the elevation to which it rises on the castle hill. On the same principle, and in a similar manner, the city of London is supplied with water from the water-works at London bridge. Had the ancients been acquainted with this simple but important principle, it would have saved them the labour and expense of rearing those stupendous works of art, the aqueducts, which consisted of numerous arches of a vast size, and sometimes piled one above another. Fig. 2. represents the syphon, the action of which depends upon the pressure of the atmosphere. If this instrument be filled with water, or any other liquid, and the shorter leg, G, plunged to the bottom of a cask, or other vessel, containing the same liquid, the water will run out at the longer leg, F, till the vessel be emptied, in consequence of the atmospheric pressure upon the surface of the liquid. On this principle, water may be conveyed over a rising ground to any distance, provided the perpendicular height of the syphon above the surface of the water in the fountain does not exceed 32 or 33 feet. On the same principle are constructed the fountain

at command, the cup of Tantalus, and other entertaining devices. The same principle, too, enables us to account for springs which are some times found on the tops of mountains, and for the phenomena of intermitting springs, or those which flow and stop by regular alternations.

may

(2.) Any quantity of fluid, however small, be made to counterpoise any quantity, however large. This is what has generally been termed the Hydrostatical Paradox; and from this principle it follows, that a given quantity of water may exert a force several hundred times greater or less, according to the manner in which it is employed. This force depends on the height of the column of water, independent of its quantity; for its pressure depends on its perpendicular height. By means of water conveyed through a very small perpendicular tube, of great length, a very strong hogshead has been burst to pieces, and the water scattered about with incredible force. On this principle, the hydrostatic press, and other engines of immense power, have been constructed.

(3.) Every body which is heavier than water, or which sinks in it, displaces so much of the wates as is equal to the bulk of the body immersed in the

water.

On this principle, the specific gravities, or comparative weight, of all bodies are determined. It appears to have been first ascertained by Archimedes, and, by means of it, he determined that the golden crown of the king of Syracuse had been adulterated by the workmen. other From this principle we learn, among many things, the specific gravity of the human body : and that four pounds of cork will preserve a person weighing 135 pounds from sinking, so that he may emain with his head completely above water.

Hydraulics, which has sometimes been treated as a distinct department of mechanical philosophy, may be considered as a branch of hydrostaics. It teaches us what relates to the motion of Auids, and how to estimate their velocity and force. On the principles of this science, all machines worked by water are constructed-as steam-engines, water-mills, common and forcing pumps, syphons, fountains, and fire-engines.

III. Pneumatics.-This branch of philosophy treats of the nature and properties of the atmosphere, and of their effects on solid and fluid bodies. From this science we learn, that air has weight, and presses on all sides, like other fluids; that the pressure of the atmosphere upon the top of a mountain is less than on a plain beneath; that it presses upon our bodies with a weight of several thousand pounds more at one time than at another; that air can be compressed into forty thousand times less space than it naturally occupies; that it is of an elastic or expansive nature, and that the force of its spring is equal to its weight; that its elasticity is increased by heat; that it is necessary to the production of sound, the support of flame and animal life, and the germination and growth of all kinds of vegetables. These positions are proved and illustrated by such experiments as the following:-The general pressure of the atmosphere is proved by such experiments as those detailed in No. II. of the Appendix. The following experiment proves that air is compressible. If a glass tube, open at one end, and closed at the other, be plunged, with the open end downwards, into a tumbler of water, the water will rise a little way in the tube; which shows, that the air which filled the tube is compressed by the water into a smaller space. The elasticity of air is proved by tying up a bladder, with a very small quantity of air within it, and putting it under the receiver of an airpump, when it will be seen gradually to inflate, till it becomes of its full size. A similar effect would take place, by carrying the bladder to the higher regions of the atmosphere. On the compression and elasticity of the air, depends the construction of that dangerous and destructive instrument, the air-gun.

