The word magnetism is derived from the Greek word μαγνητικός, a name given to the loadstone or native magnet, an ore of iron well known to the ancients. The term μαγνητικός itself is said to be derived from one Magnes, a Greek shepherd, who observed on Mount Ida the attractive power which the loadstone exercised upon his iron crook. The most probable supposition however is, that it took its name from Magnesia, a country in Lydia, where it was first discovered; and this conjecture is confirmed by the fact that the magnet was often called by the ancients Lapis Heracleus, from Heraclea, the capital of Magnesia.

The science of magnetism treats of the phenomena exhibited by magnets, whether natural like the loadstone, or artificial like bars of steel to which magnetism has been permanently communicated—of their reciprocal action upon each other; of the laws of the forces which they develop; of the methods of making artificial magnets; and of the magnetic phenomena exhibited by the globe which we inhabit.

In giving an account of this interesting science, which has made rapid progress in modern times, we shall adopt the following arrangement:

1. On the history of magnetical discovery. 2. On the general phenomena and principles of natural and artificial magnets. 3. On the magnetism of bodies not ferruginous. 4. On the development of magnetism in bodies by rotation. 5. On the influence of heat on magnetism. 6. On the action of iron spheres on the needle. 7. On the influence of magnetism on chemical action. 8. On the laws of magnetic forces. 9. On terrestrial magnetism. 10. On the different methods of making artificial magnets. 11. On magnetical instruments and apparatus. 12. On the theories of magnetism.

CHAP. I.—ON THE HISTORY OF MAGNETICAL DISCOVERIES.

The attractive power of the natural magnet or loadstone over small pieces of iron seems to have been known from the remotest antiquity. It is distinctly referred to by Homer, Pythagoras, and Aristotle. Pliny mentions a chain of iron rings suspended from one another, the first being upheld by the loadstone; and he relates that Dinoares proposed to Ptolemy Philadelphus to build, at Alexandria, a temple, the vault of which, crowned with loadstones, should suspend in the air an iron statue of Queen Arsinoë. St Augustin likewise makes mention of a statue suspended in the air in the middle of the temple of Serapis at Alexandria. From references made to the magnet by Euripides, Claudian, and others, and from the experiment with the rings mentioned by Pliny, it is not very improbable that the ancients were acquainted with the communicability of magnetism to iron bodies. The magnetical properties of the loadstone, like the electrical ones of amber, were supposed to be miraculous. Medical qualities of various kinds were ascribed to it; and even Hippocrates ranks it amongst the number of purgatives.

In order to explain the properties of the loadstone, Thales, Anaxagoras, and others, supposed it to have a soul; while some conceived that it was surrounded with an emanation, capable of creating a vacuum, into which the iron precipitated itself.

In his description of China, Duhalde has stated that the invention directive power, or polarity, of the magnet, was known to the Chinese in the earliest ages, and that the needle had been employed to guide travellers by land a thousand years before Christ; and it is stated by Humboldt, that, according to the Peuthsooyami, a treatise on Medical Natural History, written under the Soong dynasty, 400 years before Columbus, the Chinese suspended the needle by a thread, and found it to decline to the south-east, and never to rest at the true south point.

Although the common properties of the loadstone were known to the ancients, and were no doubt studied even during the dark ages, yet notwithstanding the claims of the Chinese and Arabians, the directive power of the loadstone, or of a needle touched or rubbed by it, seems to be the discovery of modern times. Are Frode, an Icelandic historian, who was born in the year 1068, and who must have written his Landnamabok, or history of the discovery of Iceland, about the end of the eleventh century, mentions, in the most unequivocal manner, the directive power of the loadstone as known in his day. He states that Floke Vilgerderson, a renowned viking or pirate, departed from Rogoland in Norway, to seek Gadersholm or Iceland, some time in the year 868. He carried with him three ravens as guides, and, to consecrate them for this purpose, he offered up a sacrifice in Smörsund, where his ship lay. "For," says Frode, "in those times seamen had no loadstone in the northern countries."

That the mariner's compass was known in the twelfth century, about the year 1150, is proved by notices of it in various authors, particularly in an old French poem called La Bible Guyot, which is contained in a curious quarto manuscript of the thirteenth century, still existing in the Royal Library at Paris. Guyot of Provins, the author of this poem, was alive in 1181. After referring to the ways by which navigators are guided in their course, and mentioning the pole star, he adds,

Un art font qui mentir ne peut, Par la vertue de la marinère, Un pierre laide et bruniere Où le fers volontiers se joint, Ont regardent lor droit point.

That is, "they possess a never-failing method, by the virtue of the marinère, an ugly and brown stone, to which iron adheres of its own accord." The author next adds, that this art consists in rubbing a needle on the marinère; and that the point of this needle turns just against the pole-star in dark nights, when neither star nor moon are seen.

Quant il nuit est ténèbre et brume, Quand ne voit estelle ne lune, Lor font à l'aiguille allumer, Puiz ne peut ils assorer, Contre l'estoile va le pointe, Par se sont il mariner cointe, De la droit voie tems; C'est un art qui ne peut mentir.

1 Sola haec materia (ferrum) vires ab eo lapide accipit, retinetque longo tempore, aliquid apprehendens ferrum, ut annulorum catena spectetur interdum, quod imperium vulgus ferrum appellat vivum. 2 Leidartstein, or Leading Stone, from which our word Loadstone is derived. Cardinal James de Vitri, who flourished about the year 1200, mentions the magnetic needle in his History of Jerusalem; and he adds, that it was of indispensable utility to those who travelled by sea.

That the mariner's compass was known to the northern nations in 1266, appears from Torricius's History of Norway, where it is mentioned that Jarl Sturla's poem on the death of the Swedish count Byrgeres was rewarded with a mariner's compass. The directive property of the magnet is also distinctly mentioned in an epistle of Petrus Peregrinus de Marcourt, written about the latter end of the thirteenth century. This letter was addressed "Ad Sigierum de Foueancourt militem de magnete." This epistle contains a description of the loadstone, the means of finding its poles, and its property of attracting iron; and it proves that the part of the magnet which is turned to the north attracts that which is turned to the south.

A Neapolitan named Flavio Gioia, who lived in the thirteenth century, has been regarded by many as the inventor of the compass. Dr Gilbert affirms that Paulus Venetus brought the compass from China to Italy in 1260. Ludi Vestomannus asserts, that about 1500 he saw a pilot in the East Indies direct his course by a magnetic needle like those now in use. One of the earliest treatises on magnetism is a Latin letter of Peter Adsgier, contained in a volume of manuscripts in the library of the university of Leyden. This letter, which appears to have been written for the instruction of a friend, is in reality a methodical treatise, in two parts, the first of which is subdivided into ten, and the second into three chapters. In the second chapter of the second part, the mariner's compass, and the method of constructing it, are clearly described; and, what is still more interesting, the author not only mentions the declination of the magnetic needle, but had observed its actual deviation from the meridian. "Take notice," says he, "that the magnet, as well as the needle which has been touched by it, does not point exactly to the poles; but that part of it which is reckoned to point to the south inclines a little to the west; and that part which looks towards the north inclines as much to the east. The exact quantity of this declination I have found, after numerous experiments, to be five degrees. However, this declination is no obstacle to our guidance, because we make the needle itself decline from the true south by nearly one point and a half towards the west. A point, then, contains five degrees." Mr Christie seems to consider the authenticity of this manuscript as doubtful, because no new observation of the declination seems to have been made for two centuries afterwards; and because the declination should be westerly in place of easterly in 1269, according to the best law of the change which can be deduced from subsequent observations.

The declination or the variation of the needle, thus distinctly described by Adsgier, if his manuscript is authentic, must be considered as well known before the time of Columbus, to whom the discovery of it has been generally ascribed. His son Ferdinand states, that on the 14th of September (13th according to Mr Irving) 1492, his father, when about two hundred leagues from the island of Ferro, noticed for the first time the variation of the needle; "a phenomenon," says Washington Irving, "which had never before been remarked." He perceived, adds this author, "about nightfall, that the needle, instead of pointing to the north star, varied but half a point, or between five and six degrees to the north-west, and still more on the following morning. Struck with this circumstance, he observed it attentively for three days, and found that the variation increased as he advanced. He at first made no mention of this phenomenon, knowing how ready his people were to take alarm; but it soon attracted the attention of the pilots, and filled them with consternation. It seemed as if the laws of nature were changing as they advanced, and that they were entering another world, subject to unknown influences. They apprehended that the compass was about to lose its mysterious virtues; and, without this guide, what was to become of them in a vast and trackless ocean? Columbus tasked his science and ingenuity for reasons with which to allay their terrors. He told them that the direction of the needle was not to the polar star, but to some fixed and invisible point. The variation was not caused by any failing in the compass, which, like the other heavenly bodies, had its changes and revolutions, and every day described a circle round the pole. The high opinion that the pilots entertained of Columbus as a profound astronomer gave weight to his theory, and their alarm subsided."

Although the details which we have already given afford sufficient proof that the variation of the needle had been discovered two hundred years before the time of Columbus, yet it is evident, from the above passage, that he had discovered the variation of the needle, or that the variation was not a constant quantity, but varied in different latitudes.

