THE power by which the lodestone is influenced, manifesting itself by certain attractive and directive virtues, and which may be understood from the following phenomena afterwards mentioned, which are common to all magnetical bodies.

CHAP. I. Phenomena and Laws of Magnetism.

§ 1. Phenomena of the Magnet.

1. A magnet, whether natural or artificial, attracts iron, and all substances which contain it in its metallic state. A pure calx of iron is but little attracted; but if the calx be heated strongly in conjunction with charcoal dust, it will then be attracted, though it has not regained its metallic splendour, and is quite delicate of malleability. The semimetal called nickel, and perhaps some others, are attracted by the magnet, though freed from iron as much as possible. From some accounts it has been suspected that brass was in a small degree affected by the magnet, and even that all very minute bodies are somewhat under its influence; but this seems not yet to be sufficiently ascertained.

2. If a magnet be suspended by a thread, nicely placed on a pivot, or set to float in a basin of water, it will turn one and constantly the same side nearly towards the north pole of the earth, the other of course turning towards the south. Hence these parts of the magnet have been called its poles, taking the designations of north and south from those parts of the world towards which they turn. This property is called the polarity of the magnet; and when it is in the act of turning itself into this position, it is said to traverse. A plane drawn perpendicular to the horizon through both poles of a magnet, after it has turned itself, is called the magnetic meridian; and the angle it makes with the meridian of the place is called the declination of the magnet or of the magnetic needle.

3. When either the north or the south poles of two magnets are placed near to each other, they repel; but a north and a south pole attract each other.

4. A magnet placed in such a manner as to be entirely at liberty, inclines one of its poles to the horizon, and of course elevates the other above it. This property is called the inclination or dipping of the magnet; and is most conspicuous in artificial magnets or needles, which may be accurately balanced before the magnetic virtue is imparted to them.

5. By proper management any magnet may be made to communicate its virtue to a piece of steel or iron, which virtue it will retain for a longer or shorter time according to circumstances.

§ 2. Of the different Substances attracted by the Magnet.

It has already been said, that iron is the only substance which the magnet particularly attracts, and that too when in its metallic state. Nevertheless this metal is so universally diffused, that there are few substances which do not contain a sufficient quantity of it to be in some degree affected by the magnet. Iron itself is attracted with different degrees of force according to the state in which it is with regard to malleability.—Even the purest calx or solution that can be made, is said to be in some degree affected by the magnet; but of all substances soft iron is attracted with the greatest force when clean and of an uniform texture. Hardened steel is attracted with much less force than iron; but the scales separated from red-hot iron, the fused globules from flint and steel, or the finery cinder, are attracted as much as iron itself. The black calx of iron is attracted but very weakly; and the red calx or rust so little, that it is generally said to be quite insensible to the magnetic attraction; though this is not found to be strictly true, even when the calx is prepared by fire, and purified in the most careful manner. Sometimes the scales and calx are capable of acquiring a polarity, though weakly. Ores of iron are attracted with greater or less force according to the state of the metal in them, and according to the quantity of it they contain; though the attraction is always manifest even when they contain such a small quantity as scarcely to deserve the name of ores. They are generally much more attracted after calcination than before; because this operation communicates to them a portion of phlogiston by which they approach to a metallic state. Ores of lead, tin, and copper, are likewise attracted, as well as native cinabar, on account of the quantity of iron they contain; and it is remarkable, that though pure lead in its metallic state is not in the least attracted, its calx is so in some degree. The calx of tin is also attracted, though in a still smaller degree than that of lead. Zinc, bismuth, and cobalt, but especially the ores of these semimetals, are attracted; but not antimony, unless it be first exposed to a gentle heat; and arsenic is not attracted at all. One kind of bismuth is said to be absolutely repelled by the magnet. Almost all other minerals are attracted, at least after having been exposed to the action Calcaceous earth is attracted less than any other kind, and the siliceous earth the most frequently. Sand, especially the black kind, is generally attracted; and amber as well as other combustible substances have the same property, after being burned. Almost every part of animal and vegetable bodies is affected by the magnet after being burned; but unburned animal or vegetable substances are very seldom if ever perceptibly attracted. It is also remarkable, that even foot, or the dust which falls upon anything left exposed to the atmosphere, are sensibly attracted. Colourless precious stones, as the diamond and crystals, are not attracted; neither the amethyst, topaz, chalcedony, or such as are deprived of their colour by fire; but all others, as the ruby, chrysolite, and tourmaline, are attracted. The emerald, and particularly the garnet, are not only attracted, but frequently acquire an evident polarity. The opal is attracted but weakly.

The attraction of so many different substances shows the universal diffusion of iron throughout almost all terrestrial substances; for to this we are with the greatest probability to ascribe the attraction of so many substances by the magnet. How small a quantity of iron indeed will give a substance this property, is evident from the following experiment related by Mr. Cavallo. "Having chosen a piece of Turkey-stone which weighed above an ounce, I examined it by a very sensible magnet needle, but did not find that it was affected in the least. A piece of steel was then weighed with a pair of scales, which would turn with the 20th part of a grain, and one end of it drawn over the stone in various directions. After this operation the steel was again weighed, and found to have lost no perceptible part of its weight; yet the Turkey-stone, which had acquired only this very small quantity of steel, now affected the magnetic needle very sensibly." In making his observations on this experiment, he proposes the magnet as a test of iron in different substances, being capable of detecting a smaller quantity than any method that chemistry can yet afford.

Our author has been at considerable pains to investigate the magnetic properties of brass and other metals; having made many experiments upon the subject, of which the following are the results: 1. Hammered brass is much more generally attracted by the magnet than other kinds; and such as is not influenced in this manner, acquires the property by being hammered. 2. A piece of brass rendered magnetic by hammering, loses the property on being made red hot so as to become softened; by a second hammering it becomes again magnetic; and thus may be made to lose its property and recover it alternately. 3. Suspecting that the magnetic property might be occasioned by a small quantity of iron abraded from the hammer, the pieces of brass were beat between two pieces of card-paper; notwithstanding which precaution, it acquired the magnetic property as before. 4 Sometimes an evident degree of magnetism was communicated by two or three strokes, and with the card-paper not above 30 strokes were given to make the brass sensibly magnetic. 5. A piece of brass was hardened by beating it between two large flints, using one for the hammer and the other for the anvil; but still it acquired a magnetic property, tho' less than with the iron hammer, which might be explained by the roughness of the flints, and their not coming into contact sufficiently with the metal. Neither of the flints was found to have acquired the smallest degree of magnetic power either before or after the experiment. 6. By melting the brass in a crucible, it was found to have entirely lost its magnetism. 7. A piece of brass deprived of its magnetic property by fire, regained it after a few strokes of the hammer, though laid between two pieces of copper. 8. Most of the pieces of brass tried by our author became magnetic by hammering; but some, though rendered equally hard with the rest, did not affect the needle in the least; but these could not originally be distinguished from such as are capable of becoming magnetic. 9. As, notwithstanding the precautions made use of in the above experiments to prevent the iron of the hammer from being in any manner of way communicated to the brass, an objection might arise, that some quantity of the calx might be diffused through the metal, and acquire phlogiston by hammering, he tried the following experiment, which seemed decisive. A piece of brass which would acquire no magnetism by hammering, was put upon an anvil with a considerable quantity of crocus martis, which had no effect upon the needle. It was then hammered for a long time, turning it frequently, so that the crocus was beaten into the substance of the brass, and gave it a red colour; nevertheless, it affected the needle in this plate no more than before. 10. A hole of about an eighth part of an inch in length, and little more than one fifth of an inch in diameter, was drilled in a piece of brass which could not be rendered magnetic by hammering; after which the hole was filled with crocus martis, and hammered as before, but still it showed no signs of magnetism (A). 11. On making this piece of brass, containing the crocus, red hot, it then affected the needle, but only in that place where the crocus was. 12. On repeating this experiment with black calx of iron instead of crocus martis, the brass was weakly attracted in that place where the calx was, and this attraction was neither augmented nor diminished by calcination. 13. On mixing a small quantity of iron with four times its weight of brass which could not be made magnetic by hammering, the whole was rendered powerfully magnetic; but on again mixing this compound with 30 times its weight of the same brass, the attraction became so weak as to be scarcely perceptible; and was neither augmented by hammering nor diminished by softening. 14. On repeating most of his experiments, by letting the pieces of brass float upon quicksilver in the manner hereafter described, he found that very few of them were not affected; and even the indifference of any of them did not seem to be

(A) These two experiments seem inconsistent with our author's assertion, that calces of iron are always affected in some degree by the magnet. Substances be very well ascertained; though these did not acquire any additional magnetism by hammering.

From all these experiments Mr Cavallo draws the following conclusions. 1. Most brafs becomes magnetic by hammering, and loses that property by annealing or softening in the fire; or at least its magnetism is so far weakened by it, as afterwards to be only discovered when floating on quicksilver. 2. The acquired magnetism is not owing to particles of iron naturally or artificially mixed with the brafs. 3. The pieces of brafs which have that property retain it without any diminution after a great number of repeated trials; but he found no method of giving magnetism to brafs which had it not naturally. 4. A large piece of brafs has generally a stronger magnetic power than a small one; and the flat surface draws the needle more powerfully than the edge or corner. 5. If only one end of a piece of brafs be hammered, then that end alone will disturb the magnetic needle. 6. The magnetic power which brafs acquires by hammering has a certain limit, beyond which it cannot be increased by farther hammering. This limit is different in different pieces of brafs, according to their thickness or quality. 7. In the course of his experiments, the following circumstance was twice observed: A piece of brafs which had the property of becoming magnetic by hammering, and of losing that property by annealing, lost its magnetic power entirely by being left in the fire till partially melted, but recovered it again on being fully so. 8. A long continuance in a strong fire, which alters the texture of the metal, making it what some workmen call rotten, generally destroys the magnetic property also; whence this property seems to be owing to some particular configuration of its parts. 9. When brafs is used in magnetical instruments, it ought either to be left entirely soft, or chosen of such a sort as will not become magnetic by hammering. 10. There are few substances in nature, which, when floated upon quicksilver, are not affected in some degree by the magnet.

Our author next proceeded to try the magnetic power of other metals, particularly the component parts of brafs, &c., copper, and zinc. With the former the result was doubtful; and though pieces of hammered copper would sometimes attract the needle, yet the attraction was always exceedingly weak. Zinc had no effect, either in its natural state or hammered as much as it could bear without breaking. A mixture of it with tin had no effect. The same was observed of a piece of a broken reflector of a telescope made of tin and copper; a mixture of tin, zinc, and copper; a piece of silver whether soft or hammered; a piece of pure gold whether soft or hammered; a mixture of gold and silver, both hard and soft; and another mixture of much silver, a little copper, and a still less quantity of gold.

The magnetic property of nickel has been mentioned by several authors; but Mr Cavallo says he has found some pieces which did not affect the needle in the least. "It is probable (says he) that these pieces were not pure nickel, and perhaps some cobalt was contained in them; but I see no reason why the nickel, when alloyed with a little cobalt, should show no attraction towards the magnet, if that property did really and essentially belong to it." Our author, lastly, made several experiments upon platinum; the magnetic properties of which were found to be very similar to those of brafs; the native grains becoming magnetic by hammering, and losing that property by heat; but the precipitate from aqua-regia, fused in a violent fire, or rather concreted together by this means, showed no sign of attraction whatever.

§ 3. Of the Attraction of the Magnet towards Iron in its various States of Existence.

I. The first experiment which naturally occurs on this subject is, Whether mere heat can make any change in the magnetic properties of iron without destroying its texture or diminishing the power of the magnet to which it is applied. Kircher says, that he tried this experiment, and found that a piece of iron heated to such a degree as to be scarcely discernible from a burning coal, was in that state as powerfully attracted as if it had been cold. Mr Cavallo found the effect directly the reverse; for, having heated a piece of steel red hot, and in that state presented it to the magnet, so as to touch it repeatedly in various places, not the least sign of attraction could be perceived. In this experiment, the redness of the iron could plainly be perceived in day-light; and our author acknowledges, that iron, tho' its redness be perceptible in the dark, will still be attracted by the magnet. The result was the same on repeating the experiment a number of times over; but the attraction became as strong as ever a little after the redness ceased in the dark. The attraction seemed to begin sooner in steel than in iron. Our author does not pretend to say, that by heating iron to a red, or even to a white heat, the attraction of the magnet for it is absolutely annihilated; but it certainly was so far diminished that it did not affect the magnetic needle.

II. It was now tried what would be the effect of decomposing iron; and with this view an earthen vessel, containing about two ounces of iron-filings, was placed near the south end of the needle of the compass, by which the latter was drawn a little out of its direction. On adding some water, and then vitriolic acid, the attraction seemed to be increased, and the needle came nearer the vessel. This superior attraction continued till the effervescence began to cease; and at last it was found to be inferior to what it had been originally. To obviate some objections which might arise from the motion of the iron-filings, the experiment was repeated with steel-wire twisted in various directions, so as to present a large surface to the acid; and being placed at a proper distance from the needle, it attracted it out of its direction from $231^\circ$ to $280^\circ$. After adding the diluted vitriolic acid, a strong effervescence ensued, and the needle was moved to $279^\circ 47'$; five minutes after that it stood at $279^\circ 35'$; and in five minutes more at $279^\circ 35'$; seeming even to come somewhat nearer in a little time after; but as it then appeared to have gained its maximum of attraction, the pot was removed, and the needle went back to its original station of $281^\circ$.

