Page images
PDF
EPUB

as llop.

so; and each end always points to the same pole. A magnet is said to traverse, according as its ends, which are called poles, point to the north or south pole of the earth. Magnetism does not subsist between the magnet and iron only; pure nickel is attracted by a magnet nearly as inuch

If a magnet be pulverized, every particle will be found to be a complete magnet, endued with two contrary poles. Heat destroys the power of a magnet, and extreme cold affects it. The attractive force of the same magnet, like the force of gravitation, varies in the inverse duplicate ratio of the distance; and the attractive force of similar magnets is proportional to their surfaces.

2. The inclination or dipping of the magnet or needle is, when a magnet is situated so as to be at liberty to move itself with sufficient freedom. Its two poles do not lie in a horizontal direction, but it generally inclines one of them towards the horizon, and of course elevates the other pole above it. When the north pole of one magnet is presented to the south of another magnet, these ends attract each other; but if the south pole of one magnet is presented to the south pole of another, or the north pole of one to the north pole of another, these ends will repel each other. A plane perpendicular to the horizon, and passing through the poles of a magnet when standing in their natural direction, is called the magnetic meridian, and the angle which the magnetic meridian makes with the meridian of the plane where the inågnet stands, is called the declination of the magnet at that place.

3. The smallest nutural magnets generally possess a greater attractive power, than those of a larger size. It frequently happens, that a natural magnet, cut off from a larger loadstone, will be able to list a greater weight of iron than the original loadstone itself. Mariner's compuss. An artificial magnet fitted up in a proper box, for the purpose of guiding the direction of a traveller, is called a magnetic needle, and the whole together is called the mariner's compass. Although the north pole of the magnet in every part of the world, when suspended, points towards the northern parts, and the south pole towards the southern parts, yet its ends seldom point exactly towards the poles of the earth. The angle in wbich it deviates from the due north and south, is called the angle of declination, or the declination of the magnetic needle, or the variation of the compass; and this declination is said to be east or west, according as the north pole of the needle is eastward or, Westward of the astronomical meridian of

the place.

[ocr errors]

4. This deviation from the meridian is not the same in all parts of the world, but is different in different places,. and it is even continually varying in the saine place. At present, the declination at London is about 24° 8' west; and it has for some years been nearly stationary. Before volcanic eruptions and earthquakes, the magnetic needle is often lubject to very extraordinary movements. It is also agitaled hefore and after the appearance of ihe aurora borealis. The north pole of a mariner's needle will be repelled by the north pole of a powerful magnet, at the distance of several feet. If the body of a man be interposed, the effect will not be at all diminished. A bar of iron, although never touched by a loadstone, will acquire a magnetic power by remaining long in a perpendicular situation. Hence a poker will generally repel or attract a mariner's needle; and very powerful magnets have been made from them, without the use of the loadstone.

[ocr errors]

CHAP. X.-ELECTRICITY. 1. From many experiments that have been made, there can remain no doubt of the existence of the electric Guid, whose singular effects have engaged the attention of Europe for more than half a century. It appears that this fluid is equally distributed through all bodies; but, like the air, is not apparent to our senses, till agitated. The equilibrium which has been interrupted by any cause, must be restored, before we can sensibly perceive the electric fluid. Hence there are two kinds of electric bodies. In the first, the electric fluid is excited and increased in them by means of friction. These are called electrics or non-conductors, and include glass; precious stones; amber; sulphur; sesinous substances ; wax; silk and cotton; f-athers, wool, and hair; paper; loaf-sugar; air when dry; oils and metallic oxyds; ashes of animal and vegetable substances; and most hard stones. The second

receive the electric power, not by friction, but by the communication with the first ;-they are termed conductors or non-electrics, and include gold, silver, pintinum ; brass; irog; tin; quicksilver; lead; the s: mi-metals; inetallic ores ; charcoal; the aniinal fiuids ; water, and other fluids, except oil; ice; snow; most saline substances; earthy substances; sinoke; steam ; and even a vacuum. Bodies of the first kind may be rendered capable of retaining the electric matter which is collected in them; in those of the second class, it is lost as soon as it is received. When a body contains a superfluous quantity of the electric fluid, it is said to be in a positive state, or electrified plus; and when it contains less than its proper share, it is said to be negalive, or electrified minus. The electric attraction also is found to vary inversely, as the squares of the distances. Theuame, electric, is derived from clectrum, amber, a substance, whose attractive power was observed at least 600 years before the Christian æra.