Notwithstanding these casual observations on the variation of the compass, no accurate measures of its amount were made till about the middle of the 16th century. In 1541 it was found that the declination of the needle from the meridian of Paris was about 7° or 8° easterly. In 1550 it was 8° or 9°, and in 1580 11½° easterly. Norman, who first observed the variation in London, made it 11° 15' easterly; and Mr Burrough, comptroller of the navy, in 1580 found it to be at an average 11° 19' E. at Limehouse. The following observations made at other places will show the gradual change in the variation.

Burrough...........1580.............11° 19' E. Limehouse. Gunter..............1612.............5 36 E. London. Gellibrand*........1633..............4 4 E. London.

Petit................1630..............4 30 E. Paris. Petit................1660..............0 10 E. Paris. Auzout..............1670..............2 0 W. Rome. Hevelius............1642..............3 5 W. Dantzig. Hevelius............1670..............7 20 W. Dantzig.

The important discovery of the dip or inclination of the needle was made in 1576, by Robert Norman, whom we have already mentioned. Having constructed many compasses, and having always balanced the needles for them before he touched them with the magnet, he invariably found, that after they were touched, the north point always inclined below the horizon, so that he was obliged to make the card of the compass level, by putting some small pieces of wire on the end of it. Having mentioned this discovery to some of his friends, he was advised to construct an instrument which would enable him to measure the greatest angle which it would make with the horizon. With this instrument, which is the dipping needle in its first and rudest form, he found the dip to be at 71° 50'; an observation which, according to Bond, must have been made about 1576.

That ferruginous substances always possess a greater or Julius Caesar, a surgeon of Rimini, first observed the conversion of iron into a magnet. In 1590 he noticed this effect on a bar of iron which had supported a piece of brickwork on the top of a tower of the church of St Augustin. The very same fact was observed about 1630, by Gassendi, on the cross of the church of St John at Aix, which had fallen down in consequence of having been struck with lightning. He found the foot of it wasted with rust, and possessing all the properties of a lodestone.

While magnetism was making slow advances by means of insulated observations, it was destined to receive a vigorous impulse from the pen of Dr Gilbert of Colchester. This eminent individual, who was physician in ordinary to Queen Elizabeth, published, in 1600, his *Physiologia Nova, seu Tractatus de Magnete et Corporibus Magnetici*, work which contains almost all the information concerning magnetism which was known during the two following centuries. It relates chiefly to the natural lodestone, and to artificial magnets, or bars of steel which have acquired similar properties. He applies the term magnetic to all bodies which are acted upon by lodestones and magnets, in the same manner as they act upon each other, and finds that all such bodies contain iron in some state or other. He considers the phenomena of electricity as having a considerable resemblance to those of magnetism, though he points out the differences by which the two classes of phenomena are marked. In treating of the directive power of the needle, he supposed, "that the earth itself being in all its parts magnetic, and the water not, wherever the land was, there would the needle turn, as to the greater quantity of magnetic matter." He regarded the earth as acting upon a magnetised bar, and upon iron, like magnet, the directive power of the needle being produced by the action of magnetism of a contrary kind to that which exists at the extremity of the needle directed towards the pole of the globe. He gave the name of pole to the extremities of the needle which pointed towards the poles of the earth, conformably to his views of terrestrial magnetism, calling the extremity that pointed towards the north the south pole of the needle, and that which pointed to the south the north pole.

About the year 1650, Mr Bond, a teacher of mathematics in London, who had been employed to superintend the publication of the popular treatises on navigation, published a work called the *Seaman's Calendar*, in which he maintains that he has discovered the true progress of the deviation of the compass; and in another book, called *The Longitude Found*, and in the *Phil. Trans.* 1668, he published a table of the computed variations for London for many years to come, extending from 1663 to 1716. The results which his table contains agree very nearly with those which were observed for the next twenty-five years, but after that the differences became very great. In a subsequent paper in the *Phil. Trans.* for 1673, Bond attempted to account for the change in the variation and dip of the needle, by supposing that the two magnetic poles revolved round the poles of the earth. He asserted that he knew the period of his revolution, as well as its cause; and he proposed to determine the longitude by means of the dip of the needle. He did not, however, think proper to communicate either his views or method to the public.

Newton, Huygens, Hooke, and some of the other philosophers who flourished about the end of the seventeenth century, were occupied, though not to a great degree, with the subject of magnetism. Some of their observations and discoveries are referred to in a manuscript volume of notes and commentaries, written by David Gregory in 1693, in a copy of Newton's *Principia*, and used by Newton in improving the second edition. Newton had supposed that the law of magnetic action approaches to the inverse triplicate ratio of the distance; but Gregory did not adopt this opinion, and invalidates the arguments which were used in its support. Newton committed another mistake in asserting, as we shall afterwards see, that red hot iron has no magnetic property.

Several interesting experiments had been made by Dr Hooke, Gilbert, on the effects of heat in destroying magnetism, and also in inducing it in substances susceptible of being impregnated. He likewise made numerous experiments with bars of iron and steel placed in the magnetic meridian and exposed to great heats. Dr Hooke took up this subject in 1684. He used rods of iron and steel about seven inches long and one fifth of an inch in diameter, and he found that they acquired permanent magnetism when strongly heated in the magnetic meridian, and allowed to cool in the same position. The permanency of the effect was greater, and the magnetism stronger, when the rods were suddenly cooled in cold water, so as to give them a very hard temper. He found that the end which was next to the north, or the lower end of a vertical bar, was invariably a permanent north pole. Even when the upper end alone was quenched, while the rest of the bar cooled slowly, that end became a sensible south pole. If the same process was adopted when the steel bar lay at right angles to the magnetic meridian, no magnetism was acquired.

The subject of terrestrial magnetism now occupied the attention of our eminent countryman Dr Edmund Halley, and in 1683 he published his *Theory of Magnetism*, which to a certain extent forms the nucleus of more modern hypotheses. He regarded the earth's magnetism as caused by four poles of attraction, two of them near each pole of the earth; and he supposes "that in those parts of the world which lie nearly adjacent to any one of these magnetic poles, the needle is governed thereby, the nearest pole being always predominant over that more remote." He supposes that the magnetic pole, which was in his time nearest Britain, was situated near the meridian of the land's end, and not above 7° from the north pole; the other north magnetic pole being in the meridian of California, and about 15° from the north pole of the earth. He placed one of the two south poles about 16° from the south pole of the globe, and 95° west from London; and the other, or the most powerful of the four, about 20° from the south pole, and 120° east of London.

In order to account for the change in the variation, Dr Halley, some years afterwards, added to these reasonable hypotheses the very extraordinary one, that our globe was a hollow shell, and that within it a solid globe revolved, in nearly the same time as the outer one, and about the same centre of gravity, and with a fluid medium between them. To this inner globe he assigned two magnetic poles, and to the outer one other two; and he conceived the change in the variation of the needle to be caused by a want of coincidence in the times of rotation of the inner globe and the external shell. "Now, supposing," says he, "such an external sphere having such a motion, we may solve the two great difficulties in every former hypothesis; for if this exterior shell of earth be a magnet, having its poles at a distance from the poles of diurnal rotation, and if the internal nucleus be likewise a magnet, having its poles in two other places, distant also from its axis, and these latter, by a gradual and slow motion, change their places in respect of the external, we may then give a reasonable account of the four magnetic poles, as also of the changes of the needle's variation." From some reasons which Dr Halley then states, he concludes "that the two poles of the external globe are fixed in the earth, and that if the needle were wholly governed by them, the variation would be always the same, with some little irregularities; but the internal sphere, having such a gradual translation of its poles, influences the needle, and directs it va- History. riously, according to the result of the attractive and directive power of each pole, and consequently there must be a period of revolution of this internal ball, after which the variation will return as before.

This theory excited so much notice, that an application was made to William and Mary, for a ship, "in order to seek, by observation, the discovery of the rule for the variation of the compass." The command of a ship of the royal navy was in consequence given to Dr Halley; and, in the accomplishment of the object which he had in view, he performed two voyages, one in 1698 and the other in 1699, in which he traversed various parts of the Pacific and Atlantic Oceans, and obtained such a number of valuable results, that he completed a chart of the variation of the needle, which exhibited to the eye the general law of its phenomena.

The very important discovery of the daily variation of the needle was made in 1722, by Mr Graham, a celebrated mathematical instrument maker in London. While the needle was advancing by an annual motion to the westward, Mr Graham found that its north extremity moved westward during the early part of the day, and returned again in the evening to the eastward, to the same position which it occupied in the morning, remaining nearly stationary during the night. Mr Graham at first ascribed these changes to defects in the form of his needles; but, by numerous and careful observations, repeated under every variation of the weather and of the heat and pressure of the atmosphere, he concluded that the daily variation was a regular phenomenon, of which he could not find the cause. It was generally a maximum between ten o'clock a.m. and 4 o'clock p.m., and a minimum between six and seven o'clock p.m. Between the 6th February and the 12th May 1722, he made a thousand observations in the same place, from which he found that the greatest westerly variation was $14^\circ 45'$, and the least $13^\circ 50'$; but in general it varied between $12^\circ 35'$ and $14^\circ$, giving $35'$ for the amount of the daily variation.