On repeating this experiment with different acids, it was found that the vitriolic increased the attraction more than either the nitrous or marine. With the former of these the maximum of attraction was sooner gained. gained and sooner lost than with the rest; and with marine acid the attraction was weakest of all; which, however, our author imputes to his not being able to raise a sufficient effervescence with this acid.

III. The degree of magnetic attraction depends upon the strength of the magnet itself, the weight and shape of the iron presented to it, the magnetic or unmagnetic state of the body, and the distance between them. A piece of clean and soft iron is more powerfully attracted than any other ferruginous substance of the same size and shape. Steel is attracted less powerfully. The attraction is strongest at the poles, diminishing according to the distance from them, and entirely ceasing at the equator or middle point betwixt the poles. It is strongest near the surface of the magnet, diminishing as we recede from it; but the proportion in which this diminution takes place has not been exactly determined. M. Mulchenbroeck made the following experiments in order to determine this point.

1. A cylindrical magnet, two inches long, and weighing 16 drams, was suspended by an accurate balance above a cylinder of iron exactly of the same shape and dimensions, and the degree of attraction betwixt the two measured by weights put into the opposite scale; the magnet being successively placed at different distances from the iron. The results were as follow:

| Distance in inches | Attraction in grains | |-------------------|---------------------| | 6 | 3 | | 5 | 3½ | | 4 | 4½ | | 3 | 6 | | 2 | 9 | | 1 | 18 | | 0 | 57 |

2. A spherical magnet of the same diameter with the cylindrical one, but of greater strength, was affixed to one of the scales of the balance, and the cylindrical magnet used in the former experiment placed upon the table with its south pole upwards, facing the north pole of the spherical magnet; when the attractions were found as follow:

| Distance in inches | Attraction in grains | |-------------------|---------------------| | 6 | 21 | | 5 | 27 | | 4 | 34 | | 3 | 44 | | 2 | 64 | | 1 | 100 | | 0 | 260 |

3. Changing the cylindrical magnet for the iron cylinder abovementioned, the result was as follows:

| Distance in inches | Attraction in grains | |-------------------|---------------------| | 6 | 7 | | 5 | 9½ | | 4 | 15 | | 3 | 25 | | 2 | 45 | | 1 | 92 | | 0 | 340 |

IV. Using a globe of iron of the same diameter with the magnet instead of the cylinder, the results were:

| Distance in inches | Attraction in grains | |-------------------|---------------------| | 8 | 1 | | 7 | 2 | | 6 | 3½ | | 5 | 6 | | 4 | 9 | | 3 | 16 | | 2 | 30 | | 1 | 64 | | 0 | 290 |

In the experiments with the cylinder, it was found that the magnet attracted a shorter cylinder with less force, but in the same proportion.—From the others, it appears, that one magnet attracts another with less force than a piece of iron, but that the attraction begins from a greater distance; whence it must follow a different law of decrease.

IV. The attraction between the magnet and a piece of iron is subject to variation from the mere shape of the latter, there being a limit in the weight and shape of the iron, in which it will attract it more forcibly than any other; but this can only be determined by actual experiment.

V. It has already been observed, that magnetic attraction takes place only between the opposite poles of two magnets: however, it frequently happens, that though the north pole of one magnet be presented to the north pole of another, that they show neither attraction nor repulsion; but that when placed very near each other, they will attract. This is explained by our author in the following manner: "When a piece of iron, or any other substance that contains iron, is brought within a certain distance of a magnet, it becomes itself a magnet, having the poles, the attractive power, and, in short, every property of a real magnet. That part of it which is nearest to the magnet acquires a contrary polarity; but it often happens that one of the magnets, being more powerful than the other, will change the pole of that other magnet in the same manner as it gives magnetism to any other piece of iron which is exposed to its influence; and then an attraction will take place between two poles apparently of the same names; though, in fact, it is an attraction between poles of different names, because one of them has actually been changed. Thus, suppose that a powerful magnet has been placed with its north pole very near the north pole of a weak magnet, it will be found, that, instead of repelling, they will attract each other, because that part of the weak magnet which before was a north pole, has been changed into a south pole by the action of the strong magnet."

VI. Neither the attraction nor the repulsion of magnetism is sensibly affected by the interposition of bodies of any sort, excepting iron or ferruginous substances in general. Thus suppose, that, when a magnet is placed at an inch distance from a piece of iron, an ounce, or any determinate weight, is required to move it; the same will be required, though a plate of metal... metal, glass, or any other substance excepting iron be interposed. Neither the absence nor presence of air has any effect upon it.

VII. By heat, the power of a magnet is weakened; and when it arrives at that degree called a white heat, it is entirely destroyed. On the other hand, the attraction is increased considerably by adding more and more weight to the magnet: for thus it will be found that the magnet will keep suspended this day a little more weight than it did the day before; which additional weight being added to it on the following day, or some day after, it will be able to suspend a weight still greater, and so on as far as a certain limit. On the other hand, by an improper situation, or by diminishing the quantity of iron appended to it, the power will decrease very considerably.

VIII. The magnetic attraction is communicable to any given piece of steel only in a certain degree; and therefore if a magnet is strong enough to give the maximum of attraction to the piece, it cannot be afterwards rendered more powerful by applying another magnet, however strong. Thus, indeed, the steel may be made stronger for a few minutes; but this overplus of attraction begins to go off as soon as the strong magnet is withdrawn; and the power, continuing gradually to diminish, settles in a short time at that degree which is its limit ever after.

IX. Some have asserted, that in the northern parts of the world, the north pole of the magnet is stronger than the south pole, and that in the southern parts the contrary takes place; others are of a quite contrary opinion, affirming, that in the northern regions the south pole is stronger than the north one: but neither of these opinions have yet been sufficiently confirmed by experience.

X. If a piece of iron be held to one of the poles of a magnet, the attractive power of the other pole will thus be augmented: Hence we may understand why a magnet will lift a greater weight from a piece of iron than from wood or any other substance, viz., that the iron appended to the magnet becomes itself a magnet while it remains in that situation; and thus, having two poles, the iron which is placed near the one increases the attractive power of the other which adheres to the magnet, and enables it to sustain a greater weight than it would otherwise do.

XI. Soft iron acquires the magnetic power by being appended to a magnet; but it lasts only while the iron remains in that situation, vanishing as soon as the magnet and iron are separated from each other. With hard iron, but especially steel, the case is quite different; and the harder the iron or steel is, the more permanent is the magnetism which it acquires; though in proportion to this same hardness it is difficult to impregnate it with the virtue.

XII. The smallest natural magnets generally possess the greatest proportion of attractive power; so that there have frequently been seen magnets not weighing more than 20 or 30 grains, which would take up 40 or 50 times their own weight; but the greatest proportion of attractive power, perhaps ever known, belonged to the magnet worn by Sir Isaac Newton in his ring. It weighed only three grains, and was able to take up 746 grains, or nearly 250 times its own weight; and Mr Cavallo has seen one which could not weigh more than six or seven grains, and yet was capable of lifting 300. A semicircular steel magnet made by Mr Canton, weighing one ounce and 13 penny-weights, took up 90 ounces; but magnets of above two pounds seldom lift more than five or six times their own weight, or indeed seldom so much. It frequently happens, that a piece cut off from a large natural magnet will lift more than the stone itself did when whole; which is to be attributed to the heterogeneous nature of the stone itself; for if part of it be impure, it is plain that this can do nothing else than obstruct the virtue of the remainder, which consequently must act more powerfully when the obstruction is removed.

13. As the two magnetic poles taken together are capable of lifting a much greater weight than a single one, and as they are generally situated in opposite parts of its surface, it has been customary to adapt two broad pieces of soft iron to them, letting the pieces project on one side of the magnet; because, in that case, the pieces themselves being rendered magnetic, another piece of iron could be conveniently adapted to their projections so as to let both poles act in concert. These pieces of iron are generally held fast upon the magnet by means of a brass or silver box; in which case the magnet is said to be armed, and the pieces of iron are called its armature. For the same purpose, and to avoid the armature, artificial magnets have been commonly made in the shape of a horse-shoe, having their poles in the two extremities. This is by far the best shape for magnets; and the horse-shoe ones are always more powerful than straight magnetic bars.

§ 4. Of the Polarity of the Magnet.

Though, properly speaking, no magnet can have more than two poles, viz. a north and a south one, yet it frequently happens that both the natural and artificial kind are divided as it were into several magnets; each of which having likewise a north and south pole, the whole appears to have a number of poles, some of one denomination and some of the other.—This plurality of poles arises sometimes from the shape, but more commonly from the heterogeneous nature, of the magnet itself: and with respect to those which have more than two poles, the following laws have been observed: 1. That the parts adjacent to one pole are endowed with a contrary polarity. 2. That the poles of one denomination are not always equal in number, but that they never differ by more than one: thus if the magnet has four south poles, it will either have three, four, or five north poles. Good and properly shaped magnets, however, have only two poles directly opposite to one another; though in truth it is always one half, or at least a great part of the magnet, that possesses one kind of polarity, the other having the contrary kind; the two points, which we call the poles, being only those where the attractive virtue is strongest. Those two points, in good magnets, are joined by a line passing through the centre, which line is called the axis of the magnet; and a circle whose plane is perpendicular to the axis encompassing the middle of the magnet is called its equator; and to complete the supposed similarity between the terrestrial globe and magnetical bodies, the latter have frequently Chap. II. Theory of Magnetism.

The phenomena of magnetism, like those of electricity, depend on a cause too little subject to the investigation of our senses, that any regular and well supported theory can as yet scarcely be expected. The subject indeed is still more difficult than that of electricity; for in the latter the fluid is often made visible and otherwise perceptible by our senses; but no experiment could ever render the cause of magnetism perceptible otherwise than by its effects. The idea of its being occasioned by a fluid entering into one pole and puffing out at another, took its rise, and became pretty general, from the following experiment: Having put a small artificial magnet among some iron-filings laid upon a piece of paper, give the table a few gentle knocks with your hand, so as to shake the filings a little, and they will dispose of themselves as represented in fig. 1.

Vol. X. Part II, than can be done without it; at the same time that the iron itself loses all magnetic power the moment it is separated from the magnet. Hardened iron, cast-iron, and steel, perform a part analogous to that of electric; for the virtue does not easily pervade them, but is retained, and may be communicated by them to other unmagnetic pieces, in like manner as the electric virtue may be communicated to bodies by means of an excited electric. With regard to other substances, they seem not to be properly conductors of magnetism, because the fluid pervades them as though nothing were present, and they cannot transmit the virtue farther than it would go without them. With soft iron it is otherwise. Thus, if to one of the poles of a magnet we append a piece of iron of considerable length, the end farthest from the magnet will likewise attract iron with much more force than the magnet could do at that distance without it, while at the same time this attractive power is plainly that of the magnet itself, and not any way inherent in the iron, as it vanishes the moment we separate them. If a piece of hard steel of an equal length with the iron be appended to the magnet by one of its ends, we will find that the distant end will not manifest any attraction, and it will be a considerable time before the magnetic virtue can diffuse itself for any distance along it; but when the separation is made, the steel will be found to be magnetic, and will preserve its virtue for a long time.

5. The electric virtue exerts itself most powerfully on points, which are found to carry it off or receive it in vast quantities. In like manner a magnet will hold a piece of iron more powerfully by a corner, or blunt point, than by a flat surface. On sharp points indeed the magnet has but little hold by reason of the deficiency of surface.

6. From some experiments related under the article Electricity, it appears possible to superinduce the negative and positive electricities upon one another; and in magnetism it is possible to do the same. Thus, if we place a wire of some length upon a pivot, so that it can turn very easily, by touching both ends of it upon the poles of a magnet it will acquire a polarity; one end being repelled by one pole and attracted by the other. If now we give the north end, for instance, a very slight touch with the north pole of the magnet, we will find that it has a small degree of south magnetism superinduced upon it, so that on approaching the south pole of the magnet it will be repelled; but by approaching the magnet nearer, or holding the wire for a little from flying away, the south magnetism of the wire will be entirely destroyed, and the north magnetism appear as before. This experiment is not very easily made; its success depends on having the first magnetism as strong and the second as weak as possible.

These are the most remarkable particulars in which magnetism and electricity are found to agree; but the differences between them are no less remarkable than those particulars. The magnetic power affects none of our senses, and most perceptibly at least attracts only iron; while electricity attracts and repels bodies of every kind indiscriminately. The electric virtue resides on the surface, but that of the magnet pervades the whole substance. A magnet loses nothing of its power by communicating its virtue to other bodies, but electricity always does; and, lastly, the magnetic virtue is permanent; whereas that of electricity, without the greatest care, is exceedingly perishable, and capable of being dissipated.

Notwithstanding these disagreements, however, the analogies between magnetism and electricity are so great, that the hypothesis of a magnetic as well as of an electric fluid has now gained general credit; and upon this hypothesis Professor Epinus has attempted to solve the phenomena of magnetism in the following manner:

1. This fluid is sufficiently subtle to penetrate the substance of all terrestrial bodies, and like the electric fluid supposed to be repulsive of itself.