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That it is capable of being rarified by heat, is proved by holding to the fire a half-blown bladder, tightly tied at the neck, when it will dilate to nearly its full size; and if either a full-blown bladder, or a thin glass bubble filled with air is held to a strong fire, it will burst. The elasticity of the air is such, that Mr. Boyle, by means of an air-pump, caused it to dilate till it occupied fourteen thousand times the space that it usually does. That air is necessary to sound, flame, animal and vegetable life, is proved by the following experiments: When the receiver of an air-pump is exhausted of its air, a cat, a mouse, or a bird, placed in it, expires in a few moments,

in the greatest agonies. A bell rung in the sam situation produces no sound; and a lighted candlis instantly extinguished. Similar experiments prove that air is necessary for the flight of birds the ascent of smoke and vapours, the explosion of gunpowder, and the growth of plants; and that all bodies descend equally swift in a place void of air; a guinea and a feather being found to fall to the bottom of an exhausted receiver at the same instant.

On the principles which this science has established, have been constructed the air-pump, the barometer, the thermometer, the diving-bell, the hygrometer, the condenser, and various other instruments, which have contributed to the comfort of human life, and to the enlargement of our knowledge of the constitution of nature.

IV. Acoustics.-This science treats of the nature, the phenomena, and the laws of souna, and the theory of musical concord and harmony. From the experiments which have been made on this subject, we learn, that air is essential to the production of sound; that it arises from vibrations in the air, communicated to it by vibrations of the sounding body; that these vibrations, or aerial pulses, are propagated all around in a spherical undulatory manner; that their density decreases, as the squares of the distances from the sounding body increase; that they are propagated together in great numbers from different bodies, without disturbance or confusion, as is evident from concerts of musical instruments; that water, timber, and flannel, are also good conductors of sound; that sound travels at the rate of 1142 feet in a second, or about thirteen miles in a minute; that the softest whisper flies as fast as the loudest thunder; and that the ut most limits, within which the loudest sounds produced by artificial means can be heard, is 180 or 200 miles;* that sound striking against an obstacle, as the wall of a house, may, like light, be reflected, and produce another sound, which is called an echo; and that, after it has been reflected from several places, it may be collected into one point or focus, where it will be more audible than in any other place. On these principles, whispering galleries, speaking trumpets, and other acoustic instruments, are constructed.

V. Optics.-This branch of philosophy treats of vision, light, and colours, and of the various phenomena of visible objects produced by the rays of light, reflected from mirrors, or transmitted through lenses. From this science we

In the war between England and Holland, in 1672, the noise of the guns was heard in those parts of Wales which were estimated to be two hundred miles distant from the scene of action. But the sounds produced by volcanoes have been heard at a much greater distance; some instances of which are stated in Chap. IV. Sect. 2. Several other facts in relation to sound are detailed in Chap. III. Art Acoustic Tunnels,

learr, that light flies at the rate of nearly twelve millions of miles every minute-that it moves in straight lines that its particles may be several thousands of miles distant from each other-that every visible body emits particles of light from its surface, in all directions-that the particles of light are exceedingly small; for a lighted candle will fill a cubical space of two miles every way with its rays, before it has lost the least sensible part of its substance; and millions of rays, from a thousand objects, will pass through a hole not larger than the point of a needle, and convey to the mind an idea of the form, position, and colour of every individual object-that the intensity or degree of light decreases, as the square of the distance from the luminous body increases; that is, at two yards' distance from a candle, we shall have only the fourth part of the light we should have at the distance of one yard; at three yards' distance, the ninth part; at four yards, the sixteenth part, and so on-that glass lenses may be ground into the following forms, plano-convex, plano-concave, double convex, double convex, double concave, and meniscus, that is, convex on one side, and concave on the other-that specula, or mirrors, may be ground into either a spherical, parabolical, or cylindrical form-that, by means of such mirrors and lenses, the rays of light may be so modified as to proceed either in a diverging, converging, or parallel direction, and the images of visible objects represented in a variety of new forms, positions, and magnitudes-that every ray of white light may be separated into seven primary colours: red, orange, yellow, green, blue, indigo, and violet-that the variegated colouring which appears on the face of nature is not in the objects themselves, but in the light which falls upon them that the rainbow is produced by the refraction and reflection of the solar rays in the drops of falling rain-that the rays of light are refracted, or bent out of their course, when they fall upon glass, water, and other mediums-that the light of the sun may be collected into a point or focus, and made to produce a heat more intense than that of a furnace*-that the rays from visible objects, when reflected from a concave mirror, converge to a focus, and paint an image: of the objects before it, and that when they pass through a convex glass, they depict an image behind it.