The law of the magnetic force, or the rate at which it varies with the distance, had, as we have seen, occupied the attention of Sir Isaac Newton and David Gregory. Numerous experiments were made by various authors for the same purpose, a large collection of which have been published by Scarcella, in his treatise De Magnete, published at Brescia in 1759. Muschenbroeck made a great number of experiments with the same view; but as the joint action of the four poles was never considered, the precise law of variation remained unknown. Mr Hauksbee and Dr Brooke Taylor employed a much better method, namely, the deviation of a compass-needle from the meridian, produced by the action of a magnet at different distances; but the magnets which they used had improper shapes, and the conclusion which they drew from their experiments was, that the magnetic force decreased much quicker at great distances than at small ones, and that it is different in different loadstones.

Notwithstanding this strange conclusion, the observations to which we have referred were of great value; and Mr Michell succeeded in deducing from them, in 1750, the true law of magnetic action. "There have been," says Mr Michell, "some who have imagined that the decrease of the magnetic attraction and repulsion is inversely as the cubes of the distances; others, as the squares; and others, that it follows no certain ratio at all, but that it is much quicker at great distances than at small ones, and that it is different in different stones. Among the last is Dr Brooke Taylor and Muschenbroeck, who seem to have been pretty accurate in their experiments. The conclusions of these gentlemen were drawn from their experiments, without their being aware of the third property of magnets just mentioned, which, if they had made proper allowances for, together with the increase and diminution of power in the magnets they tried their experiments with, all the irregularities they complained of (as far as appears from their relations of them) might very well be accounted for, and the whole of their experiments coincide with the squares of the distances inversely."

It is to Mr Michell also that we owe the introduction of the torsion balance, for measuring small forces; an instrument which, as we shall see, was employed with singular success and dexterity by Coulomb in his electrical, magnetic, and hydrodynamical researches; and the science of magnetism is no less indebted to Mr Michell for his invention of the method of double touch, as it is called, by which artificial magnets may be made with greater strength than could have been obtained from the previous method of Duhamel.

The hypothesis of Descartes, who explained the polarity of the needle by means of currents moving rapidly from the equator to the poles, was adopted and defended by Euler and Daniel Bernoulli; but we cannot afford any space for such useless speculations. Euler afterwards occupied himself more advantageously for science in attempting to investigate mathematically the direction of the needle on every part of the earth's surface. Perceiving the intricacy which would arise from the adoption of four poles, as imagined by Halley, he tried the effect of employing two poles not diametrically opposite; and he found, that when a proper position was given them, the variation under the same meridian might be both easterly and westerly, as in Halley's chart. The solution which he has given is founded on the principle, "that the magnetic direction on the earth follows always the small circle which passes through the given place, and the two magnetic poles of the earth;" or that the horizontal needle is a tangent to the circle passing through the place of observation, and through the two points on the earth's surface where the dipping needle becomes vertical, or the horizontal needle loses its directive power. In the application of this principle, Euler makes four different suppositions respecting the magnetic poles: 1. That they are diametrically opposite to each other; 2. that they are in opposite meridians, but not in opposite parallels; 3. that they are on the same meridian; and 4. that they have every other situation whatever. The first of these suppositions he finds to be quite irreconcilable with the observed phenomena, but in the other three he finds that the variation may be both easterly and westerly in the same meridian. By successive approximations he finds the position of the two magnetic poles in 1757 to be as follows: The north pole in latitude $76^\circ$ north, and longitude $96^\circ$ west from Teneriffe; and the south pole in latitude $58^\circ$ south, and longitude $158^\circ$ west from Teneriffe. To this dissertation Euler has added a chart of the curves of equal variation, calculated on the preceding principles, and suited to 1757; and their general accordance with observation is very surprising. In a subsequent dissertation Euler endeavoured to improve his theory, by supposing the two magnetic poles to be at the surface of the earth. The chord joining these poles he calls the magnetic axis, and the middle point of that chord its magnetic centre. Then, if we draw a line from the place of observation to the magnetic centre, and consider this as the base of an isosceles triangle, one of whose sides is the magnetic axis, the other side will be the direction of a freely suspended needle. This hypo-

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1 Phil. Trans. No. 368, 396. 2 A Treatise of Artificial Magnets, 8vo, Lond. 1750, p. 19. 3 Berlin Memoirs, 1757, 1766. Magnetism.

The law of magnetic action occupied the particular attention of M. Lambert, the celebrated Prussian philosopher, who has published an account of his labours in the Memoirs of the Academy of Berlin for 1756. Having placed a mariner's compass at various distances from a magnet, and in the direction of its axis, he observed the declination of the needle produced by the magnet, and the obliquity of the magnet to the needle's axis. From several observations at different obliquities, he found that the action of magnetism on a lever was proportional to the sine of the angle of its obliquity to the axis of the lever or needle. M. Lambert then proceeded to study the effect of distance, and he discovered that the force of a magnet is proportional to the distance of the nearest pole of the magnet from the centre of the needle, diminished by the square of a constant quantity, nearly equal to two thirds of the length of the needle. This result he found to be true with magnets ten times larger, and needles twice as short; but as the law led him to a strange result, as if the action on a magnet were exerted from a centre beyond itself, he was therefore obliged to take another method of determining the law of action, namely, by a series of experiments on the directive power of the magnet, from which he inferred, "that the force of each transverse element of a magnet is as its distance from the centre, and its action as a particle of another magnet inversely as the square of the distance." By means of this law he calculates the position of a very small needle, and draws three of the curves to which it should be a tangent, and these coincide very accurately with some of those which he had observed.

Our learned countryman Dr Robison had been pursuing similar inquiries before he had seen Lambert's experiments. He got some magnets made, composed of two balls connected by a slender rod; and after magnetising them strongly, he found that the force of each pole resided nearly in the centre of the ball. In this way the attractive and the directive powers of the magnets were easily computed, and the result was, that the force of each pole was inversely as the square of the distance. In no case did the error of this hypothesis amount to one fifth of the whole, and in the calculation for the phenomena of the directive power the errors were still smaller. When Dr Robison had seen Lambert's second memoir, he repeated all his former experiments in Lambert's manner, taking the precaution of keeping the needle in its natural position, which he had not previously attended to; and the results which he now obtained were still more conformable to his conjecture as to the law of variation. Dr Robison tried another method of ascertaining the law of magnetic action. In 1769 or 1770 he constructed a needle of two balls joined by a slender rod, and having touched it with great care, so as to keep the whole strength of the poles near the centre of the balls, he counted the number of oscillations which it performed horizontally in a given time by the force of the earth's magnetism. "He then placed it on the middle of a very fine and large magnet, placed with its poles in the magnetic meridian, the north pole pointing south. In this situation he counted the vibrations made in a given time. He then raised it up above the centre of a large magnet, till the distance of its poles from those of the great magnet was changed in a certain proportion. In this situation its vibrations were again counted. It was tried in the same way in a third situation, considerably more remote from the great magnet. Then having made the proper reduction of the forces corresponding to the obliquity of their action, the force of the poles of the great magnet was computed from the number of vibrations." The results of these experiments were the most consistent with each other of any that Dr Robison made for determining the law of the magnetic force, and it was chiefly from them that he thought himself authorized to say with some confidence, that it is inversely as the square of the distance. When Dr Robison, however, observed, some years afterwards, that Æpinus, in 1777, conceived the force to vary inversely as the simple distance, he repeated the experiments with great care, and added another set made with the same magnet, and the same needle placed at one side of the magnet instead of above it. By this arrangement, which greatly simplified the process, the result of the whole was still more satisfactory. The inverse law of the square of the distance was therefore well established.

Various speculations respecting the cause of the phenomena of magnetism had been hazarded by different authors; but it was reserved for M. Æpinus to devise a rational hypothesis, which embraced and explained almost all the phenomena which had been observed by previous authors. This hypothesis, which he has explained at great length in his Tentamen Theoria Electricitatis et Magnetismi, published in 1759, may be stated in the following manner.

1. In all magnetic bodies there exists a substance which may be called the magnetic fluid, whose particles repel each other with a force inversely as the distance. 2. The particles of this fluid attract the particles of iron, and are attracted by them in return with a similar force. 3. The particles of iron repel each other according to the same law. 4. The magnetic fluid moves through the pores of iron and soft steel with very little obstruction; but its motion is more and more obstructed as the steel increases in hardness or temper, and it moves with the greatest difficulty in hard-tempered steel and the ores of iron.

The method of making artificial magnets, which was practised by the philosophers of the seventeenth century, of making magnets was a very simple but a very inefficacious one. It consisted in merely rubbing the steel bar to be magnetised, upon one of the poles of a natural or artificial magnet, in a direction at right angles to the line joining the poles of the magnet. Towards the middle of the eighteenth century, however, the art of making artificial magnets had excited general attention; and it is to Dr Gowin Knight, an English physician, that we are indebted for the discovery of a method of making powerful magnets. This method he kept secret from the public, but it was afterwards published by Dr Wilson. Duhamel, Canton, Michell, Antheaume, Savery, Æpinus, Robison, Coulomb, Biot, Scoresby, and others, made various improvements on this art, as will be minutely described when we arrive at that part of our subject.