2. There is a mutual attraction between the magnetic fluid and iron, but an indifference between it and all other bodies.

3. There is a great resemblance between ferruginous bodies and electrics, as the magnetic fluid passes with difficulty through the former.

4. Iron and all ferruginous substances contain a quantity of magnetic fluid equably dispersed through their substance when those bodies are not magnetic. In this state they show neither attraction nor repulsion, because the repulsion between the particles of magnetic fluid is balanced by the attraction between the matter of those bodies and the fluid; in which case these bodies are said to be in a natural state: but when in a ferruginous body the quantity of magnetic fluid is driven to one, then the body becomes magnetic; one extremity of it being now overcharged with magnetic fluid and the other undercharged. Bodies thus constituted, viz. rendered magnetic, exert a repulsion between their overcharged extremities in virtue of the repulsion between the particles of that excess of magnetic fluid, which is more than overbalanced by the attraction of their matter. There is an attraction exerted between the overcharged extremity of one magnetic body and the undercharged extremity of the other, on account of the attraction between that fluid and the matter of the body: but to explain the repulsion which takes place between their undercharged extremities, we must either imagine that iron when deprived of the magnetic fluid is repulsive of itself, or that the undercharged extremities appear to repel each other only because either of them attracts the opposite overcharged extremities.

A ferruginous body, therefore, according to this hypothesis, is rendered magnetic by having the equable diffusion of magnetic fluid through its substance disturbed, so as to have an overplus of it in one or more parts and a deficiency in others, its magnetism remaining as long as its impermeability prevents the restoration of the balance between the overcharged and undercharged parts. A piece of iron is rendered magnetic by the vicinity of a magnet; because when the overcharged part or pole of the magnet is presented to it, the overplus of the magnetic fluid in that pole repels the fluid away from the nearest extremity of the iron; which therefore becomes undercharged, or possessed of the contrary polarity, to the most remote part of the iron, which consequently becomes overcharged, or possessed of the same polarity as the presented pole of the magnet. When the piece of iron is rendered magnetic by presenting it to the undercharged extremity or pole of the magnet, then the Theory.

part of the iron which is nearest to it becomes overcharged, &c., because that part of the magnet, being deprived of its magnetic fluid, attracts the magnetic fluid of the iron to that extremity of the iron which lies nearest to itself.

Hence, in order to give magnetism to a piece of steel, the strength of the magnet employed must be such as to overcome the resistance which the substance of the steel makes against the free passage of the magnetic fluid; hence a piece of soft steel is rendered magnetic more easily than a hard one, and a strong magnet will render magnetic such bodies as a weak one cannot affect. When two magnets of equal power have their opposite poles presented to each other, they mutually preserve and strengthen the powers of each other; but when poles of the same denomination are forced together, if the powers are equal, they mutually weaken each other; or if unequal, the weaker will have its poles altered, or perhaps its attractive power entirely destroyed in a short time.

Before we make any remarks upon this hypothesis, it will be necessary to take notice of another, which Mr. Cavallo considers as so well established, "that there can hardly be a philosopher sceptical enough to doubt of its truth." This is, that the earth itself is a magnet; which position, he says, is proved almost to a demonstration in the following manner.

1. Almost all the phenomena which may be exhibited with a common magnet may also be exhibited with the earth, as far as it can be tried. And,

2. Vast masses of iron or ferruginous matter actually magnetic are dug out of the earth almost in every part of it.

In support of the above position, he adduces the phenomena of the compass, dipping-needle, and the magnetism (to be afterwards explained) which soft iron receives when properly situated. All these may be imitated by a common magnet or terrella. An objection, however, occurs, that the most remarkable phenomenon of all, viz., the attraction of iron, is wanting. No experiment has yet shown that this metal is attracted more powerfully near the poles than at the equator itself; yet this ought very notably to be the case in such a large magnetic body. Our author indeed is of opinion, that if the experiment were tried with sufficient accuracy, the weight of the iron would be augmented by proceeding a considerable way either southward or northward. But besides that this hypothesis is as yet entirely unsupported by experiment, the difference he even supposes is quite trifling and insignificant. The dipping of the needle may indeed show that in this hemisphere there is a superiority of attraction between one end of the needle and the earth; but it remains to be proved whether this superiority resides in the needle or in the earth itself.

The following consideration indeed seems evidently to show that the power, whatever it is, resides in the needle itself; namely, that at the equator, the needle ought to remain in an east and west direction, if so placed; because of the equal attraction of the north and south poles. Were the needle carried to the pole itself, we can only suppose that it would point perpendicularly downwards; in every other case, the attraction will not be perpendicular, but oblique: and supposing us to recede from the point of perpendicular attraction only a few miles, the obliquity would become so great, that no attraction or repulsion towards that point would be distinguishable from an horizontal direction. The inclination of the needle therefore shows, that it is not actuated by the influence of a distant point in the earth; but by some power in the atmosphere immediately acting upon the needle, and directing its course either to the earth, or from it, in a certain position.

Those who maintain the magnetism of the earth, have been considerably embarrassed with some of the natural phenomena. The variation of the compass first showed that the needle was not influenced by those points on which the earth turns round in its diurnal course: but this was easily solved by another hypothesis, viz., that the earth had two magnetic poles by which the needle is influenced, and two others round which it turns on its axis. This hypothesis was likewise embarrassed by the continual shifting of the variation either to the eastward or westward. Hence another supposition was made by Dr. Halley; namely, that there is a large magnet inclosed within the body of the earth, which not being fixed to the external part, moved with respect to it, and of consequence occasioned the variation. This was likewise overthrown, by observing that the variation of the compass was irregular, and differed so much in different parts of the world, that it could not be owing to any regular cause diffused over the whole. Four magnetic poles were then supposed to lie within the earth, and to be moveable with respect to each other; and that therefore the variation, whose theory would now be very intricate, ought to be derived from all their actions conjointly: but, notwithstanding all this complication of poles, it might still be objected, that some kind of regularity, not observed in the variation of the magnetic compass, ought to have taken place. So that as yet there is no theory which seems to explain the variation with any kind of certainty.

The different hypotheses on this subject are more fully considered under the article Variation: here we shall only observe, that with respect to the magnetism of the earth, the particulars already related seem to decide against its existence. The most unequivocal proof we have of the existence of magnetism is the attraction of iron; and this capital mark is deficient, or at least has never been improved, in the earth. The poles of all the magnets, we know, are fixed and invariable; nor are we obliged to have recourse to magnets within magnets, or other unorthodox suppositions, to account for their phenomena: if the earth is a magnet, therefore, the magnetism it possesses must be of a kind so different from the property usually distinguished by that name, that we can in no respect determine them to be the same.

Mr. Cavallo is of opinion that "the magnetism of the earth arises from the magnetism of all the magnetic substances contained in it, and intermixed with other bodies; that the magnetic poles of the earth may be considered as the centres of the polarities of all the particular aggregates of the magnetic substances; and that those principal poles must change place relatively to the surface of the earth, according as the particular aggregates of magnetic substances within the earth are in some manner or other altered, so as to have their power diminished, increased, approached, Magnetism.

Theory.

proached, or removed from the principal poles." But this seems not by any means sufficient to account for the phenomena. The magnetic needle is indeed affected by iron at a distance, but that distance is by no means considerable. A magnet or needle in a house in one street will not be affected by a smith's shop or iron warehouse in another; and there is an undoubted certainty that the magnetic needle is affected on some parts of the sea where no magnetic bodies can be present unless at a great distance on land, or below the unfathomable depths of the ocean. Besides, let us imagine as many of these bodies as we please within the surface of the earth, they must be supposed, in order to account for the phenomena of the needle, to have their poles lying all nearly the same way; which can by no means be proved to be the case: not to mention that the attraction of iron would in some places be very perceptible, which has never yet been experienced in any part of the world.

Lastly, the hypothesis of the magnetism of the earth seems to be entirely overthrown by the following curious method of giving magnetism instantaneously to an iron-bar. Take a bar of soft iron two or three feet long, and between an half and two inches thick; which description is very well answered by some kitchen pokers. Place it in the magnetical line, i.e., the posture assumed by the dipping needle; or if a needle of this kind is not at hand, place it straight up in any degree of north or south latitude beyond 40°, or horizontally if nearer to the equator. Present then a magnetic needle to various parts of the bar; and it will be found, that in this country the lower half of the bar will repel the north end of the needle, and the upper half attract it. In south latitudes the case will be reversed; for the lower end will attract the north pole of the needle, and the upper end repel it. If the bar be not very short, its extremities will also attract small bits of iron, as filings, &c. On turning it upside down, the end which repelled the north pole of the needle before will now attract it; the reason of which is, that in the northern hemisphere the end which is nearest the earth always becomes a north pole, and in the southern hemisphere a south one. Now it is plain, that considering the distance of both poles of the earth from the iron-rod, any kind of posture in which we can place it must make a difference so trifling, that we cannot suppose the one to influence it more than the other. The whole phenomenon shows that there is in the atmosphere a current of fluid either going into the earth, or coming out from it, which influences iron when held in the direction in which itself moves. That it does not influence the metal when lying horizontally, may be owing to its want of sufficient breadth to render the effect perceptible. The earth therefore is not a magnet, but is surrounded by a fluid whose motion is productive of magnetism in iron; and most probably, though it produces this as it were accidentally, will be found to answer much more important purposes in the economy of nature. The next question then is with regard to the fluid itself; and this, from many articles in this work, will appear to be the same with that of electricity. Under the article Aurora Borealis, Earthquake, Electricity, &c., it is shown, that the solar light, absorbed by the equatorial regions of the earth, becomes subject to new laws of motion, acting in short as if it were another fluid, in which state we call it electricity, or the electric fluid. In this state it passes through the substance of the earth from the equator towards the polar regions, getting out again in the vicinity of the poles, ascending into the high atmospherical regions, and then returning to the equatorial parts from whence it came. On this supposition, which appears to be greatly confirmed by various natural phenomena, it is easy to see, why in the northern and southern parts the direction of the currents issuing from the earth should always become more and more perpendicular to the earth as we approach the poles, and on the contrary why their direction must be horizontal or nearly so in the equatorial parts. The discovery of this general cause therefore seems to be the nearest approach we can as yet make to the knowledge of the origin of magnetical phenomena. In what manner iron more than other metals is influenced by this fluid, or why the direction of a current of electric matter either to or from the earth, should cause such strong attractions as magnetical bodies are sometimes endowed with, we have as yet no data for understanding.

Aepinus's theory of an accumulation of the electric fluid in one pole, and a deficiency of it in the other, seems not to be tenable in any respect. It is impossible to show why the mere turning of a bar upside down should accumulate the fluid, unless it was a gravitating one in the end next the earth; and though we should even make this extravagant supposition, it will be as difficult to account for the very same fluid being repelled by the earth in the southern hemisphere: for if we account the north magnetism an accumulation, we must count the south one a deficiency; or if the south magnetism is an accumulation, the opposite one must be a deficiency; and whichever supposition we adhere to, the difficulties are equally great and unsurmountable.

Chap. III. Practice of Magnetism.

This consists in communicating the magnetic virtue from one body to another; making artificial magnets, compasses, dipping-needles, &c.; and investigating the various phenomena resulting from bodies placed in different situations.

§ 1. To communicate Magnetism by the Loadstone.

Magnetism is communicated merely by presenting a piece of iron or steel to one of the poles of a magnet or loadstone, even without touching it; though a strong and permanent power cannot be given without contact, or even stroking the one upon the other for a number of times. In this operation, that part of the ferruginous body which touches the pole of the magnet acquires the contrary magnetism; that is, if it touches the north pole, it will turn towards the south, et vice versa. The power acquired is strongest when soft iron is applied, weaker with hardened iron, and weakest of all with hard steel: but the permanency of it follows just the reverse of this rule; for steel or hardened iron will preserve its virtue for many years, but soft iron loses it the moment we withdraw the magnet. When we desire a strong and permanent virtue, therefore, it is best to use the hardest steel, and to impregnate it by means of one or more pow- The operation of communicating magnetism to pieces of steel or iron, is called touching them; and as this is of the utmost utility in navigation, for the purpose of giving polarity to needles, very considerable pains have been bestowed upon the subject, in order to discover the methods of giving them the magnetic virtue in the most effectual and permanent manner.

When only one magnetic bar is to be made use of, one of its poles must be applied as represented fig. 2, where CD represents the needle or steel bar to be impregnated. The magnet AB is then to be drawn all along the surface of it, till it reaches the extremity D. The magnet being then removed, must be applied to the extremity C, and drawn over the needle as before. Thus the needle must be rubbed several times; by which means it will acquire a considerable degree of magnetism. In this method, that other extremity of the needle which the magnet touched last acquires the contrary magnetism; that is, if B be the north pole of the magnet, C will be the north pole, and D the south of the needle. This method, however, is never found to be equally effectual with that in which two magnets, or both poles of one magnet, are made use of.