2. Machines have been invented, in which, by means of a wheel, a rapid motion is given to a glass globe or cylinder, which, in turving round, rubs against the hand or cushion. Through this friction, the globe preserves its electric virtue, which may be extended at pleasure by wires or chains, which communicate with the glass globe. If

, while the machine is working, the hand touch one of these wires, a smart shock is felt; and if the place be darkened, a luminous spark will be perceived. If any number of persons take hands, and form a circle, they will all receive the electric shock in the same moment, which may

be made more or less violent. We can give the electric fire power sufficient to kill, not only sparrows, and other little birds, but also hens, geese, cocks, and even sheep.

3. This experiment succeeds only, where there are several glass jars or bottles, nearly filled with water, and united with chains to each other, and in communication with the glass globe, working as before described. The water communicates to the internal surface of the bottles a large quantity of the electric matter; the outward surface at the same time losi:g an equal quantity, by means of its communication with soine conducting body. A violent

[ocr errors]

tush, a loud esplosion, and violent agitation, ignition of coinbustible unatter, and the death of the animal, are the consequences of this experiment.

5. There are other effects, which are common to al} kinds of experiments; such as a sulphureous sinell, an agitation of the air, and the electric matter acquiring a new property. It has been observed, that soine experiments have faileil, because the wires which served as the conductors of the electric matter, were too angular and pointed. It was then supposed, the electric matter became dissipated by the points. This conjecture was confirmed on putting the face or band near the angles and points, for a copious stream of the electric fluid was perceiver to issue from them. It was then conjectured, that these points which cast off the electric maiter, inight also attract it; and a number of experiments have confirmed the truth of this conclusion.

5. Thunder and Lightning. There is no longer any doubt that the cause of thunder is the same with that which produces the ordinary phenomena of electricity; the resemblance between them is indeed so great, that we cannot believe thunder itself to be any other than a grander species of electricity. A cloud in a thunder-storm may be considered as a great conductor, actually insulated and electrified; and it may be supposed to have the same effect upon those non-electrics which it meets with in its course, as our common conductors have, upon those which are presented to them. If a cloud of this kind meets with another which is not electrified, or less so than itself, the electric matter flies off from all parts towards this cloud; hence proceed flashes of lightning, and the formidable report of thunder. The discovery of Dr. Franklin, which established the identity of lightning with the eleetric fluid, suggested the invention of metallic conductors, for securing buildings.

6. The aurora boreales, or northern lights, are occasioned by the passage of the electric fluid through a highly rarefied atmosphere. Earthquakes, whirlwinds, and waterspouts, are also dependent upon electrical phenomena. The pedo and two other kinds of fish have the power of giving an electrical shock, and communicate it to any number of persops, either by immediate touch, or by means

of a metalline rod. Electricity has been employed, with great success, in medicine; particularly in the cure of paralytic affections, and of deafness.

CHAP. XI.-GALVANISM. 1. GALVANISM is but another mode of exciting electricity.' Its name is derived from professor Galvani of Bologna, who, in the year 1791, made some important discoveries on the subject. He found that a dead frog, skinned, is capable of having its muscles brought into action by means of electricity; and that independent of any apparent electricity, the same motions may be produced in the dead animal, or even in a detached limb, by communication between the nerves and muscles, with electrical conductors. The action of electricity on a dead frog, as well as other animals, occasions a tremulous motion of the muscles, and generally an extension of the limbs. If the legs of a frog recently dead, be skinned, and a small part of the spine be attached to them, but separated from the rest of the body, and a part of the nerve proceeding from this limb, be wrapped in a bit of tin foil, or laid upon zinc, and a piece of silver laid with one end upon

the bare muscle, and with the other upon the tin or zinc, the motion of the limb will be very vigorous. The two metals may be placed not in contact with the preparation, but in any

other part of the circuit, which may be completed by means of other conductors, as water, &c.

2. In some surgical operations, the animal body and its amputated limbs, have been convulsed by the application of metals. But the living animal body may become sensible of the action of metallic application in a very

harmless way. If a thin plate of zinc be placed on the upper surface of the tongue, and a half-crown, shilling, or silver tea-spoon, be laid on the lower surface of the tongue, and both metals, after a short space of time, be brought into contact, a peculiar sensation similar to taste, will be perceived at the moment, when the mutual touch happens. If the silver be put beneath, and the zinc upon the same sensation will arise, but in a weaker degree, resembling diluted aminoniac. If a silver probe be intro

the tongue,

« PreviousContinue »