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• This is produced by means of lenses, or mirrors of a large diameter, called burning-glasses. By these instruments the hardest metals, on which common fires, and even glass-house furnaces, could produce no effect, have been melted in a few seconds. M. Villette, a Frenchman, nearly a century ago, constructed a mirror, three feet eleven inches in diameter, and three feet two inches in focal distance, which meited copper ore in eight seconds, iron ore in twentyfour seconds, a fish's tooth in thirty-two seconds, cast iron in sixteen seconds, a silver sixpence in seven seconds, and tin in three seconds. This mirror condensed the solar rays 17,257 times, a degree of heat which is about four hundred and ninety times greater than common fire. Mr. Parker, of London, constructed a lens three feet in diameter,

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On these and other principles demonstrated by this science, the Camera Obscura, the Magic Lantern, the Phantasmagoria, the Kaleidoscope, the Heliostata, the Micrometer-Spectacles, Opera-Glasses, Prisms, single, compound, lu cernal, and solar Microscopes, reflecting and re fracting Telescopes, and other optical instru: ments, have been constructed by means of which the natural powers of human vision have beer wonderfully increased, and our prospects into the works of God extended far beyond what former ages could have conceived.

VI. Electricity.-This name has been given to a science which explains and illustrates the operations of a very subtile fluid called the electric fluid, which appears to pervade every part of nature, and to be one of the chief agents employed in producing many of the phenomena of the material world. If a piece of amber, sealing wax, or sulphur, be rubbed with a piece of flannel, it will acquire the power of attracting small bits of paper, feathers, or other light sub stances. If a tube of glass, two or three feet in length, and an inch or two in diameter, be rubbed pretty hard, in a dark room, with a piece of dry woollen cloth, besides attracting light substances, it will emit flashes of fire, attended with a crackling noise. This luminous matter is called electricity, or the electric fluid. If a large globe or cylinder of glass be turned rapidly round, and made to rub against a cushion, streams and large sparks of bluish flame will be elicited, which will fly round the glass, attract light bodies, and produce a pungent sensation if the hand be held to it. This glass, with all its requisite apparatus, is called an electrical machine. It is found, that this fluid will pass along some bodies, and not along others. The bodies over which it passes freely are, water, and most other fluids, except oil and the aerial fluids; iron, copper, lead, and in general all the metals, semimetals, and metallic ores; which are, therefore, called conductors of electricity. But it will not pass over glass, resin, wax, sulphur, silk, baked woods, or dry woollen substances; nor through air, except by force, in sparks, to short distances. These bodies are, therefore, called non-conductors.

The following facts among others, have been ascertained respecting this wonderful agent:That all bodies with which we are acquainted possess a greater or less share of this fluid-tha the quantity usually belonging to any body produces no sensible effects; but when any surface and six feet eight inches focus, which weighed 212 pounds. It melted twenty grains of gold in four seconds, and ten grains of platina in three seconds. The powef burning glasses is as the area of the lens directly, and the square of the focal distance in versely-or, in other words, the broader the mirror or lens, and the shorter the focal distance, the more intense is the heat produced by such instruments. A globular decanter of water makes a powerful burning-glass; and house furniture has been set on fire by incautiously exposing it to the rays of the

sun.