To communicate magnetism by means of two magnetic bars, place the bar or needle AB, fig. 3, upon a table; then set the two magnetic bars CD, EF, straight upright upon it at a little distance, equal on both sides from the middle of the bar AB, and in such a manner that the south pole D of one of the bars may be nearest to that end of the bar AB which is to become the north pole, &c. These two bars must then be slid gradually towards one extremity of the bar, keeping them constantly at the same distance from each other; and when one of them, for instance CD, is arrived at A, then they must be slid the contrary way, till EF arrives at B; and thus the bar AB must be rubbed a greater or smaller number of times, till it will be found by trial to have acquired a considerable power. When the magnetic bars are powerful, and the bar AB of very good steel, and not very large, a dozen of strokes are fully sufficient; but when the bars are to be removed from the bar AB, care must be taken to bring them to the same situation where they were first placed; viz. at a little and equal distance from the middle of the bar AB, from whence they may be lifted up.

If it be required to communicate the greatest magnetic power possible, we may proceed in the following manner: 1. The magnetic bars may be joined at top, as in fig. 4, interposing a piece of wood, or any other substance excepting iron; for thus the opposite poles being contiguous in the upper part, strengthen each other, and of consequence the lower ones are also strengthened. 2. The bar to be rendered magnetic may be placed between the bars of soft iron, as shown in the same figure. 3. The magnetic bars may be inclined the contrary way, as recommended by Mr. AEpinus, making an angle of about 15 degrees with the bar AB. See fig. 5. In the same manner may a bar be rendered magnetic by an armed or horse-hoe magnet. In any of the methods hitherto mentioned, however, the bar to be rendered magnetic must be stroked on every side; and to let the magnetic centre fall just in its middle, care must be taken to stroke one half of the bar just as much as the other. Whenever a steel bar, or, in general, any piece of ferruginous matter, is rendered magnetic by the application of two bars, or by the two poles of one magnet, the operation is called the double touch, but the single touch when only one bar is applied.

Artificial magnets of a semicircular form, or shaped like a horse-shoe, have the magnetism communicated to them in the same manner with those which are straight, only the magnetic bars used for this purpose must follow the curvature of the bar to be impregnated. Thus, suppose it is required to impregnate the crooked piece of steel ABC, fig. 6, lay it flat on a table, and to its extremities apply the magnets DF, EG, joining their extremities FG with the conductor or piece of soft iron FG. Apply then the magnetic bars HI to the middle of the piece ABC, and stroke it with them from end to end, following the direction of the bent steel, so that on one side of it the magnetic bars may stand as represented by the dotted lines LK. When the piece of steel has been thus rubbed a sufficient number of times on one side, it is then to be turned, and rubbed in like manner on the other, until it has acquired a sufficient degree of magnetism.

From considering that soft iron, or soft steel, acquires magnetism very easily, though it loses it with equal facility, Mr. Cavallo was induced to suppose, that if magnetism were to be communicated to a piece of hard steel while softened by heat, and the metal were then to be hardened by pouring cold water upon it while in the act of receiving the magnetism, it was possible the virtue might be first communicated to them in a very high degree, and then be fixed by means of the hardening of the steel. To determine this matter, six magnetic bars were placed in an oblong earthen vessel, in such a manner that the north poles of three of them might be opposite to the south poles of the three others, forming two parcels of bars lying in the same direction, and about three inches asunder, which was nearly the length of the steel-bar intended to be rendered magnetic. The bar was made quite red hot, and in that state was placed between the magnetic bars. Cold water was then immediately poured upon it; by which it was hardened to such a degree that the file could not touch it; but though it had thus received a considerable degree of magnetism, the power was not superior to what might have been communicated in the ordinary way. On repeating the experiment with steel-bars of different sizes, it was found that short bars receive a proportionally greater degree of power than long ones, and that because the latter cannot be sufficiently penetrated by the magnetic power when the magnets are placed at their ends; and if a number of magnets be placed along the sides, in order to communicate a greater degree of virtue, it frequently happens that the bar acquires a number of poles. Our author is nevertheless of opinion, that this method is of considerable use; though by it we cannot communicate... communicate any extraordinary degree of magnetism, it is yet very useful in constructing large artificial magnets. For thus they will acquire a considerable degree of power, without any additional trouble to the workman, and may then be fully impregnated in the usual way, which cannot be done without a great deal of labour when the operation is begun upon bars which have no virtue at all.

§ 2. To communicate the Magnetic Virtue without any Magnet either natural or artificial.

This may be done with a soft iron-bar in the manner already related, viz. by turning it in a position perpendicular to the surface of the earth, or any other excepting a line directly perpendicular to the dipping needle. The magnetism thus acquired, however, is always weak, and is instantaneously lost; while a steel bar will not receive any perceptible degree of magnetism by this method. But if an iron-bar be made red hot, and left to cool in the magnetic line, or if it be repeatedly struck with a hammer while in that line, it will acquire a small degree of permanent magnetism; though this also will soon vanish by leaving the bar in an improper position, or by inverting and striking it again. The magnetism lasts longer in proportion to the hardness of the iron; but a longer time will be required to give it the degree of virtue it is capable of receiving by this method. If an iron bar is left for a long time in the direction of the magnetic line, or even in a perpendicular posture, it will sometimes acquire a great degree of power. Mr Boyle makes mention of an iron-bar, ten feet long, which had acquired so much virtue by standing in this posture, that it exceeded a loadstone of three pounds and an half weight, and would turn the needle at eight or ten feet distance. Even tongs, pokers, and other kitchen utensils, by being often heated, and set to cool again in an erect posture, are frequently observed to gain a magnetic virtue. Sometimes iron-bars, which were not capable of receiving permanent magnetism on account of their softness, have, merely by exposure to the atmosphere for a great length of time, acquired a considerable degree of power; at the same time it has been remarked, that these bars became much harder by this exposure; the cause of which has not yet been discovered.

Iron or steel acquires a very perceptible degree of magnetism by drilling, hammering, or other methods by which they are put into violent action. The cause of this magnetism Mr Cavallo looks for in the earth itself, the changeable nature of the metal by heat or cold, and the vibratory motion into which its parts are accidentally put. "For the same reasons (says he) it seems that magnetism, in certain cases, is produced by electricity; the particulars observed concerning which are the following:—When the bar or needle is laid horizontally in the magnetic meridian, whichever way the shock of an electric jar or battery enters, the end of the needle which lies towards the north acquires the north polarity, viz. the power of turning towards the north when freely suspended, the other end acquiring the south polarity. If the bar before it receives the shock has some polarity, and is placed with its poles contrary to the usual direction, then its original polarity is always diminished, and sometimes reversed. When the needle is struck standing perpendicularly in this hemisphere, the lower end becomes the north pole, even when it had some magnetism before, and receives the shock while standing with its south pole downwards. When all other circumstances are alike, the degree of magnetism received seems to be the same, whether the needles are struck while standing horizontally in the magnetic meridian or perpendicular to the horizon. When a needle is placed in the magnetic equator, a shock through its length very seldom renders it magnetic; but if the shock be passed through its width, it acquires the virtue, the extremity which lay towards the west generally becoming the north pole. If a needle or bar strongly magnetic, or a natural magnet, be struck by the electric shock, its power is thereby diminished. When the shock is too strong, so that the needle is thereby rendered considerably hot, it acquires either no magnetism at all or a very small degree of it. Hence a stroke of lightning often renders pieces of iron or steel magnetic, as well as those bodies which naturally contain iron, as some bricks, &c."

There are various methods of communicating a permanent magnetism to ferruginous bodies, by means of a bar rendered magnetic by the earth; of which the most simple is that described by Mr Marcel, whose experiments were made in the year 1726. Being employed in making some observations on the magnetic power which he found in great pieces of iron, he took a large vice weighing 90 pounds, in which he fixed a small anvil weighing 12 pounds. The steel to which he wished to give the magnetic virtue was laid upon the anvil in a north and south position, which happened to be the diagonal of the square surface of the latter. He then took a piece of iron an inch square, and 33 inches long, weighing about eight pounds, having one end rounded and brightly polished, the other being tapered. Holding then the steel fast upon the anvil with one hand, he took the iron-bar in the other, and holding it perpendicularly, he rubbed the steel hard with the rounded part towards him from north to south, always carrying the bar far enough round about to begin again at the north. Having thus given 10 or 12 strokes, the steel was turned upside down, and rubbed as much on the other side. Proceeding in this manner till it had been rubbed 400 times, the steel was as strongly magnetic as if it had been touched by a powerful loadstone. The place where he began to rub was always the north pole. In these experiments it sometimes happened that the virtue was imparted by a few strokes; nay, by a single one, a small needle was made to receive a very considerable power. Thus he imparted to two compass needles such a degree of magnetic power, that one took up this and another a whole ounce of iron; and though these needles were anointed with linseed oil to keep them from rusting, and a hard coat was thus formed upon them, they nevertheless retained their virtue. Thus also a knife was made so strongly magnetic, that it would take up an ounce and three quarters of iron. Four small pieces of steel, each an inch long and 1/8th of an inch broad, as thin as the spring of a watch, were thus impregnated with the magnetic virtue, and then joined into a small artificial magnet; which at its first formation took up eight times its own own weight of iron; and after being six years kept in the most careless manner, was found to have rather gained than lost any thing of its virtue. In the course of his experiments, Mr Marcel found, that the end at which he began to rub was always the north pole, whatever position the steel was laid in. On rubbing a piece of steel from one end to the middle, and then from the other end to the middle, it acquired two north poles, one at each end, the middle being a fourth pole. Beginning to rub from the middle towards each end, he found a north pole in the middle and a south pole at each extremity.

Magnetism may be communicated to a small piece of soft steel in the following manner. Take two iron bars of about an inch square, and upwards of three feet in length, keep them in the magnetical line, or in a perpendicular posture, as represented fig. 7. Let the piece of steel CB be either fastened to the edge of a table or held by an assistant; and placing the lower extremity of the bar AB, and the upper extremity of the bar CD, on opposite sides, and in the middle of the steel, from the latter from the middle towards its extremities, moving both bars at the same time. When both are arrived at the extremities of the steel, remove them from it, and apply them again to the middle. Do so for 40 or 50 times, and the steel will be found to have a considerable degree of magnetic power. Care, however, must be taken, in removing the bars, not to draw them along the surface of the steel, or the experiment will not succeed, because the magnetism is destroyed by the contrary strokes.

The late Dr Godwin Knight possessed a surprising skill in magnetism, being able to communicate an extraordinary degree of attractive or repulsive virtue, and to alter or reverse the poles at pleasure; but as he refused to discover his methods upon any terms whatever (even, as he said, though he should receive in return as many guineas as he could carry), these curious and valuable secrets have died with him. In the 69th volume of the Philosophical Transactions, however, Mr Benjamin Wilson hath given a process which at least discovers one of the leading principles of Dr Knight's art, and may perhaps be a means of discovering the whole to those who shall be less reserved. The doctor's process, according to Mr Wilson, was as follows. Having provided himself with a great quantity of clean iron filings, he put them into a large tub that was more than one third filled with clean water; he then, with great labour, worked the tub to and fro for many hours together, that the friction between the grains of iron by this treatment might break off such smaller parts as would remain suspended in the water for a time. The obtaining of these very small particles in sufficient quantity seemed to him to be one of the principal desiderata in the experiment. The water being by this treatment rendered very muddy, he poured the same into a clean iron vessel, leaving the filings behind; and when the water had stood long enough to become clear, he poured it out carefully, without disturbing such of the sediment as still remained, which now appeared reduced almost to impalpable powder. This powder was afterwards removed into another vessel in order to dry it; but as he had not obtained a proper quantity thereof by this one step, he was obliged to repeat the process many times. Having at last procured enough of this very fine powder, the next thing was to make paste of it, and that with some vehicle which would contain a considerable quantity of the phlogistic principle: for this purpose, he had recourse to linseed oil in preference to all other fluids. With these two ingredients only he made a stiff paste, and took particular care to knead it well before he moulded it into convenient shapes. Sometimes, while the paste continued in its soft state he would put the impression of a seal upon the several pieces; one of which is in the British Museum. This paste was then put upon wood, and sometimes on tiles, in order to bake or dry it before a moderate fire, at about the distance of a foot or thereabouts. He found that a moderate fire was most proper, because a greater degree of heat made the composition frequently crack in many places. The time required for the baking or drying of this paste was generally about five or six hours before it attained a sufficient degree of hardness. When that was done, and the several baked pieces were become cold, he gave them their magnetic virtue in any direction he pleased, by placing them between the extreme ends of his large magazine of artificial magnets for a few seconds or more as he saw occasion. By this method the virtue they acquired was such, that, when any of those pieces were held between two of his best ten-guinea bars, with its poles purposely inverted, it immediately of itself turned about to recover its natural direction, which the force of those very powerful bars was not sufficient to counteract.

In the 66th volume of the Philosophical Transactions we have the following account, from Dr Fothergill, of Dr Knight's method of imitating natural magnets, but which is by Mr Cavallo supposed to be owing to some mistake or misinformation. "I do not know (says he), that ever the doctor (Dr Knight) left behind him any description of a composition he had made to form artificial lodestones. I have seen in his possession, and many other of his friends have likewise seen, such a composition; which retained the magnetic virtue in a manner much more fixed than either any real lodestone or any magnetic bar however well tempered. In the natural ones he could change the poles in an instant, so likewise in the hardest bars; but in the composition the poles were immovable. He had several small pieces of this composition which had strong magnetic powers. The largest was about half an inch in breadth, very little longer than broad, and near a quarter of an inch thick. It was not armed, but the ends were powerfully magnetic; nor could the poles be altered, tho' it was placed between two of his largest bars, and they were very strongly impregnated. The mass was not very heavy, and had much the appearance of a piece of black lead, though not quite so shining. I believe he never divulged this composition; but I think he once told me, the basis of it was filings of iron reduced by long continued attrition to a perfectly impalpable slate, and then incorporated with some phlogistic matter to give it due consistence."