The science of magnetism owes many obligations to Mr John Canton, one of the most active experimental philosophers who adorned the middle of the eighteenth century. In or previous to the year 1756, he made no fewer than 4000 observations on the diurnal variation of the needle, with the view of determining its amount, and investigating its origin. He found the daily change differ-

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1 See an excellent paper on Terrestrial Magnetism, in the Magazine of Popular Science, May 1836, No. iv. p. 223, 224. 2 Memoirs of the Berlin Academy, vol. xxii. He found also that the time of minimum westerly variation at London was between eight and nine o'clock A.M., and the time of maximum between one and two o'clock P.M., the needle returning to its morning position about eight or nine in the evening. A series of similar observations were made with nearly the same results by Mr Van Swinden; but this excellent observer discovered, that some time before the hour in the morning when the westerly minimum took place, and after the same hour in the evening, a motion of the needle both to the eastward and westward took place; that is, the morning westerly variation is sometimes preceded by a small easterly variation, and the principal easterly variation in the evening is followed by a small westerly variation.

Canton explained the westerly variation of the needle, and the subsequent easterly motion, by supposing that the heat of the sun, acting upon the eastern parts of the earth, weakens their influence, as heat is known to do that of a magnet, and consequently the needle will move to the westward. In the same way, as the sun warms the western side of the earth in the afternoon, the needle will then take a contrary direction.

One of the ablest cultivators of the science of magnetism was the celebrated Coulomb, who, by the application of the principle of torsion, first used by Michell, determined the correct law of magnetic attractions and repulsions. After measuring with great nicety, by the torsion balance, the force requisite to make a magnetic bar suspended horizontally deviate any number of degrees from a given position, he was enabled to verify the discovery of Lambert already mentioned, that the effect of terrestrial magnetism is proportional to the sine of the angle which the magnetic meridian forms with the axis of the magnet upon which it acts. By making the homologous poles of two magnetised wires repel each other, he observed the force of torsion which was necessary to overcome certain quantities of their mutual repulsion, and, at the distances 12°, 17°, and 24°, he found that the repulsive forces were as the numbers 3312, 1692, and 864, deviating little from 3312, 1650, and 828, which they would have been had the repulsive force varied in the inverse ratio of the square of the distance. The excess of 42 and 36 in the experimental numbers was owing to the circumstance that it was not a particle, but a portion, of each wire, from which the repulsive force emanated, so that the force of the other particles being exerted less obliquely, and therefore being stronger at greater distances, ought to produce an excess such as that actually observed. A similar result was obtained when the contrary poles of the magnetised wires were made to attract each other; so that Coulomb concluded that the attractive and repulsive forces exercised by two magnetic particles are inversely as the square of their distances, a result which he confirmed by several other methods than that which we have noticed.

Provided with such a delicate instrument as the torsion balance, Coulomb was enabled to apply it with singular advantage to almost every branch of the science. His first object was to determine the law according to which magnetism is distributed in a magnetic bar. It was of course well known that the magnetism in the middle of the bar was imperceptible, and that it increased according to a regular law, and with great rapidity, towards each of its poles. By suspending a small proof needle with a silk fibre, and causing it to oscillate horizontally, opposite different points of a magnetic bar placed vertically, Coulomb computed the part of the effect which was due to terrestrial magnetism, and the part which was due to the action of the bar; and in this way he obtained the following results, which show the extreme rapidity with which magnetism is increased towards the poles.

| Distances from the north end of the Bar | Intensity of the Magnetism at these distances | |----------------------------------------|-----------------------------------------------| | 0 inches | 165 | | 1 | 90 | | 2 | 48 | | 3 | 23 | | 4.5 | 9 | | 6 | 6 |

In examining the distribution of electricity in a circular plane, Coulomb found that the thickness of the electric stratum was almost constant from the centre to within a very small distance of the circumference, when it increased all on a sudden with great rapidity. He conceived that a similar distribution of magnetism took place in the transverse section of a magnetic bar; and, by a series of nice experiments with the torsion balance, he found this to be the case, and established the important fact, that the magnetic power resides on the surface of iron bodies, and is entirely independent of their mass.

The effect of temperature on magnets was another subject to which Coulomb directed his powerful mind; but he did not live to give an account of his experiments, which were published after his death by his friend M. Biot. Coulomb found that the magnetism of a bar magnetised to saturation diminished greatly by raising its temperature from 12° of Reaumur to 680°; and that when a magnetic bar was tempered at 780°, 860°, and 950° of Reaumur, the development of its magnetism was gradually increased, being more than double at 900° of what it was at 780°. He found also that the directive force of the bar reached its maximum when it was tempered at a bright cherry-red heat at 900°; and that at higher temperatures the force diminished. It is to Mr Barlow, however, as we shall presently see, that we are indebted for the complete investigation of the influence of temperature on the development of magnetism.

Coulomb made many valuable experiments on the best methods of making artificial magnets, and he subjected all the various processes that had been previously employed to the test of accurate measurement. His experiments on the best forms of magnetic needles are equally valuable; but the most interesting of his researches, and the last to which he devoted his great talents, were those which relate to the action of magnets upon all natural bodies. Hitherto, iron, steel, nickel, and cobalt, had been regarded as the only magnetic bodies; but, in the year Universal 1802, Coulomb announced to the Institute of France, that all bodies whatever are subject to the magnetic influence, even to such a degree as to be capable of accurate measurement. The substances employed by Coulomb were in the form of a cylinder or small bar, about one third of an inch in length and one thirtieth in thickness, and they were suspended by a single fibre of silk between the opposite poles of two powerful steel magnets, placed in the same straight line, and having their opposite poles at a distance exceeding by a quarter of an inch the length of the cylinders. The cylinders were then made to oscillate between the poles of the magnets, and were

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1 See our article Electricity, p. 586, where the torsion balance is minutely described. The result of these experiments was, that whatever was the substance of the cylinders, they always arranged themselves in a line joining the poles of the magnets, and returned to that position whenever they were deflected from it. These experiments were made with cylinders of gold, silver, copper, lead, tin, glass, wood, chalk, bone, and every variety of substance, organic and inorganic.

The only explanation which Coulomb could give of these phenomena was, either that all bodies whatever were susceptible of magnetism, or that they contained small portions of iron or other magnetic metals, which communicated to them the property of obeying the magnet. In order to investigate this subject, MM. Sage and Guyton prepared highly purified needles of the different metals, and M. Coulomb found that the momenta of the forces with which they were solicited by the magnets were as follows:

| Substance | Momentum | |-----------|----------| | Lead | 0.00674 | | Tin | 0.00591 | | Silver | 0.00520 | | Gold | 0.00406 | | Copper | 0.00406 |

The momentum of torsion alone, for all the needles, being 0.00136, a little more than a fourth of the action which the magnets exert upon the needles.

In order to determine if these phenomena were owing to particles of iron disseminated through the bodies, Coulomb fabricated needles out of three different mixtures of white wax and iron filings, and he found that the forces exerted by magnets upon these needles were proportional to the absolute quantities of iron which they contain. Coulomb now tried a needle of silver, purified by upellation, and another needle of silver alloyed with 1/10th part of iron, and he found that the action of the magnet upon the former was 415 times less than upon the latter. Hence there will be 415 times less iron in the pure than in the impure silver; and since the latter contains 3/10th part of its weight of iron, the first will contain 3/10th of 3/10th, or 9/100th, or it will contain 32799 parts of pure silver and one of iron, a quantity of alloy beyond the reach of chemical detection.

Amongst the scientific travellers who contributed to our knowledge of terrestrial magnetism, Baron Alexander Humboldt was one of the most distinguished. Himself an accurate and scientific observer, and possessed of nice instruments and methods of observation, he made numerous accurate observations on the dip and variation of the needle in various parts of the earth, and particularly near the magnetic equator; and, by means of these valuable data, M. Biot was enabled to throw much light on the subject of terrestrial magnetism. Hitherto the magnetic poles had been considered as either on or very near the surface of the earth; but as it had been found impossible to deduce the phenomena of the variation and dip of the needle, philosophers were led to consider the situation of these poles as indeterminate. M. Biot was the first to adopt this view of the subject; and after numerous comparisons, he came to the conclusion, that the nearer these poles were placed to each other, the greater was the agreement between the computed and observed results; and by considering the two poles as indefinitely near each other in the centre of the earth, the computed and observed measures approximated as closely as could be expected. Hence it was inferred that the phenomena of terrestrial magnetism were not such as are produced by permanently magnetic bodies, but those rather that arise from simple iron or ferruginous masses, which are only temporarily magnetic. In this manner M. Biot was led to express the law of terrestrial magnetism in a complicated formula, which represented the observations with wonderful accuracy.