becomes possessed of more or less than its natural share, it exhibits certain appearances, in the form of light, sound, attraction, or repulsion, which are ascribed to the power called electricthat there are two different species of the electrical fluid, or, at least two different modifications of the same general principle, termed positive and negative electricity-that positive and nega tive electricity always accompany each other; for if a substance acquire the one, the body with which it is rubbed acquires the other-that it moves with amazing rapidity; having been transmitted through wires of several miles in length, without taking up any sensible space of time; and, therefore, it is not improbable, that were an insulated conducting substance extended from one continent to another, it might be made to fly to the remotest regions of the earth in a few seconds of time-that it has a power of suddenly contracting the muscles of animals, or of giving a shock to the animal frame-that this shock may be communicated, at the same instant, to a hundred persons, or to an indefinite number who form a circle, by joining their hands together that it may be accumulated to such a degree as to kill the largest animals-that vivid sparks of this fluid, attended with a crackling noise, may be drawn from different parts of the human body, when the person is insulated, or stands upon a stool, supported by glass feet-that electricity sets fire to gunpowder, spirits of wine, and other inflammable substances-that it melts iron wire, and destroys the polarity of the magnetic needlethat it augments the natural evaporation of fluids, promotes the vegetation of plants, and increases the insensible perspiration of animals, and can be drawn from the clouds by means of electrical kites, and other elevated conductors. By means of the electrical power, small models of machinery have been set in action: orreries to represent the movements of the planets, have been put in motion; and small bells have been set a ringing for a length of time; and, in consequence of the knowledge we have acquired of the mode of its operation in the system of nature, the lightnings of heaven have been arrested in their course, and constrained to descend to the earth, without producing any injurious effects.

From these, and a variety of other facts and experiments, it is now fully ascertained, that lightning and electricity are identical; and that it is the prime agent in producing the awful phenomena of a thunder-storm; the lightning being the rapid motion of vast masses of electric matter, and thunder the noise, with its echoes, produced by the rapid motion of the lightning through the atmosphere. There can be little doubt that, in combination with steam, the gases, and other agents, it also produces many of the terrific phenomena of earthquakes, volcanoes, whirlwinds, water-spouts, and hurricanes, and the sublime coruscations of the aurora borealis. In the ope

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rations of this powerful fluid we behold a stry king display of the sovereignty and majestic agency of God. In directing its energies, "his way way is in the whirlwind and the storm, and the clouds are the dust of his feet; the heavens are covered with sackcloth, the mountains quake before him, the hills melt, the earth is burned at his presence, and the rocks are thrown down by him:" Nah. i. 3-6. It is easy to conceive, that, by a few slight modifications produced by the hand of Omnipotence, this powerful fluid might become the agent of producing either the most awful and tremendous, or the most glorious and transporting scenes, over every region of our globe. As it now operates, it is calculated to inspire us rather with awe and terror than with admiration and joy and to lead our thoughts to a consideration of the state of man as a depraved intelligence, and a rebel against his Maker.

VII. Galvanism is intimately connected with electricity, though it is generally considered as a branch of chymistry. It is only another mode of exciting electrical action. In electricity the effects are produced chiefly by mechanical action; but the effects of galvanism are produced by the chymical action of bodies upon each other. If we take a piece of zinc, and place it under the tongue, and lay a piece of silver, as big as a halfcrown, above it; by bringing the outer edges of these pieces in contact, we shall immediately experience a peculiar and disagreeable tase, like that of copper. The same thing may be noticed with a guinea and a piece of charcoal. If a person, in the dark, put a slip of tinfoil upon one of his eyes, and a piece of silver in his mouth, by causing these pieces to communicate, a faint flash will appear before his eyes. If a living frog or fish, having a slip of tinfoil pasted upon its back, be placed upon a piece of zinc, by forming a communication between the zinc and tinfoil, the spasms of the muscles are excited. These and similar effects are produced by that modification of electricity which has been termed galvanism. Three different conductors, or what is called a galvanic circle, are requisite to produce such effects. A piece of copper, a piece of flannel, moistened with water or acid, and a piece of zinc, laid upon one another, form a circle; and if this circle be repeated a number of times, a galvanic pile or battery may be formed capable of giving a powerful shock. The most common and convenient form, however, of a battery, is found to be a trough of baked wood, three or four inches deep and as many wide. In the sides are grooves, opposite to each other, into each of which is placed a double metallic plate, of zinc and copper soldered together, and the cells are then filled either with salt and water, or with a solution of nitrous acid and water.

By means of the galvanic agency, a variety of surprising effects have been produced. Gun. powder, cotton, and other inflammable sup