From these accounts it appears that the basis of Dr Knight's artificial lodestones was the black powder to which iron filings are reduced by water, and which is known among the apothecaries by the name of Martial Ethiops; whence Mr Cavallo gives the following receipt for imitating the natural magnets.—"Take some martial ethiops, or, which is more easily easily procured, reduce into very fine powder the scales of iron, which fall from red-hot iron when hammered, and are found abundantly in smiths shops. Mix this powder with drying linseed oil, so as to form it into a very stiff paste, and shape it in a mould so as to give it any form you require; whether of a terracotta, a human head, or any other. This done, put it into a warm place for some weeks, and it will dry so as to become very hard; then render it magnetic by the application of powerful magnets, and it will acquire a considerable power."

As to the method of making artificial magnets of steel, none has succeeded in it better than Mr Canton, whose process is as follows.

Procure a dozen of bars; fix of soft steel, each three inches long, one quarter of an inch broad, and one twentieth of an inch thick; with two pieces of iron, each half the length of one of the bars, but of the same breadth and thickness: also six pieces of hard steel, each five inches and a half long, half an inch broad, and three-twentieths of an inch thick; with two pieces of iron of half the length, but the whole breadth and thickness of one of the hard bars; and let all the bars be marked with a line quite round them at one end. Then take an iron poker and tongs (fig. 8.), or two bars of iron, the larger they are and the longer they have been used, the better; and fixing the poker upright between the knees, hold to it, near the top, one of the soft bars, having its marked end downwards, by a piece of sewing silk, which must be pulled tight by the left hand, that the bar may not slide; then grasping the tongs with the right hand, a little below the middle, and holding them nearly in a vertical position, let the bar be stroked by the lower end from the bottom to the top, about ten times on each side, which will give it a magnetic power sufficient to lift a small key at the marked end: which end, if the bar was suspended on a point, would turn towards the north, and is therefore called the north pole; and the unmarked end is, for the same reason, called the south pole. Four of the soft bars being impregnated after this manner, lay the two (fig. 9.) parallel to each other, at the distance of one fourth of an inch, between the two pieces of iron belonging to them, a north and a south pole against each piece of iron: then take two of the four bars already made magnetic, and place them together so as to make a double bar in thickness, the north pole of one even with the south pole of the other; and the remaining two being put to these, one on each side, so as to have two north and two south poles together; separate the north from the south poles at one end by a large pin, and place them perpendicularly with that end downward on the middle of one of the parallel bars, the two north poles towards its south and the two south poles towards its north end: slide them backward and forward three or four times the whole length of the bar, and removing them from the middle of this, place them on the middle of the other bar as before directed, and go over that in the same manner; then turn both the bars the other side upwards, and repeat the former operation: this being done, take the two from between the pieces of iron; and, placing the two outermost of the touching bars in the room, let the other two be the outermost of the four to touch these with; and this process being repeated till each pair of bars have been touched three or four times over, which will give them a considerable magnetic power, put the half-dozen together after the manner of the four (fig. 10.), and touch them with two pair of the hard bars placed between their irons, at the distance of about half an inch from each other: then lay the soft bars aside; and with the four hard ones let the other two be impregnated (fig. 11.), holding the touching bars apart at the lower end near two tenths of an inch; to which distance let them be separated after they are set on the parallel bar, and brought together again before they are taken off: this being observed, proceed according to the method described above, till each pair have been touched two or three times over. But as this vertical way of touching a bar will not give it quite so much of the magnetic virtue as it will receive, let each pair be now touched once or twice over in their parallel position between the irons (fig. 12.), with two of the bars held horizontally, or nearly so, by drawing at the same time the north pole of one from the middle over the south end, and the south of the other from the middle over the north end of a parallel bar; then bringing them to the middle again, without touching the parallel bar, give three or four of these horizontal strokes to each side. The horizontal touch, after the vertical, will make the bars as strong as they possibly can be made, as appears by their not receiving any additional strength, when the vertical touch is given by a great number of bars, and the horizontal by those of a superior magnetic power. This whole process may be gone through in about half an hour; and each of the large bars, if well hardened, may be made to lift 23 Troy ounces, and sometimes more. And when these bars are thus impregnated, they will give to an hard bar of the same size its full virtue in less than two minutes; and therefore will answer all the purposes of magnetism in navigation and experimental philosophy much better than the loadstone, which is known not to have a sufficient power to impregnate hard bars. The half dozen being put into a case (fig. 13.) in such a manner as that two poles of the same denomination may not be together, and their irons with them as one bar, they will retain the virtues they have received; but if their power should, by making experiments, be ever so far impaired, it may be restored without any foreign assistance in a few minutes. And if, out of curiosity, a much larger set of bars should be required, these will communicate to them a sufficient power to proceed with; and they may, in a short time, by the same method, be brought to their full strength.

To expedite the process of making magnets, the bars should be fixed in a groove, or between brass pins, to prevent them from sliding; or they may be kept steady by means of a weight and ruler, as in fig. 11.

§ 3. Apparatus for making Experiments in Magnetism, with an Account of various Experiments tending to illustrate and prove the Laws already laid down.

The apparatus necessary in magnetics is but small; consisting only of a few magnets or magnetic bars, a magnetic horizontal needle or compass, and a dipping needle. For those who do not intend to be very accurate, a common artificial horse-hoe magnet and a few sewing needles may be sufficient; but where greater accuracy is required, it will then be necessary to have have a good set of magnetic bars, commonly six; a few small magnetic needles, a larger needle in a box with a graduated circle, and a dipping needle; to which may be added some pieces of steel-wire, a few bars of soft iron, &c.

The magnetic bars ought to be made of the best steel, and tempered quite hard. There is not, however, any method known as yet by which we can distinguish the kind of steel which is best for magnetical purposes. It will be proper, therefore, previous to the construction of the bars, to try the quality of the metal in the following manner: Take a piece of it about three inches long and a quarter of an inch thick, no matter whether round or square; make it red-hot, and in that condition plunge it into cold water, which hardens it so that a file will not touch it. Apply then two powerful magnetic bars; holding the north pole of one to one extremity of the steel, and the south-pole of the other magnet to the other extremity of the steel. Having kept them in this position for about a minute, separate them from the steel, and then try whether it will keep suspended a key or other piece of iron which may be at hand. By treating in this manner pieces of different steel, it will easily be perceived which is capable of lifting the greatest weight, and consequently the most proper for the construction of the bars.

Having determined the quality of the material, the next thing to be considered is the shape of the bars; for unless the length and breadth of them bear a certain proportion to each other, they will not be capable of receiving their utmost power. The best shape, according to Mr Cavallo, is when the length is ten times the breadth and twenty times the thickness. The usual dimensions are five inches in length, half an inch in breadth, and a quarter of an inch in thickness. Cylindrical bars are less convenient.—It is not absolutely necessary to polish these bars; though it will be better to do so, they being in this state much less liable to rust. One extremity is generally marked with a line all round, to distinguish one pole from another; and it is the north pole which is usually marked in this manner. When kept together, the magnetic bars must be placed alternately with the marked end of one contiguous to the unmarked end of the other. Two pieces of soft iron called supports always belong to each set of bars. Each of these is equal in size to the half of one of the bars; so that when placed contiguous to one another in one direction, they may equal one of the bars. These are useful when other bodies are to be rendered magnetic. For the construction of the Compass and Dipping-Needle, see these articles.

Experiments with the above described Apparatus.

1. To determine whether any substance is attracted by the magnet or not.—If the substance to be examined contains iron, the attraction will evidently show itself on bringing near it one of the magnetic bars. The quantity of attraction will always be known by the force requisite to separate them, and its proportion is estimated by the degree of that force. Thus if two ounces are required to separate a magnet from any substance, the degree of attraction is reckoned double to that which requires only one ounce to separate them. If the attraction be so small that it cannot be perceived in this way, it must be put to swim upon water in an earthen or wooden vessel, by means of a piece of wood or cork. In this way the attraction will be much more easily manifested by the body coming towards the magnet when approached to it. It will sometimes be necessary to bring the magnet within one-tenth part of an inch of the body to be attracted; and as the latter advances, care must be taken to withdraw the magnet; for if they be suffered to strike against each other, the body, if hard, will generally recede; and it will likewise be proper to present the magnet to the body when the latter is at rest.

By letting the substances to be attracted swim upon quicksilver, a still smaller degree of attraction can be perceived. In using this fluid, the following particulars must be attended to. 1. The aperture of the vessel in which the quicksilver is kept must be at least five inches in diameter. The reason of this is, that, as the surface of the quicksilver descends near the sides of the vessel, the curvature of surface formed by that descent is proportionably greater in the narrow vessels than larger ones. If the vessel is only three or four inches in diameter, the body to be attracted will perpetually run from one side to another; a common soup-plate, however, will be found a very convenient vessel for this purpose. 2. It will be necessary to have the quicksilver very pure; and as it is very difficult to preserve it in that state, it must be frequently passed through a piece of writing paper rolled up conically, and having a small aperture of about half of an inch diameter in the lower part. 3. The neighbouring air must not be disturbed, that the body may be kept without motion; and, while in this state, one of the poles of the magnet is to be presented to it in the same manner as when the experiment is tried with water. It was in this manner that Mr Cavallo made his experiments on the magnetism of brass and other metals, of which we have already given an account.

If it be suspected that the given body have some magnetism already, the very same process is required; only observing to present a piece of soft and clean iron to the body when swimming upon water or quicksilver. A piece of iron about half an ounce weight, and an inch in length, will be very proper for this purpose.

2. To find the poles of a magnetic body.—Present the various parts of the body successively to one of the poles of a magnetic needle, and it will soon be discovered which parts of the given body are possessed of a contrary polarity by the needle's standing perpendicularly towards them. One of the poles being thus discovered, turn the opposite pole of the magnetic needle towards the body, and it will soon find out its other pole. When the magnetism of the body to be examined is very weak, there will be danger of reversing the polarity by bringing the needle too near; and as the distance at which this effect will take place cannot be determined, it will always be proper to keep it so far distant that it can only sensibly affect the needle. Where there are only two poles, they may be found out merely by sprinkling some iron-flakes upon the body; for these will stand erect upon the polar points. They may be distinguished by setting the body to float in water, or tying it to a thread and letting it hang freely, freely, so that one may turn towards the north and the other towards the south. This method, however, will not succeed when there are more than two poles, nor even very well in that case, unless they lie in parts directly opposite to one another.

3. Effects of the magnet on soft iron.—Having placed a magnetic needle upon a table, bring a bar of soft iron about eight inches long and a quarter of an inch thick, so near that it may draw one end of the needle a little out of the way. In this situation approach gradually the north pole of a magnet to the other extremity of the bar, and the north end of the needle will recede from the bar more and more in proportion as the magnet is brought nearer the bar. If the experiment be repeated with the other pole of the magnet, the north end of the needle will then be attracted by the bar. The reason of this is, that when we bring the north pole of the magnet towards one end of the bar, the latter acquires a south polarity, and the other one of course a north polarity. Hence the needle is repelled, because magnetic poles of the same kind repel one another; but when the south pole is brought near the end of the bar, that end which it approaches receives the north polarity, and the other of course the south; whence the needle, instead of being repelled, is now attracted. By approaching a small magnetic needle to different parts of the bar, it will be found that one half of it possesses one kind of polarity, and the other the contrary kind; the magnetic centre, however, or the limit betwixt the two polarities, is not always in the middle of the bar, but is generally nearer that end which is presented to the magnet. The difference increases as the bar is lengthened; and when the latter exceeds a certain length, it acquires several poles. This depends on the strength of the magnet; and when it happens, the first magnetic centre comes very near to the end of the bar which stands next the magnet, and successive centres are formed betwixt every two poles. Thus, supposing the north pole of a magnet to be brought to the end of such a bar, the end it touches becomes a south pole; a few inches farther a north polarity takes place, after that a south polarity, and so on. The poles become weaker and weaker as they recede from the end which the magnet touches; so that if the bar be of considerable length, they totally vanish long before they come to the other end. Hence, by applying a magnet to one end of a long bar, we will not thereby give any magnetism to the other; and this will happen when a magnet capable of lifting two pounds of iron is applied to a bar of about an inch square and five feet long.

4. The action of magnetism shown by the repulsion of two pieces of wire.—Tie two pieces of soft wire each to a separate thread, and having suspended them close by each other, bring one of the poles of a magnet under them, and they will immediately repel; the divergency becoming greater as the magnet is brought nearer within a certain limit, and will decrease as the magnet is removed. If steel-wires or common sewing needles be used, the repulsion will continue for a considerable time after the magnet is removed; and this divergency will even be greater after the removal of the magnet, as its attraction tends to draw them nearer each other; and, if brought too near, no repulsion will be shown by them. The experiment may be agreeably diversified by using four or more needles, and presenting a north pole to one pair and a south pole to another, &c.