In the year 1809, Professor Krafft of St Petersburg undertook the very same inquiry, and after comparing the Krafft's same observations which were used by Biot, with the respective situations of the places where they were obtained, he arrived at the following simple law: "If we suppose a circle circumscribed about the earth, having the two extremities of the magnetic axis for its poles, and if we consider this circle as a magnetic equator, the tangent of the dip of the needle, in any magnetic latitude, will be equal to double the tangent of this latitude." Upon re-examining his former formula, M. Biot found that it was reducible to the above simple law, a coincidence which may be considered as giving it additional confirmation.

One of the most zealous and successful cultivators of magnetical science is Professor Hansteen of Christiania, who published, in 1817, an able work on the magnetism of the earth. The Royal Society of Denmark proposed, in 1811, the prize question, "Is the supposition of one magnetic axis sufficient to account for the magnetical phenomena of the earth, or are two necessary?" Professor Hansteen's attention had been previously drawn to this subject by seeing a terrestrial globe, on which was drawn an elliptical line round the south pole, and marked Regio Polaris magnetica, one of the foci being called Regio fortior, and the other Regio debilior. As this figure professed to be drawn by Wilcke, from the observations of Cook and Furneaux, Hansteen was led to compare it with the facts; and the result of the comparison being favourable, he was induced to study the theory of Halley, which had previously appeared to him wild and extravagant. The result of his researches, however, was favourable to that part of Halley's theory which assumes the existence of four poles and two magnetic axes. Hansteen's Memoir, which was crowned by the Danish Society, forms the groundwork of the larger volume which he published in 1817. In his fifth chapter, on the Mathematical Theory of the Magnet, he deduces the law of magnetic action from a series of experiments similar to those of Hauksbee and Lambert. Assuming that the attraction or repulsion between any two magnetic particles is directly as the intensity of the force, and inversely as some unknown power \( t \) of the mutual distance of these particles, and supposing that the magnetic intensity of any particle is proportional to some power \( r \) of its distance \( a \) from the centre of the magnet, he finds the following expression for the effect \( F \), which a linear magnet would have upon a magnetic point situated anywhere upon the axis produced:

\[ F = \int \frac{x^t dx}{(a-x)^t} - \int \frac{x^t dx}{(a+x)^t}; \]

\( x \) being the length of half the axis of the linear magnet, and \( F \) (multiplied by a constant quantity, depending on the degrees of magnetism which the point and line possess) representing the force. In conducting the experiments, Hansteen placed a very sensible compass upon a horizontal table, so that the needle pointed to 0°. From beneath the centre of this needle, and perpendicular to its direction, or to the magnetic meridian, he drew a straight line upon the table, and divided it into portions, so that ten of them were equal to the half axis \( a \) of the artificial magnet. This magnet was then placed on the line at different distances from the needle, and the deviation of the needle from the magnetic meridian which it produced was accurately observed for each distance. Upon com-

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1 Unternehmungen über den Magnetismus der Erde, 4to. Christiania, 1817. paring the results, and calculating them by the formula, upon the supposition that \( t \) was 1 or 3, the differences Hansteen's were very great; but by making \( t = 2 \), the calculated and observed results agreed remarkably well, as the following table shows:

| Values of \( s \), or Distances in half Axes of the Magnet. | Values of \( F \), or Increase of the Force. | |----------------------------------------------------------|-----------------------------------------------| | | Calculated Values. | | | Observed Values. \( r = 1 \) \( r = 2 \) \( r = 3 \) | | 11 | 1-000 1-000 1-000 | | 10 | 1-306 1-334 1-334 1-325 | | 9 | 1-834 1-835 1-836 1-836 | | 7 | 3-947 3-938 3-945 3-949 | | 5 | 11-015 11-072 11-119 11-154 | | 4 | 22-441 22-245 22-411 22-530 |

From this remarkable coincidence between the observed and the computed results, Hansteen concludes that "the attractive or repulsive force with which two magnetic particles affect each other, is always directly as their intensities, and inversely as the squares of their mutual distance." He shows that the undetermined value of \( r \) produces almost no effect at considerable distances; and he is inclined to think that \( r = 2 \), or that the absolute intensity of any magnetic particle, situated in the axis, is proportional to the square of its distance from the middle point of that axis.

Mr Hansteen has also demonstrated that the distance from the middle of a magnet being the same, the force opposite the poles, or in the direction of the axis, is double of the force in the magnetic equator. If a globe contains at its centre an infinitely small magnet, Hansteen shows that, near the magnetic equator, the dip must increase twice as rapidly as the magnetic latitude, and, near the pole, half as rapidly; and that the increment of the dip must be equal to the alteration of the latitude of that part of the globe where the dip is 54° 44′. Our author also states, that if the earth had only one magnetic axis, whose centre coincided with that of the earth, the lines of equal dip would coincide with those of equal intensity; but as this is far from being the case, his opinion that there are two magnetic axes becomes more probable.

The most valuable part, however, of Professor Hansteen's work is that which relates to the number, position, and revolution of the magnetic poles. Having collected all the observations of value that had been made on the variation of the needle, he proved that there were four points of convergence among the lines of variation, viz. a weaker and a stronger point in the vicinity of each pole of the globe. The strongest poles N, S, lie almost diametrically opposite to each other, and the same is true of the weaker poles \( n, s \). These four poles he found to have a regular motion obliquely, the two northern ones N, \( n \), from west to east, and the two southern ones S, \( s \), from east to west. The following he found to be their periods of revolution, and their positions, in 1830:

| Pole N | 69° 30′ N. | 87° 19′ W. | 1740 years. | | Pole S | 68 44 S. | 131 47 E. | 4609 do. | | Pole \( n \) | 85 6 N. | 144 17 E. | 860 do. | | Pole \( s \) | 78 29 S. | 137 45 W. | 1304 do. |

Since the publication of these results, Professor Hansteen had access to the valuable series of magnetical observations made during the British voyages of discovery to the arctic regions; and, after a diligent comparison of them, he obtained new and more accurate determinations of the positions and periods of revolution of the four magnetic poles, but still differing so little from his previous determinations, as to give a high degree of probability to the correctness of the results. He finds from Captain Parry's observations, that in 1819 the pole N must have been situated in north lat. 71° 27′, and the time of its revolution 1890 (in place of 1740, as formerly calculated). The period of revolution of S he changed from 4609 to 4605, and that of \( s \) from 1304 to 1303, that of \( n \) remaining at 860 as before. Professor Hansteen considers N and S as the terminating points of one magnetic axis, and \( n \) and \( s \) those of the other axis; and he remarks that these two axes cross each other without intersection, or without passing through the centre of the earth. In reference to the causes of these singular changes, our author considers it possible that the illumination and heating of the earth, during one revolution about its axis, may produce a magnetic tension, as it produces the electrical phenomena. In support of his hypothesis of four poles, Professor Hansteen has shown very clearly that the changes in the variation and dip of the needle in both hemispheres may be well explained by their motion; but we cannot here enter into these details.

With the view of discovering the nature of the forces by which the phenomena of terrestrial magnetism are produced, Professor Hansteen resolved to ascertain, at different parts of the earth's surface, the intensity of its magnetism, and to determine the form of the lines of equal intensity, or, as he calls them, the isodynamical magnetic lines. By means of the same needle intrusted to different philosophers, he had observations on the number of its oscillations in a given time made in every part of Europe; and he afterwards undertook a journey to Siberia to make the observations himself in that interesting magnetical region. From these observations he deduced the following law, according to which the magnetic intensity varies with the dip of the needle:

| Magnetic Dip. | Magnetic Intensity. | |--------------|---------------------| | 0° | 1-0° | | 24 | 1-1 | | 45 | 1-2 | | 64 | 1-3 | | 73 | 1-4 | | 76° | 1-5 | | 81 | 1-6 | | 86 | 1-7 |

Professor Hansteen's journey to Siberia was attended with secondary consequences of great value to science. The attention of the Russian government, and the Academy of Sciences at St Petersburg, was thus called to the subject of magnetism; and, on the recommendation of Baron Humboldt, the emperor liberally agreed to erect magnetic observatories in suitable stations, for determining, every ten years, the exact position of the two lines of the variation which pass through his empire.

In determining the intensity of terrestrial magnetism, Professor Hansteen observed that the time of vibration of a horizontal needle varied during the day. Graham had previously suspected a change of this kind, but his methods were not accurate enough to prove it. Hansteen, however, found that the minimum intensity took place between 10 and 11 A.M., and the maximum between 4 and 5 P.M. He concluded also that there was an annual variation, the intensity being considerably greater in winter near the perihelion, than in summer near the aphelion; that the greatest monthly variation was a maximum when the earth is in its perihelion or aphelion, and a minimum near the equinoxes; and that the greatest daily variation... is least in winter and greatest in summer. He found also that the aurora borealis weakened the magnetic force, and that the magnetic intensity is always weakest when the moon crosses the equator. In making experiments in the round tower at Copenhagen, he found that the magnetic intensity increased regularly towards the top, where it was a maximum; and having extended his observations, he obtained the general result, that at the foot of any vertical object the needle oscillates quicker at the north side of it, and slower at the south side; whereas at the upper end it oscillates quicker at the south side, and slower at the north side.

In the aerostatic ascent of MM. Gay-Lussac and Biot, they were unable to detect any change in the intensity of terrestrial magnetism at the height of 4000 metres. Saussure, however, had found that the intensity was considerably less on the Col du Géant than at Chamouni and Geneva, the difference in the levels of these places being in the one case 10,000 and in the other 7800 feet, but his observations contradict his conclusion. M. Kupffer has more recently obtained a similar result by observations on Mount Elbrouz, having found a decrease of intensity in rising 4500 feet above his first station; and he explains the result obtained by MM. Gay-Lussac and Biot by supposing that an increase of intensity was produced by the diminution of temperature. Mr Henwood, on the other hand, has made observations at the surface of Dolcoath mine, at 1320 feet beneath its surface, and on a hill 710 feet above the level of the sea, without being able to detect any difference in the intensity. To the late Captain Foster we owe many valuable observations on the magnetic intensity made at Spitzbergen and elsewhere. From these he concluded, that the diurnal change in the horizontal intensity is principally, if not wholly, owing to a small change in the amount of the dip. The maximum took place at about 3h. 30' A.M., and the minimum at 2h. 47' P.M., its greatest change amounting to one eighty-third of its mean value. Captain Foster is of opinion that these changes have the sun for their primary agent, and that his action is such as to produce a constant inflexion of the pole towards the sun during the 24 hours, an idea which Mr Christie had previously stated.

About the year 1818, Professor Barlow of Woolwich turned his attention to the subject of magnetism, with the view principally of calculating the effect of a ship's guns on the compass. In trying the effects of different iron balls, he was led to the curious facts, that there exists round every globe and mass of iron a great circle inclined to the horizon at an angle equal to the complement of the dip of the needle; that the plane of this circle is a plane of no attraction upon a needle whose centre is in that plane; that if we regard this circle as the magnetic equator, the tangent of the deviation of the needle from its north or south pole will be proportional to the rectangle of the sine of the double latitude, and cosine of the longitude; that when the distance of the needle is variable, the tangent of deviation will be reciprocally proportional to the cube of the distance; and that, all things else being the same, the tangents of deviation will be proportional to the cubes of the diameters of the balls or shells, whatever be their masses, provided their thickness exceed a certain quantity.

These results were published in the first edition of Mr Barlow's Essay on Magnetic Attractions; but in the second edition of that work, he has published some curious results respecting the relative magnetic power of different descriptions of iron and steel, and on the effect of temperature on the quality and quantity of the attractive power of iron. The results of the first of these series of experiments were as follow, the numbers expressing the proportional magnetic power of the different descriptions of iron and steel.

- Malleable iron: 100 - Shear steel, hard: 53 - Blistered steel, soft: 67 - Cast iron: 48 - Blistered steel, hard: 53 - Cast steel, soft: 74 - Shear steel, soft: 66 - Cast steel, hard: 49

In his experiments on the effects of temperature, Mr Barlow found that every kind of iron and steel possessed a greater capacity for the development of its magnetism when softened by heat than when cold; from which he concludes that its complete development when cold is prevented only by the hardness or resisting power of the metal. At a white heat he found that iron lost entirely its magnetic power, a result apparently inconsistent with the preceding conclusion; but, what was a still more extraordinary circumstance, when the white heat, at which there was no magnetism, began to subside into a bright red, or red heat, an attractive power showed itself, the reverse of what it had when cold; and after it had passed through these two shades of colour, it resumed the same attractive power which it had when cold, the passage from the negative attraction of red passing into the positive attraction of the cold metal at the point of a red heat, the maximum, however, taking place at a blood-red heat.

The experimental laws of attraction of an iron shell or Mr C. Bonnycastle, obtained by Mr Barlow, were first examined theoretically by Mr Charles Bonnycastle, who deduced them mathematically from the theory of Apinus, which supposes the two magnetic fluids to be accumulated in the poles of magnets. This theory, however, led to some improbable consequences, and therefore Mr Barlow was induced to adopt that of Coulomb and Biot, with the modification, that the magnetic power all resides on the surface of iron bodies, and is independent of the mass; a modification which enabled Mr Barlow to obtain a general analytical expression of the disturbing power of an iron ball at its surface, as compared with that of the earth, and from which he deduced theoretically all his experimental laws.

These important discoveries enabled Mr Barlow to invent a most ingenious method of correcting the error of the compass, arising from the attraction of all the iron on board ships. This source of error had been noticed by Mr Wales, Mr Downie in 1794, and by Captain Flinders; but it is to Mr Bain* that we owe the distinct establishment and explanation of this source of error. As a hollow shell of iron about four pounds in weight acts as powerfully at the same distance as a solid iron ball of 200 pounds weight, Mr Barlow happily conceived that a plate of five or six pounds weight might be made to represent and counteract the amount of the attraction of all the iron on board a vessel, and therefore leave the needle as free to obey the action of terrestrial magnetism as if there were no iron in the ship at all. After this ingenious contrivance had been submitted to the admiralty, it was tried in every part of the world; and even in the regions which surround the magnetic pole, where the compass becomes useless, it never failed to indicate the true magnetic direction when the connecting plate was properly applied. At Port Bowen, where the dip is 88°, and the magnetic intensity which acts upon a horizontal needle extremely weak, the azimuth compass on board Captain Parry's ship gave the very same variation as that observed on shore. "Such an invention as this," says Captain Parry, "so sound in principle, so easy of application, and so universally beneficial..." in practice, needs no testimony of mine to establish its merits; but when I consider the many anxious days and sleepless nights which the uselessness of the compass in these seas had formerly occasioned me, I really should have esteemed it a kind of personal ingratitude to Mr Barlow, as well as great injustice to so memorable a discovery, not to have stated my opinion of its merits, under circumstances so well calculated to put them to a satisfactory trial." For this beautiful invention, the Board of Longitude conferred upon Mr Barlow the highest reward of L500; and the Emperor of Russia, who was never attentive to the interests of science, sent him a fine gold watch and a rich dress chain, for the same contrivance.

A series of beautiful discoveries was made about this time by M. Arago, Mr Christie, and Mr Barlow, on the influence of rotation on bodies both magnetic and non-magnetic. Mr Barlow, so early as 1818 or 1819, had found, that when a plate of iron was made to turn upon its centre, different parts of its circumference had different degrees of magnetic action on the compass; but here there was no effect discovered as due to rotation. In 1821, Mr Christie, in a series of experiments on iron plates, not only found that different parts in the circumference of the same plate had different attracting powers; but that the same part had a different influence, according as the same plate was made to revolve to the right or left hand. Mr Christie therefore discovered that there was a deviation due to rotation, and that magnetical effects were produced which were nearly independent of the velocity of rotation, and which continued after the rotation had ceased. When the rotation was very rapid, the forces exerted upon the needle were always in the same direction as the forces derived from the slowest rotation, and which continue to act after the rotation has ceased, the former being to the latter nearly as three to two. From all the observations made by Mr Christie, he considers that the direction of the magnetic polarity acquired by rotation, whether at right angles to the line of the dip or not, has always a reference to the direction of the terrestrial magnetic force; and he infers that this magnetism is communicated to it from the earth. "It does not therefore appear from this," says Mr Christie, "that a body can become polarised by rotation alone, independently of the action of another body; so that, if from these experiments we might be led to attribute the magnetic polarity of the earth to its rotation, we must at the same time suppose a source from which magnetic influence is derived. Is it not, then, possible that the sun may be the centre of such influence, as well as the source of light and heat, and that, by their rotation, the earth and other planets may receive polarity from it?" When these experiments were repeated at Port Bowen in 1825 by Captain Foster, the phenomena were exhibited on a more striking scale.

In December 1824, Mr Barlow began a series of experiments, with the view of ascertaining whether magnetism, as produced by various processes with iron, could be excited or disturbed by rapid rotation. They were completed in January, but their publication was delayed till June, that an account of them might appear along with those of Mr Christie above mentioned. Mr Barlow's first experiments were made on a 13-inch shell attached to a lathe turned by a steam-engine, the mean speed of which was about 640 revolutions in a minute. The deviation of a needle exposed to its action increased with the velocity, and remained constant while the velocity continued constant, the needle always returning exactly to its original position the moment the motion of the ball ceased. This, therefore, is a phenomenon different from that observed by Mr Christie; a temporary effect wholly dependent on the velocity of rotation, whereas that observed by Mr Christie was permanent, and nearly independent of the number of revolutions. In examining the direction of the new force impressed upon the iron shell, he found it to be in every case equivalent to a polarisation at right angles to the axis of rotation.

Previous to the publication of these experiments, and without any knowledge of them, M. Arago had made the remarkable discovery, that if plates of copper and other substances are put into rapid rotation beneath a magnetised horizontal needle freely suspended, the rotatory plate will first cause the needle to deviate from its true direction; and by increasing the velocity, the deviation will increase, till the needle passes the opposite point, when it will continue to revolve, and at last with such velocity that the eye is unable to distinguish it.