5. In what circumstances a magnet can lift the greatest weight.—By means of a crooked wire we may show that the power of a magnet varies according to circumstances. Thus, let a piece of wire about a quarter of an inch in diameter, and four or five inches long, be bent in the manner represented by ACB, fig. 14, with a sharp corner at C. Tie it fast to a cross bar, or let it be held by an affitant with the corner downwards. Then apply either pole of the magnet DE to one of its extremities; and if in this situation a small piece of iron, as H, be put to the corner C, it will remain suspended. On applying the contrary pole of another magnet to the other extremity of the wire, the piece of iron will immediately fall off; but if a pole of the same kind be applied, it will not only be still kept suspended, but be more strongly attracted than before.

In the case just mentioned, the first magnet is assisted by the action of the second; but in order to strengthen a magnet in this manner, it does not appear necessary to use a magnet at all. Thus, having found by trial how much a magnetic bar can lift, procure an oblong piece of iron about four inches long, and somewhat heavier than the bar can bear. Apply one end of this to the pole of the bar, holding it with your hand till you place under the other end a larger piece of iron. It will then be found that the magnet will support the piece of iron which it could not do before. The lower piece of iron is to be placed between an half and three quarters of an inch below the under part of the oblong piece which hangs at the magnet. The same effect will be produced by the opposite pole of another magnet; but a pole of the same denomination would weaken the attraction.

6. The generation of poles, and of magnetic centres in the parts of a broken magnet.—Take a magnetic bar about six or eight inches long and a quarter of an inch diameter, whose magnetic centre will be in the middle, or near it. Break off about one third part by a smart stroke of an hammer, and it will be found that the broken part, though in the magnet it had but one polarity, will now have acquired a north and south pole, with a magnetic centre, as if it were a distinct magnet. The experiment may be diversified as follows: Having made a steel bar about six inches long and a quarter of an inch thick quite hard, break it into two unequal parts. Join these, and press them hard together, giving it the magnetic virtue at the same time by means of two powerful magnets: while the parts remain in this position, so that the bar looks as if it had not been broke, it will have only two poles; but as soon as they are separated, each part will be found to become a distinct magnet, having a north and south pole proper to itself.

7. To remove the magnetic centre in a magnet.—This may be done in various ways; as, by striking a magnetic bar repeatedly, heating it, hard rubbing, &c.; but in all these methods the magnetism of the bar is diminished at the same time that the centre is removed; so that they ought not to be continued beyond what is necessary to produce a sensible removal of the magnetic centre. 8. The disadvantages of using magnets of unequal power, and of steel not properly hardened.—Having communicated the magnetic virtue to a steel-bar by means of a magnet of any given power, then rub it with a weaker magnet, and it will be found, that the power of the bar, instead of being augmented, will now be diminished; being no stronger than if it had been rubbed only with the weak magnet. The impropriety of using soft steel in making artificial magnets may be understood from the following example: Take two wires about 14 inches long, and one eighth of an inch in thickness; let one be of very hard steel, the other of soft steel or iron, though not of the softest fort; then, by means of magnetic bars, give the virtue to those wires, treating them both in the same manner, and it will be generally found that the hard wire will have only two poles, but the other a greater number.

9. To weaken or destroy the magnetism of a wire by bending.—Having communicated the magnetic virtue to an iron or soft steel wire of about four or five inches long and one-twentieth of an inch in diameter, roll it round a stick so as to make four or five revolutions. When taken off the stick it will be found to have its virtue quite destroyed, or at least very much weakened by the bending. This effect cannot be produced but when the texture of the wire is strained by the bending; for if it be of such an elastic nature as to recover its straightness after being once rolled round the stick, little change is made on the magnetic power. When only the middle of the wire is bent, little or no change takes place in the magnetic power. If a piece of magnetic wire be cleaved, or split lengthwise, the parts will sometimes have the same poles, and sometimes the contrary; but when one part is much thinner than the other, the slender part will generally have its poles reversed.

10. To improve natural magnets.—This may be done by the same methods which are used to communicate the virtue to steel-bars or to iron-ores; but the natural magnets being generally very short, we can seldom do more than place them between two strong magnetic bars: However, when they are of sufficient length, they must be rubbed with other bars besides those between which they are put; using the same precautions as in making artificial magnets. When subjected to this operation, it will always be proper to remove the armature from them.

11. To arm natural or artificial magnets.—The first step towards this operation is to find out the poles of the magnet, after which it is to be properly shaped; that of a parallelopipedon is the best: in which case care must be taken to let the poles fall about the middle of two opposite surfaces; and in this direction the magnet ought to have the greatest length possible; for a natural magnet is weakened much more by having a part cut off from its length than its breadth. This being done, provide two plates of soft iron, equal in breadth to those surfaces where the poles stand, and projecting a little on one side of the stone, as shown by fig. 15. The projections marked DD must be much narrower than the breadth of the plates; from a quarter to half an inch being sufficient for the larger magnets, and about one tenth of an inch for small ones, for the purpose of applying to them the surface of the iron F. The thickness of the plates CD CD must be proportioned to the strength of the magnet A B; and this proportion cannot easily be determined without an actual experiment. The best method, therefore, is to make them somewhat thick at first, and then keep filing them down as long as the power of the magnet increases; after which the filing is to be discontinued. The armature may be kept on either by tying or by a box; which last is the preferable method. The armature of spherical magnets must be adapted to their shape, and each large enough to cover a quarter of it. In like manner may artificial magnets be armed, and thus a compound magnet may be produced much more powerful than any single one. Thus Dr Knight constructed two very powerful artificial magnets, or magazines of magnetic bars, which are now in the repository of the Royal Society. Each of these consists of 240 bars disposed in four lengths, so as to form a parallelopipedon, each length containing 60 bars. They are all kept together by iron braces, and the whole suspended on pivots, with a wooden pedestal or carriage, by which they may be easily placed in any required position. If the artificial magnets be made in the shape of a horse-hoe or a semicircle, they have no occasion for armature, it being sufficient to join them either by rivetting or by means of a box; and indeed even when straight bars are used, a compound magnet may be made without armature; but then as the poles cannot act in the same plane, it is necessary to have two magazines in order to give magnetism the more conveniently to other bodies. The power of a magnet is rather augmented by being armed, for the same reason that it is increased by a piece of iron affixed to it. E is a brass ring, by which it may be suspended with the iron adhering to it, which is the best method for preserving its virtue.

12. Magnetism requires some time to penetrate through iron. Having placed a bulky piece of iron, suppose one weighing 40 or 50 pounds, so near a magnetic needle as to draw it a little out of its direction, apply one of the poles of a strong magnet to the other extremity of the iron, and you will find that it requires some seconds before the needle can be affected by it. The interval is greater or less according to the size of the iron and the strength of the magnet.

**Chap. IV. Entertaining Experiments.**

Construction of the Magnetic Perspective-Glass.] Provide an ivory tube, about two inches and a half long, and of the form expressed in fig. 16, CCLXXVI. The sides of this tube must be thin enough to admit a considerable quantity of light. It is to open at one end with a screw; at that end there must be placed an eye-glass of about two inches focus, and at the other end any glass you please. Have a small magnetic needle, like that placed on a compass. It must be strongly touched, and so placed at the bottom of the tube that it may turn freely round. It is to be fixed on the centre of a small ivory circle C, of the thickness of a counter, which is placed on the object-glass D, and painted black on the side next it. This circle must be kept fast by a circular rim of pasteboard, that the needle may not rise off its pivot, after the same manner as in the compass. This tube will thus become a compass sufficiently transparent to show... show the motions of the needle. The eye-glass serves more clearly to distinguish the direction of the needle; and the glass at the other end, merely to give the tube the appearance of a common perspective. It will appear from the laws of magnetism already laid down, that the needle in this tube, when placed over, and at a small distance from, a magnet, or any machine in which it is contained, will necessarily place itself in a position directed by that magnet, and consequently show where the north and south pole of it is placed; the north end of the needle constantly pointing to the south end of the magnet. This effect will take place, though the magnet be inclosed in a case of wood; or even metal, as the magnetic effluvia penetrates all bodies. You must observe, however, that the attracting magnet must not be very far distant from the needle, especially if it be small, as in that case its influence extends but to a short distance. This tube may be differently constructed, by placing the needle in a perpendicular direction, on a small axis of iron, on which it must turn quite freely, between two small plates of brass placed on each side the tube: the two ends of the needle should be in exact equilibrium. The north and south ends of this needle will, in like manner, be attracted by the south and north ends of the magnetic bar. The former construction, however, appears preferable, as it is more easily excited, and the situation of the needle much more easily distinguished.

Exp. I. The magnetic paradox.

Having placed a small piece of iron wire not above a tenth part of an inch long upon a table A.B., fig. 17. Hold the magnetic bar E.F. about four or five inches above the table, with either of its poles pointing to the table, so that the perpendicular let fall from the pole may touch the table at G, two or three inches from the wire; which distances, however, are subject to variations arising from the power of the magnet.—When the magnet is held in a proper position with respect to the iron, the latter will elevate one of its ends, as is shown at C.D., forming an angle with the table, which is larger the nearer the wire comes to the point G, where it stands quite upright. Knock the table gently, and the wire C.D. will gradually proceed towards G, every knock making it jump up and advance a little way. This will naturally be attributed to the attraction of the magnet; which not being sufficiently strong to draw the wire directly towards it, is just able to bring it gradually towards G when the motion of the table lifts it up. But if, instead of holding the magnet over the table, it be placed below it at H.I., the wire will now make an obtuse angle towards G; as is shown at K.L., and, on knocking the table, will recede from the magnet as if repelled, though in truth it is as much attracted as before.

The cause of this seeming repulsion will be understood from fig. 18, where the wire is represented by K.L. and the magnet by H. The former being rendered magnetic by the proximity of the magnet H, is inclined to it according to the laws already laid down; but, by reason of its weight, and being supported only at one end, it inclines less than it would do if it were freely suspended by its centre. Let M.N. be a line passing through the centre of the wire; then, by the motion of the table, the wire being lifted up, the end K will be at liberty to descend farther in the direction in which it is attracted by the magnet than it was before. It will then take the position represented by r.Q., its centre remaining nearly in the same perpendicular M.N. We say nearly, because the action of the magnet will undoubtedly move the whole wire somewhat nearer to itself; and the motion of the centre will be a diagonal compounded of the forces of gravity and of the magnet. The latter, however, being much smaller, will, by conspiring with the action of gravity, draw down the nearest end of the wire r so far, that a perpendicular line P.O. let fall from the extremity of it will touch the table in a point farther distant from the magnet than K. In this perpendicular the wire will depend very nearly, and then refuse its proper situation, parallel, or nearly so, to K.L.; when a second knock will remove it a little farther off, for the reason already assigned. The former part of the experiment may be easily explained upon the same principles. The whole may be diversified by using iron-filings instead of the wire. In this case, when the magnet is held over the table, they will be gradually collected about the point G, and dispersed from it while the magnet is held under.

2. The communicative crown.

Take a crown-piece, and bore a hole in the side of it; in which place a piece of wire, or a large needle, well polished, and strongly touched with a magnet. Then close the hole with a small piece of pewter, that it may not be perceived. Now the needle in the magnetic perspective before described, when it is brought near to this piece of money, will fix itself in a direction correspondent to the wire or needle in that piece. Define any person to lend you a crown-piece, which you dexterously change for one that you have prepared as above. Then give the latter piece to another person, and leave him at liberty either to put it privately in a snuff-box, or not; he is then to place the box on a table, and you are to tell him, by means of your glass, if the crown is or is not in the box. Then bringing your perspective close to the box, you will know, by the motion of the needle, whether it be there or not; for as the needle in the perspective will always keep to the north of itself, if you do not perceive it has any motion, you conclude the crown is not in the box. It may happen, however, that the wire in the crown may be placed to the north, in which case you will be deceived. Therefore, to be sure of success, when you find the needle in the perspective remain stationary, you may make some pretence to define the person to move the box into another position, by which you will certainly know if the crown-piece be there or not.—You must remember, that the needle in the perspective must here be very sensible, as the wire in the crown cannot possibly have any great attractive force.

3. The magnetic table.

Under the top of a common table place a magnet that turns on a pivot; and fix a board under it, that nothing may appear. There may also be a drawer under the table, which you pull out to show that there is nothing concealed. At one end of the table there must be a pin that communicates with the magnet, and by which it may be placed in different positions: this pin must be so placed as not to be visible to the spectators. Strew some steel-filings or very small nails over that part of the table where the magnet is. Then ask any one to lend you a knife, or a key, which will then attract part of the nails or filings.

Then placing your hand in a careless manner on the pin at the end of the table, you alter the position of the magnet; and giving the key to any person, you desire him to make the experiment, which he will then not be able to perform. You then give the key to another person; at the same time placing the magnet, by means of the pin, in the first position, when that person will immediately perform the experiment.

4. The mysterious watch.

You desire any person to lend you his watch, and ask him if he thinks it will or will not go when it is laid on the table. If he say it will, you place it over the end of the magnet, and it will presently stop (a). You then mark with chalk, or a pencil, the precise point where you placed the watch; and moving the position of the magnet, as in the last experiment, you give the watch to another person, and desire him to make the experiment; in which he not succeeding, you give it to a third person, at the same time replacing the magnet, and he will immediately perform the experiment.