M. Arago was led to this beautiful discovery by a previous series of experiments of great interest. He found that a magnetic needle oscillating above or near any body whatever, such as a plate of metal or a surface of water, gradually oscillated in arcs of less and less amplitude, as if it had been placed in a resisting medium; and, what was particularly remarkable, the number of oscillations performed in a given time was not changed. This curious fact was announced to the Academy of Sciences in Paris on the 22nd of November 1824; and he was hence conducted to the still more remarkable discovery of the effects of rotation which we have already mentioned.

M. Seebeck of Berlin repeated the experiments of M. Arago on the influence of plates of metal and other substances in diminishing the amplitude of oscillation; but we must reserve our account of them till we come to the chapter on that subject.

The experiments of M. Arago on the rotation of metallic plates were described and repeated by M. Gay-Lussac in London in the month of March or April 1825; and they excited so much attention, from their connection with the effects observed by Mr Barlow, that Mr Babbage and Mr Herschel immediately erected an apparatus for repeating them. In their first trial, the deviation of the needle did not exceed 10° or 11° with a revolving plate of copper. In order to enlarge the visible effect, they reversed the experiment, in order to try whether discs of copper and other non-magnetic substances might not be set in motion if suspended over a revolving magnet. A horse-shoe magnet, capable of lifting twenty pounds, was made to revolve rapidly about its axis of symmetry placed vertically. A circular disc of copper, six inches in diameter and one twenty-fifth of an inch thick, was suspended centrally over it, by a silk thread just capable of supporting it. A sheet of paper being interposed, and the magnet set in motion, the copper began revolving in the same direction, at first slowly, but with an accelerating velocity. On reversing the motion of the magnet, the velocity of the copper was destroyed gradually. It stopped for an instant, and then immediately began to revolve in the opposite direction. Screens of paper, glass, wood, copper, tin, zinc, lead, bismuth, were interposed betwixt the magnet and the copper, but they exerted no sensible interceptive power. But when tinned iron plate was interposed, the magnetic influence was greatly diminished by one plate, and almost annihilated by two thicknesses of it. A piece of iron connecting the two poles of the revolving magnet produced the same effect. The substances in which signs of magnetism were developed by the revolving magnet were copper, zinc, silver, tin, lead, antimony, mercury, gold, bismuth, and carbon in the state in which it is precipitated from carburetted hydrogen in gas-works. By getting plates of different metals cast in the same mould, they found that the proportional intensity of magnetic action for each respectively was as follows:

| Substance | Intensity | |-----------|-----------| | Zinc | 1·11 | | Copper | 1·00 | | Tin | 0·51 | | Lead | 0·25 | | Antimony | 0·01 | | Bismuth | Inappreciable | M. Arago had observed the very remarkable fact, that if the disc of copper be cut from the circumference towards the centre, like radii, but without taking away the metal, the action upon the needle is greatly diminished. After verifying this result, Messrs Babbage and Herschel ascertained that re-establishing the metallic contact with other metals, restored, either wholly or very nearly, the original power of the plate, even though the soldering metal had a very feeble magnetic power. The law of the force, with a decrease of distance, they found to vary between the square and the cube. "The rationale," says Messrs Babbage and Herschel, "of these phenomena, as well as of those observed by Mr Barlow in the rotation of iron, which form only a particular case (though certainly the most prominent of any) of the class in question, seems to depend on a principle which, whether it has or has not been before entertained, or distinctly stated in words, it may be as well, once for all, to assume here, as a postulation, viz. that in the induction of magnetism, time enters as an essential element, and that no finite degree of magnetic polarity can be communicated to or taken from any body whatever, susceptible of magnetism, in an instant.

The preceding results were verified by Mr Christie, who found, that when a thick plate of copper revolved under a small magnet, the force which deflected the needle varied inversely as the fourth power of the distance; but when the copper discs were small, and the magnets large, the power of the distance was between the square and the cube; when the plates were of different weights, the force was nearly in the ratio of the weights at small distances, but at smaller distances it varied in a higher ratio.

The discovery of two poles of maximum cold on opposite sides of the north pole of the earth, which was announced by Sir David Brewster in 1820, led him and other authors to the opinion that there might be some connection between the magnetic poles and those of maximum cold. "Imperfect," says he, "as the analogy is between the isothermal and magnetic centres, it is yet too important to be passed over without notice. Their local coincidence is sufficiently remarkable, and it would be to overstep the limits of philosophical caution to maintain that they have no other connection but that of accidental locality; and if we had as many measures of the mean temperature as we have of the variation of the needle, we might determine whether the isothermal poles were fixed or moveable." And he concludes his paper on the mean temperature of the globe with the following paragraph: "Having thus endeavoured to establish a new law of the distribution of heat over the surface of the globe, it might be no uninteresting inquiry to investigate the causes which have modified in so remarkable a manner the influence of the solar rays. The subject, however, is too comprehensive and too hypothetical to be discussed at present. How far the general form and position of the continents and seas of the northern hemisphere may disturb the natural parallelism of the isothermal lines to the equator,—to what extent the current through Behring's Strait, transporting the waters of warmer climates across the polar seas, may produce a warm meridian in the direction of its motion, and throw the coldest parts of the globe to a distance from the pole,—whether or not the magnetic, or galvanic, or chemical poles of the globe (as the important discoveries of Mr Oersted entitle us to call them), may have their operations accompanied with the production of cold, one of the most ordinary effects of chemical action,—or whether the great metallic mass which crosses the globe, and on which its magnetic phenomena have been supposed to depend, may not occasion a greater radiation of heat in those points where it develops its magnetic influence,—are a few points which we may attempt to discuss when the progress of science has accumulated a greater number of facts, and made us better acquainted with the superficial condition as well as the internal organization of the globe."

The two poles of maximum cold, which will likely perform an important part in the future history of terrestrial magnetism, are situated, according to Sir David Brewster, as follows, according to the best observations made both near them and at a distance. The American pole is situated in latitude 73° north, and longitude 100° west from Greenwich, a little to the east of Cape Walker; and the Asiatic pole in latitude 73° north, and longitude 80° east, between Siberia and Cape Matzol on the Gulf of Oby. Hence the two warm meridians will be in west longitude 10° and east longitude 170°, the latter passing through Lord Mulgrave's range, and the former between St Helena and Ascension Island. The two cold meridians, or those which pass through the poles of maximum cold, will be in west longitude 100° and east longitude 80°, the latter passing near Mexico and through Bathurst Island, and the former through Colombo in Ceylon, Berar in Hindustan, and crossing the Oby a little to the west of Narym in Siberia. The following is the formula which the same author has given for the mean temperature at any point of the globe, T being the mean temperature required, t the maximum equatorial temperature, r the minimum temperature at each of the cold poles, and δ, θ the distances of the place from the two cold poles.

\[ T = (t - r) \left( \sin^n \delta \cdot \sin^n \theta \right) + r. \]

The distances δ, θ are found from the formula

\[ \cos \delta = \frac{\cos L (\cos L - l)}{\cos \theta} \quad \text{and} \]

\[ \tan \phi = \cos M \tan L; \]

in which L is the colatitude of the pole of maximum cold, l the colatitude of the place, and M the difference of longitude between the place and the pole of maximum cold. The values of t and r have been determined with considerable accuracy, t being nearly 82°8 Fahrenheit, and r from 0° to —3½°. The exponent n is nearly 4ths, but future observations may induce us to increase or diminish it.

Now it is a remarkable circumstance that the same formula, mutatis mutandis, expresses the magnetic intensity of magnetism at any point of the earth's surface, the intensity at the two magnetic poles being supposed equal. If we call S the maximum number of seconds in which any number n of oscillations are performed which takes place at the island of St Thomas on the west coast of Africa, and s the minimum number of seconds in which n oscillations are performed which takes place at the magnetic poles, then the intensity I will be

\[ I = (S - s) \left( \sin^n \delta \cdot \sin^n \theta \right) + s, \]

δ and θ being determined by the formula already given, adopting the position of the poles in the preceding page. The values of S and s, according to Captain Sabine and Hansteen, will be about 370° and 262°. This formula will give for the isodynamical lines, or those of equal intensity, a series of returning curves of the nature of Lemniscates, almost similar to those drawn by Captain Sabine, and given in a future figure, and exactly like the polar isothermal lines.

The connection thus indicated between the heat and the magnetism of the earth has been studied by succeeding authors, and the general principle has been adopted by many distinguished philosophers. Dr Traill expressed the opinion, "that the disturbance of the equilibrium of the temperature of our planet by the continual action of the sun's rays on its intertropical regions, and by the polar ices, must convert the earth into a vast thermo-magnetic apparatus."