5. The magnetic dial.

Provide a circle of wood or ivory, of about five or six inches diameter, as fig. 19, which must turn quite free on the stand B (fig. 20), in the circular border A: on the circle must be placed the dial of pasteboard C (fig. 19,) whose circumference is to be divided into 12 equal parts, in which must be inscribed the numbers from 1 to 12, as on a common dial. There must be a small groove in the circular frame D, to receive the pasteboard circle: and observe, that the dial must be made to turn so free, that it may go round without moving the circular border in which it is placed. Between the paste-board circle and the bottom of the frame, place a small artificial magnet E (fig. 21.), that has a hole in its middle, or a small protuberance. On the outside of the frame place a small pin P, which serves to show where the magnetic needle I, that is placed on a pivot at the centre of the dial, is to stop. This needle must turn quite free on its pivot, and its two sides should be in exact equilibrium. Then provide a small bag, that has five or six divisions, like a lady's work-bag, but smaller. In one of these divisions put small square pieces of pasteboard, on which are wrote the numbers from 1 to 12, and if you please you may put several of each number. In each of the other divisions you must put 12 or more like pieces; observing, that all the pieces in each division must be marked with the same number. Now the needle being placed upon its pivot, and turned quickly about, it will necessarily stop at that point where the north end of the magnetic bar is placed, and which you previously knew by the situation of the small pin in the circular border. You therefore present to any person that division of the bag which contains the several pieces on which is wrote the number opposite to the north end of the bar, and tell him to draw any one of them he pleases. Then placing the needle on the pivot, you turn it quickly about, and it will necessarily stop, as we have already said, at that particular number.

Another experiment may be made with the same dial, by desiring two persons to draw each of them one number out of two different divisions of the bag; and if their numbers, when added together, exceed 12, the needle or index will stop at the number they exceed it; but if they do not amount to 12, the index will stop at the sum of those two numbers. In order to perform this experiment, you must place the pin against the number 5, if the two numbers to be drawn from the bag be 10 and 7; or against 9 if they be 7 and 2.—If this experiment be made immediately after the former, as it easily may, by dexterously moving the pin, it will appear the more extraordinary.

6. The dexterous painter.

Provide two small boxes, as M and N (fig. 22), four inches wide, and four inches and a half long. Let the box M be half an inch deep, and N two-thirds of an inch. They must both open with hinges, and shut with a clasp. Have four small pieces of light wood, (fig. 23, 24, 25, 26,) of the same size with the inside of the box M (fig. 22.), and about one third of an inch thick. In each of these let there be a groove, as AB, EF, CD, GH; these grooves must be in the middle, and parallel to two of the sides. In each of these grooves place a strong artificial magnet, as fig. 27. The poles of these magnets must be properly disposed with regard to the figures that are to be painted on the boards; as is expressed in the plate. Cover the bars with paper, to prevent their being seen; but take care, in pasting it on, not to wet the bars, as they will thereby rust, which will considerably impair their virtue. When you have painted such subjects as you choose, you may cover them with a very thin clear glaze. At the centre of the box N, place a pivot (fig. 28.), on which a small circle of pasteboard OPQR (fig. 29.) is to turn quite free; under which is to be a touched needle S. Divide this circle into four parts, which are to be disposed with regard to the poles of the needle, as is expressed in the figure. In these four divisions you are to paint the like subjects as are on the four boards, but reduced to a smaller compass. Cover the inside of the top of this box with a paper M, (see fig. 22.) in which must be an opening D, at about half an inch from the centre of the box, that you may perceive successively, the four small pictures on the pasteboard circle just mentioned. This opening is to serve as the cloth on which the little painter is supposed to draw one of the pictures. You may cover the top of the box, if you please, with a thin glaze. Then give the first box to any person, and tell him to place any one of the four pictures in it privately, and, when he has closed it, to give it you. You then place the other box over it; when the moveable circle, with the needle, will turn till it comes in the same position with the bar in the

(a) To perform this experiment, you must use a strong magnetic bar; and the balance of the watch must not be of brass, but steel. 7. The cylindric oracle.

Provide a hollow cylinder of about six inches high and three wide, as AB, fig. 30. Its cover CD must be made to fix on any way. On one side of this box or cylinder let there be a groove, nearly of the same length with that side; in which place a small steel bar (fig. 31.) that is strongly impregnated, with the north pole next the bottom of the cylinder. On the upper side of the cover describe a circle; and divide it into ten equal parts, in which are to be wrote the numbers from 1 to 10, as is expressed in fig. 32. Place a pivot at the centre of this circle, and have ready a magnetic needle. You are then to provide a bag, in which there are several divisions, like that described in exper. 5. In each of these divisions put a number of papers, on which the same or similar questions are wrote. In the cylinder put several different answers to each question, and seal them up in the manner of small letters. On each of these letters or answers is to be wrote one of the numbers of the dial or circle at the top of the box. You are supposed to know the number of the answers to each question. You then offer one of the divisions of the bag, observing which division it is, to any person, and desire him to draw one of the papers. Next put the top on the cylinder, with that number which is wrote on the answer directly over the bar. Then placing the needle on the pivot, you turn it briskly about, and it will naturally flop at the number over the bar. You then desire the person who drew the question to observe the number at which the needle stands, and to search in the box for a paper with the same number, which he will find to contain the answer.—You may repeat the experiment by offering another division of the bag to the same or another person; and placing the number that corresponds to the answer over the magnetic bar, proceed as before.

It is easy to conceive of several answers to the same question. For example, suppose the question to be, Is it proper to marry?

Answer 1. While you are young, not yet; when you are old, not at all.

2. Marry in haste, and repent at leisure.

3. Yes, if you can get a good fortune; for something has some favour, but nothing has no flavour.

4. No, if you are apt to be out of humour with yourself; for then you will have two persons to quarrel with.

5. Yes, if you are sure to get a good husband (wife); for that is the greatest blessing of life. But take care you are sure.

6. No, if the person you would marry is an angel; unless you will be content to live with the devil.

8. The enchanted ewer.

Fix a common ewer, as A, (fig. 33.) of about 12 inches high, upon a square stand BC; in one side of which there must be a drawer D, of about four inches square and half an inch deep. In the ewer place a hollow tin cone, inverted, as AB, fig. 34. of about four inches and a half diameter at top, and two inches at bottom; and at the bottom of the ewer there must likewise be a hole of two inches diameter.

Upon the stand, at about an inch distance from the bottom of the ewer, and directly under the hole, place a small convex mirror H, of such convexity that a person's visage, when viewed in it, at about 15 inches distance, may not appear above two inches and a half long.

Upon the stand likewise, at the point I, place a pivot of half an inch high, on which must be fixed a touched needle RQ, inclosed in a circle of very thin pasteboard OS, fig. 35, of five inches diameter. Divide this pasteboard into four parts, in each of which draw a small circle; and in three of these circles paint a head as x, y, z, the dres of each of which is to be different, one, for example, having a turban, another a hat, and the other a woman's cap. Let that part which contains the face in each picture be cut out, and let the fourth circle be entirely cut out; as it is expressed in the figure. You must observe, that the poles of the needle are to be disposed in the same manner as in the plate.

You are next to provide four small frames of wood or pasteboard, n° 1, 2, 3, 4, each of the same size with the inside of the drawer. On these frames must be painted the same figures as on the circular pasteboard; with this difference, that there must be no part of them cut out. Behind each of these pictures place a magnetic bar, in the same direction as is expressed in the plate; and cover them over with paper, that they may not be visible. Matters being thus prepared, you first place in the drawer the frame n° 4, on which there is nothing painted. You then pour a small quantity of water into the ewer, and desire the company to look into it, asking them if they see their own figures as they are. Then you take out the frame n° 2, and give the three others to any one, desiring him to choose in which of those dresses he would appear. Then put the frame with the dress he has chosen in the drawer; and a moment after, the person looking into the ewer will see his own face surrounded with the dress of that picture. For, the pasteboard circle (divided, as above described, into four parts, in three of which are painted the same figures as on three of the boards, and the fourth left blank) containing a magnetic needle, and the four boards having each a concealed magnet; therefore, when one of them is put in the drawer under the ewer, the circle will correspond to the position of that magnet, and consequently the person looking into the top of the ewer will see his own face surrounded with the head-dress of the figure in the drawer.—This experiment, well performed, is highly agreeable. As the pasteboard circle can contain only three heads, you may have several such circles, but you must then have several other frames; and the ewer must be made to take off from the stand.

9. The box of metals.

Provide a wooden box, about 13 inches long and seven wide, as ABCD (fig. 36.). The cover of this box should be as thin as possible. Have six small boxes or tables, about an inch deep, all of the same size and form, as EFGHIK, that they may indifferently nately go into similar holes made in the bottom of the large box. In each of these tablets is to be placed a small magnetic ball, and their poles are to be disposed as expressed in the figure. Cover each of these tablets with a thin plate of one of the following metals, viz. gold, silver, copper, iron, pewter, and lead. You must also have a magnetic perspective, at the end of which is to be two circles, one divided into six equal parts, and the other into four, as in fig. 37, from the centre of which there must be drawn an index N, whose point is to be placed to the north. Therefore, when you are on the side CD of the box, and hold your perspective over any one of the tablets that are placed on the holes E, F, G, so that the index drawn on the circle is perpendicular to the side AB, the needle in the perspective will have its south pole directed to the latter that denotes the metal contained in that tablet. When you hold the perspective over one of the boxes placed in the holes H, I, K, so that the index drawn on the circle is perpendicular to the side CD, the south pole of the needle will in like manner express the name of the metal inclosed. If the under side of any one of the tablets be turned upwards, the needle will be slower in its motion, on account of the greater distance of the bar. The gold and silver will still have the same direction; but the four other metals will be expressed by the letters on the interior circle. If any one of the metals be taken away, the needle will not then take any of the above directions, but naturally point to the north; and its motion will be much slower. You therefore give the box to any one, and leave him at liberty to dispose all the tablets in what manner and with what side upward he pleases, and even to take any one of them away. Then, by the aid of your perspective, you tell him immediately the name of the metal on each tablet, and of that he has taken away.

This box of metals will, on comparison, be found far to exceed that which has been publicly exhibited: for that, being composed of six tablets, of which two only differ in form, admits but of six different dispositions, whereas in this the tablets may be placed 720 different ways. In the other, you must also know the particular side of the box, which in this is not at all necessary. Nay, you may here distinguish each metal, though the box be completely covered with paper; for the effect of the needle will be always the same. The experiments with this box are therefore much more extraordinary, and its construction at the same time more simple.

10. The magnetic planetarium.

Construct a round box, ILMN, (fig. 38,) of eight or nine inches diameter, and half an inch deep. On its bottom fix a circle of pasteboard, on which draw the central circle A, and the seven circumjacent circles B, C, D, E, F, G, H. Divide the central circle into seven equal parts by the lines AB, AC, AD, AE, AF, AG, and AH, which must pass through the centres of the other circles, and divide each of them into two equal parts. Then divide the circumference of each of those circles into 14 equal parts, as in the figure. You are likewise to have another pasteboard of the same figure, and divided in the same manner, which must turn freely in the box by means of an axis placed on a pivot; one end of which is to be fixed in the centre of the circle A. (See fig. 39.) On each of the seven smaller circles at the bottom of the box, place a magnetic bar, two inches long, in the same direction with the diameters of those circles, and their poles in the situations expressed in the figure. There must be an index O (fig. 39.), like that of the hour-hand of a dial, which is to be fixed on the axis of the central circle, and by which the pasteboard circle in the box may be turned about. There must be also a needle P, which must turn freely on the axis, without moving the circular pasteboard.—In each of the seven divisions of the central circle write a different question; and in another circle, divided into 12 parts, you may write the names of the 12 months. In each of the seven circles write two answers to each question, observing that there must be but seven words in each answer; in the following manner. In the first division of the circle G fig. 37, which is opposite to the first question, write the first word of the first answer. In the second division of the next circle, write the second word; and so on to the last word, which will be in the seventh division of the seventh circle. In the eighth division of the first circle, write the first word of the second answer; in the ninth division of the second circle, write the second word of the same answer; and so on to the 14th division of the seventh circle, which must contain the last word of that answer. The same must be done for all the seven questions; and to each of them must be assigned two answers, the words of which are to be dispersed through the seven circles. At the centre of each of these circles place a pivot; and have two magnetic needles, the pointed end of one of which must be north, and the other south, QR, (fig. 39.) Now, the index of the central circle being directed to any one of the questions, if you place one of the two magnetic needles on each of the seven lesser circles, they will fix themselves according to the direction of the bars on the correspondent circles at the bottom of the box, and consequently point to the seven words that compose the answer. If you place one of the other needles on each circle, it will point to the words that are diametrically opposite to those of the first answer, the north pole being in the place of the south pole of the other.—You therefore present this planetarium to any person, and desire him to choose one of the questions there wrote; and you then set the index of the central circle to that question, and putting one of the needles on each of the seven circles, you turn it about; and when they all settle, they will point to the seven words that compose the answer. The two answers may be one favourable and the other unfavourable; and the different needles will serve to diversify the answers when you repeat the experiment.

There may be also a moveable needle to place against the names of the months; and when the party has fixed upon a question, you place that needle against the month in which he was born, which will give the business an air of more mystery. On the centre of the large circle may be the figure of the sun; and on each of the seven smaller circles one of the characters of the five planets, together with the earth and moon. This Magnetism.

11. The sagacious swan.

Provide a box XY, 18 inches long, 9 wide, and 2 deep, the top of which is to slide on and off at the end Y. Toward the end X, describe a circle of six inches diameter, round which are to be fixed six small vases of wood or ivory, of one inch and a half high; and to each of them there must be a cover. At the end Y place an egg B, of ivory or other matter, of about three inches and a half high, with a cover that shuts by a hinge, and fastens with a spring. It must be fixed on the stand C; through which, as well as the bottom of the egg, and the part of the box directly underneath, there must pass a hole of one-third of an inch in diameter. In this cavity place an ivory cylinder F, that can move freely, and rises or falls by means of the spring R. You must have a thin copper basin A, of six inches diameter, which is to be placed on the centre of the circle at X, and consequently in the middle of the six vases. Let a proper workman construct the movement expressed by fig. 41, which is composed of a quadrant G, that has 16 teeth, and is moveable about an axis in the stand H, that has an elbow, by which it is screwed to the bottom of the box at L. To the quadrant there must be joined the straight piece K. The horizontal wheel M has 24 teeth; and is supported by the piece S, which is screwed to the end of the box next Y. On the axis of this wheel place a brass rod OP, five inches long; and at the part O place a large bar or horse-shoe, of a semicircular form, and about two inches and a half diameter, strongly impregnated. The steel rod V, takes at one end the teeth of the quadrant G, by the pinion F; and at the other end the wheel M, by the perpendicular wheel N, of 30 teeth; the two ends of this rod are supported by the two stands that hold the other pieces. Under the piece K, that joins to the quadrant, must be placed the spring R, by which it is raised, and pushes up the cylinder that goes through the stand C into the egg. You must also have six small etwees or cases, as Y, Y, Y, Y, Y, Y. They must be of the same circumference with the cylinder in the stand, and round at their extremities: their length must be different, that, when they are placed in the egg, and the lower end enters the hole in which is the cylinder, they may thrust it down more or less, when the top of the egg, against which they press, is fastened down, and thereby lower the bar that is fixed to the end of the quadrant, and consequently, by means of the pinion Z and wheels NM turn the horse-shoe that is placed upon the axis of the last wheel. The exact length of these etwees can be determined by trials only; which trials, however, may be made with round pieces of wood. In each of these etwees place a different question, wrote on a slip of paper and rolled up, and in each of the vases put the answer to one of the questions; as you will know, by trials, where the magnetic bar or horse-shoe will stop. Lastly, provide a small figure of a swan, or what other you please, made of cork or enamel, in which you must fix a touched needle, of the largest size of those commonly used in sewing.

Being thus prepared, you offer a person the six etwees, and desire him to choose any one of them himself, and conceal the others, or give them to different persons. He is then to open his etwee, read the question it contains to himself, and return the etwee to you, after replacing the question. You then put the etwee in the egg, and placing the swan upon the water in the basin, you tell the company she will presently discover in which of the vases the answer is contained. The same experiment may be repeated with all the etwees.

12. The multifarious verse.

The eight words that compose this Latin verse,

Tot sunt tibi dotes, quot celi sidera, virgo (A),

being privately placed in any one of the different combinations of which they are susceptible, and which are 40320 in number, to tell the order in which they are placed.

Provide a box that shuts with hinges, and is eight inches long, three wide, and half an inch deep. Have eight pieces of wood about one third of an inch thick, two inches long, and one and a half wide, which will therefore, when placed close together, exactly fill the box. In each of these pieces or tablets place a magnetic bar, with their poles as is expressed in the figure. The bars being covered over, write on each of the tablets, in the order they then stand, one of the words of the foregoing Latin verse. On a very thin board of the same dimensions with the box, draw the eight circles, A, B, C, D, E, F, G, H, (fig. 43,) whose centres should be exactly over those of the eight tablets in the box when the board is placed upon it. Divide each of those circles into eight parts, as in the figure; and in each of those divisions write one of the words of the Latin verse, and in the precise order expressed in the plate; so that, when the board is placed over the box, the eight touched needles placed at the centre of the circles may be regulated by the poles of the bars in the box, and consequently the word that the needle points to in the circle be the same with that inscribed on the tablet. Cover the board with a glass, to prevent the needles from rising off their pivots, as is done in the sea-compass. Over the board place four plates of glass, I, L, M, N, fig. 44, which will give the machine the figure of a truncated pyramid, of eight inches high. Cover it with a glass, or rather a board, in which are placed two lenses, O O, of eight inches focus, and distant from each other about half an inch. Line the four plates of glass that compose the sides with very thin paper, that will admit the light, and at the same time prevent the company from seeing the circles on the board.

These preparations being made, you give the box to any one; and tell him to place the tablets on which the words are wrote, privately, in what position he thinks proper, then to close the box, and, if he pleases, to wrap it up in paper, seal it, and give it you. Then placing the board with the pyramid upon it, you immediately tell him the order in which the tablets are placed, by reading the words to which the needles on the circles point.

(A) i.e. Thy virtues, virgin, are as numerous as the stars of heaven. Animal Magnetism, a sympathy lately supposed by some persons to exist between the magnet and the human body; by means of which the former became capable of curing many diseases in an unknown way, something resembling the performances of the old magicians.

The fanciful system, to call it by no worse name, of animal magnetism, appears to have originated, in 1774, from a German philosopher named Fuerbr Hehl, who greatly recommended the use of the magnet in medicine. M. Mesmer, a physician of the same country, by adopting the principles of Hehl, became the direct founder of the system; but, afterwards deviating from the tenets of his instructor, he lost his patronage, as well as that of Dr Ingenhouz, which he had formerly enjoyed. Mesmer had already distinguished himself by "A Dissertation on the influence of the Stars upon the human body," which he publicly defended in a thesis before the university of Vienna; but he was so unable to stand before the opposition of Hehl and Ingenhouz, that his system fell almost instantly into disrepute. Mesmer appealed to the academy of sciences at Berlin; but they rejected his principles as destitute of foundation, and unworthy of the smallest attention. He then made a tour through Germany, publishing everywhere the great cures he performed by means of his animal magnetism, while his enemies everywhere pursued him with detections of the falsehood of his assertions.

Mesmer, still undaunted by so many defeats, returned to Vienna; but meeting there with no better success than before, he retired to Paris in the beginning of the year 1778. Here he met with a very different reception. He was first patronized by the author of the Dictionnaire des Merveilles de la Nature; in which work a great number of his cures were published, Mesmer himself receiving likewise an ample testimony of his candour and solid reasoning. Our physician soon collected some patients; and in the month of April 1778 retired with them to Creteil, from whence he in a short time returned with them perfectly cured. His success was now as great as his disappointment had been before. Patients increased so rapidly that the Doctor was soon obliged to take in pupils to assist him in his operations. These pupils succeeded equally well as Mesmer himself; and so well did they take care of their own emolument, that one of them, named M. Defflon, realized upwards of L. 100,000 Sterling. In 1779 Mesmer published a memoir on the subject of Animal Magnetism, promising afterwards a complete work upon the same, which should make as great a revolution in philosophy as it had already done in medicine.

The new system now gained ground daily; and soon became so fashionable, that the jealousy of the faculty was thoroughly awakened, and an application concerning it was made to government. In consequence of this a committee was appointed to inquire into the matter, consisting partly of physicians and partly of members of the royal academy of sciences, with Dr Benjamin Franklin at their head. This was a thunderstroke to the supporters of the new doctrine.—Mesmer himself refused to have any communication with the committee; but his most celebrated pupil Vol. X. Part II.

Defflon was less scrupulous, and explained the principles of his art in the following manner:

1. Animal magnetism is an universal fluid, constituting an absolute plenum in nature, and the medium of all mutual influence between the celestial bodies and betwixt the earth and animal bodies.

2. It is the most subtle fluid in nature; capable of a flux and reflux, and of receiving, propagating, and continuing all kinds of motion.

3. The animal body is subjected to the influences of this fluid by means of the nerves, which are immediately affected by it.

4. The human body has poles and other properties analogous to the magnet.

5. The action and virtue of animal magnetism may be communicated from one body to another, whether animate or inanimate.

6. It operates at a great distance without the intervention of any body.

7. It is increased and reflected by mirrors; communicated, propagated, and increased by sound; and may be accumulated, concentrated, and transported.

8. Notwithstanding the universality of this fluid, all animal bodies are not equally affected by it; on the other hand, there are some, though but few in number, the presence of which destroys all the effects of animal magnetism.

9. By means of this fluid nervous disorders are cured immediately, and others mediately; and its virtues, in short, extend to the universal cure and preservation of mankind.

From this extraordinary theory, Mesmer, or M. Defflon, had fabricated a paper, in which he stated that there was in nature but one disease and one cure, and that this cure was animal magnetism; and lastly, M. Defflon engaged, 1. To prove to the commissioners, that such a thing as animal magnetism existed; 2. To prove the utility of it in the cure of diseases; after which he was to communicate to them all that he knew upon the subject. The commissioners accordingly attended in the room where the patients underwent the magnetical operations. The apparatus consisted of a circular platform made of oak, and raised about a foot and a half from the ground; which platform was called the baguet. At the top of it were a number of holes, in which were iron rods with moveable joints for the purpose of applying them to any part of the body. The patients were placed in a circle round, each touching an iron rod, which he could apply to any part of the body at pleasure; they were joined to one another by a cord passing round their bodies, the design being to increase the effect by communication. In the corner of the room was a piano forte, on which some airs were played, occasionally accompanied with a song. Each of the patients held in his hand an iron rod ten or twelve feet long; the intention of which, as Defflon told the commissioners, was to concentrate the magnetism in its point, and thus to render its effects more sensible. Sound is another conductor of this magnetism; and in order to communicate the magnetism to the piano forte, nothing more is necessary than to bring the iron rod near it. Some magnetism is also furnished by the person who plays it; and this magnetism is transmit- ted to the patients by the founds. The internal part of the platform was said to be so contrived as to concentrate the magnetism, and was the reservoir whence the virtue diffused itself among the patients. Its structure, however, is not mentioned; but the committee satisfied themselves, by means of a needle and electrometer, that neither common magnetism nor electricity was concerned.

Besides the different ways of receiving the magnetism already mentioned, viz. by the iron, cord, and piano forte, the patients also had it directly from the Doctor's finger, and a rod which he held in his hand, and which he carried about the face, head, or such parts of the patient as were diseased; observing always the direction of what he called the poles. The principal application of magnetism, however, was by pressure of the hands or fingers on the hypochondria or lower regions of the stomach.

The effects of these operations upon Deflon's patients were very different. Some felt nothing, neither had the magnetism any effect whatever upon them. Some spat, coughed, sweat, and felt, or pretended to feel, extraordinary heats in different parts of the body. Many women, but very few men, had convulsions, which Deflon called their crises, &c.—The commissioners at last found that they could come to no satisfactory conclusion while they attended in this public way, and therefore determined to try the experiments themselves privately. As the fluid itself, however, was totally imperceptible by any of the senses, they could only ascertain themselves of its existence by ultimately curing diseases, or by its observable effects upon the human body. Being well assured, however, that though many diseases were cured, it would not amount to any proof of the existence of animal magnetism, they determined to observe its effects on the animal economy. For this purpose they made the following experiments:

1. They tried it upon themselves, and felt nothing. 2. Seven of Deflon's patients were magnetized at Dr Franklin's house, four of whom felt nothing; three felt, or affected to feel something. 3. Several persons in a higher sphere of life were magnetized, and felt nothing. 4. The commissioners, now determined to discover what share imagination had in this business, blindfolded several of the common people, and made them sometimes think that they were magnetized, at other times they magnetized them without letting them know that they did so: the consequence was, that when they supposed themselves magnetized, the patients likewise thought they felt something, and vice versa. 5. A magnetized tree was said to produce convulsions; a young man, blindfolded, fell into convulsions when he imagined himself near the tree, though he was really at a considerable distance from it. Deflon accounted for this on the principle of all trees being magnetic: but in this case, every one, susceptible of magnetism, would be seized with convulsions when he approached a tree. The same influence of imagination was observed in a woman accustomed to have convulsions when magnetized. They came on when nothing was done to her, on being told, when blinded, that she was magnetized.

Other instances are given, from which it was evident, either that the patients were impostors, or in such a most wretched state of debility both of mind and body, that the most trifling effects of the former had the most powerful effects on the latter. The commissioners therefore entirely disapproved of the whole. The touch, imitation, and imagination, they concluded, were the great causes of the effects produced by Mr Deflon's operations; and by means of these they supposed that convulsions, which in themselves are a very violent disorder, might be spread much farther than could be wished, even through a whole city. It was observed that the operator sometimes pressed strongly, and for a length of time, upon different parts of the body, particularly the hypochondria and pit of the stomach; and it is well known that a strong pressure on these parts will produce disagreeable sensations in those who enjoy perfect health.

It is needless to add more upon this subject, than that Mefiner complained of the report of the commissioners, petitioned parliament, was by them commanded to discover the mysteries of his doctrine; and that it is now exploded by every man of sense.—The conclusion of the academicians concerning it was, that it is not entirely useless even to philosophy; as it is one fact more to be confided to the history of the errors and illusions of the human mind, and a signal instance of the power of imagination.