1 Edinburgh Transactions, 1820. History. tus;" and "that the disturbance of the equilibrium of temperature even in stony strata may elicit some degree of magnetism." Mr Christie thinks it "not improbable that difference of temperature may be the primary cause of the polarity of the earth, though its influence may be modified by other circumstances." M. Ampère, who ascribes magnetism to transverse electrical currents, thinks that the strata of our globe may form considerable galvanic arrangements, and that the electric currents may be affected by the rotation of the earth. M. Oersted remarks, in a recent treatise on thermo-electricity, "that the most efficacious excitation of electricity upon the earth appears to be produced by the sun producing daily evaporation, deoxidation, and heat, all of which excite electrical currents." After stating that the sun daily produces electric currents, and these currents magnetise, he observes, that "thus the earth seems to have a constant magnetic polarity, produced in the course of time by the electrical currents which surround it, and a variable magnetism, produced immediately by the same current." As the sun produces different effects on water and solid bodies, Oersted supposes that the intensity will vary in the same parallel, and the direction of the needle will be oblique to the equator, in consequence of the lines of equal electro-magnetic intensity being twice bent by the influence of the two great masses of continent. "The yearly and daily change," he observes, "must occasion yearly and daily variations. As to the variations comprehended in greater periods, we might perhaps attribute them to a motion of the coolest points in such continents, which, it appears, cannot remain the same for ever, because the currents of warmer air must principally be directed to such points." Analogous views have been recently stated by M. Kupffer, in a memoir read in 1829 to the Russian Academy, in which he adopts explicitly Sir David Brewster's opinions of the existence of two cold poles distant from the pole of revolution. "But this distribution of temperature," says he, "appears also to have a great influence on the distribution of the intensity of terrestrial magnetism. This would no doubt be the case if it is true, as I have tried to show in another memoir, that terrestrial magnetism resides at the surface of the globe. We have here the choice between two hypotheses; either the earth should be considered as a magnet existing by itself, and then the intensity of its magnetism will be the inverse of its temperature; or it receives its influence from without, and is only like a piece of soft iron, to which the presence of a distant body communicates magnetism, and then the intensity of its magnetism will increase with its temperature. Though the first of these hypotheses has been hitherto generally adopted, yet the second acquires some probability from the discovery of the magnetic influence of the solar rays, and of the known relation between the diurnal variations of the declination of the needle and the course of the sun." The connection between the poles of maximum cold and those to which the isodynamical magnetic lines are related, is considered by Dr Dalton as a probable supposition. "If the idea," says he, "suggested by Sir David Brewster in the Transactions of the Royal Society of Edinburgh, vol. ix. 1821, be correct (and there seems great reason to believe it to be so), namely, that there are two poles of greatest cold in the northern hemisphere, the above observation will enable us to see the natural cause of this remarkable fact. The lands within the arctic circle, in the absence of the sun, must depend upon the south-west winds from the two great oceans for their winter heat. Those parts of the eastern and western continents which are most remote from the ocean, as measured along the curvilinear tracts of the current of air, must receive that air in great measure deprived both of its vapour and its temperature. Accordingly it is found that the temperature of the north-east parts of such continents exhibits the extreme of cold. Probably a latitude of 75° north, and a longitude of 90° east and 90° west, would be found nearly equally cold, and to exceed any other place on the surface of the globe in this respect; and it would be a curious coincidence if Professor Hansteen's two supposed northern magnetic poles should be found in the same positions as the two poles of extreme cold."

In a general history of magnetical discoveries, it may be proper to take some notice of the very curious experiments which have been made respecting the influence of the solar rays in the production of permanent magnetism, although, according to the generally received opinion, the existence of such an influence has not been established; but if the propriety of doing this had been doubtful, the observation just made by M. Kupffer, as connecting this supposed property of violet light with terrestrial magnetism, would have removed the doubt. Dr Morichini, an eminent physician in Rome, was the first who announced it as an experimental fact, that an unmagnetised needle could be rendered magnetic by the action of the violet rays of the sun. His experiments were successfully repeated by Dr Carpi at Rome, and the Marquis Ridolfi at Florence; but M. d'Hombre Firmas at Alais in France, Professor Configliachi of Pavia, and M. Bérard of Montpellier, failed in obtaining decided magnetic effects from the violet rays. In 1814 Dr Morichini exhibited the actual experiment to Sir Humphry Davy, and in 1817 Dr Carpi showed it to Professor Playfair. A few months after Sir Humphry witnessed the experiment, the writer of this article met him at Geneva, and learned from him the fact, that he had paid the most diligent attention to one of Morichini's experiments, and that he saw, with his own eyes, an unmagnetised needle rendered magnetic by violet light. The following account of the experiment made by Dr Carpi was given to us verbally by Professor Playfair, who approved of the statement of it which we drew up at the time. "The violet light was obtained in the usual manner, by means of a common prism, and was collected into a focus by a lens of a sufficient size. The needle was made of soft wire, and was found upon trial to possess neither polarity nor any power of attracting iron filings. It was fixed horizontally upon a support, by means of wax, and in such a direction as to cut the magnetic meridian at right angles. The focus of violet rays was carried slowly along the needle, proceeding from the centre towards one of the extremities, care being taken never to go back in the same direction, and never to touch the other half of the needle. At the end of half an hour after the needle was exposed to the action of the violet rays, it was carefully examined, and it had acquired neither polarity nor any force of attraction; but after continuing the operation twenty-five minutes longer, when it was taken off and placed on its pivot, it traversed with great alacrity, and settled in the direction of the magnetical meridian, with the end over which the rays had passed turned towards the north. It also attracted and suspended a fringe of iron filings. The extremity of the needle that was exposed to the action of the violet rays repelled the north pole of a compass-needle. This effect was so distinctly marked as to leave no doubt in the minds of any who were present, that the needle had received its magnetism from the action of the violet rays." In this state of the subject, Mrs Somerville made some simple and well-conducted experiments, which seemed to settle the question at rest, from the distinct and decided

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1 Meteorological Observations and Essays, second edition, 1834, p. 215. character of the results. A sewing needle, an inch long, and devoid of magnetism, had one half of it covered with paper, and the other exposed to the violet rays of the spectrum, five feet distant from the prism. In two hours it acquired magnetism, the exposed end exhibiting north polarity. The indigo rays produced an equal effect, and the blue and green the same in a less degree. The yellow, orange, and red rays had no effect even after three days' exposure to their action. Pieces of blue watch-springs received a higher magnetism. When the sun's light fell upon the exposed end through blue coloured glass, or through blue or green ribbon, the same magnetic effects were produced.

The experiments of Mr Christie, an account of which was read to the Royal Society a short time before Mrs Somerville's, confirmed her results to a certain degree, by a different mode of observation. He found that the compound solar rays possessed magnetic influence, and exhibited it in their effect of diminishing the vibrations of magnetised and unmagnetised steel needles, and also needles of copper or of glass, by making them oscillate in the sun's white ray. Mr Christie, however, has recently remarked, that as his experiments have not succeeded on repetition by Mr Snow Harris, when made in a vacuum, his results must have been owing to currents of air. In justice to Mr Christie, however, we must mention, that Professor Zantedeschi repeated Mr Christie's experiments at Pavia, under an Italian sun, with a needle a Paris foot long, and obtained a striking result. This needle, when drawn from its position of equilibrium, through an area of 90°, performed four oscillations in 30°, the last of which had a semiamplitude of 70°. In the solar rays it performed in 30° four oscillations, the last of which had only a semiamplitude of 60°. When he exposed to the sun the north pole, the semiamplitude of the last oscillation was 6° less than that of the first, while by exposing the south pole this last oscillation became greater than the first.

The experiments of Baumgartner and Barlocei tended to confirm these results. The former found that iron wires polished on a part of their length are magnetised by white solar light, exhibiting a north pole on the polished part; and the latter has shown that an armed natural loadstone, which carried 1½ Roman pound, exhibited, after three hours' exposure to the strong light of the sun, an increase of energy equivalent to 2 ounces or ¼th of a pound, while another larger one, which carried 5 pounds 5 ounces, had its strength nearly doubled by two days' exposure. Zantedeschi tried an artificial horse-shoe loadstone, which carried 18½ ounces; after three days' exposure to the sun it carried 3½ ounces more, and by continuing its exposure its power increased to 31 ounces. An oxidated magnet gained most power, and a polished one none. He found also that the north pole of a loadstone exposed to the sun's rays concentrated by a lens acquires strength, while its south pole, similarly exposed, loses it.

Notwithstanding all these results, the general opinion seems now to be, that light does not exercise any decided effect in producing magnetism. The experiments of MM. P. Ries and Moser were made with needles both polished and oxidated, and also with wires half-polished; and polarised as well as common light was made to fall upon them in a concentrated state, but no decided effect upon their number of oscillations could be observed; and they state that they think themselves justly entitled to reject totally a discovery which, for seventeen years, has at different times disturbed science.

In 1827, Mr Snow Harris communicated to the Royal Society of Edinburgh his Experimental Inquiries concerning the Laws of Magnetic Forces, made with a beautiful and accurate instrumental apparatus, invented by the author for examining the phenomena of induced magnetism. With this apparatus he found that the magnetic development in History masses of iron by induction is, ceteris paribus, directly proportional to the power of the inductive force, and inversely as the distance; and that the forces which magnets develope in a mass of iron at a given distance, within certain limits, may be taken as a fair measure of their respective intensities. From another series of experiments he has shown, that the absolute force of attraction exerted between a magnet and a piece of iron, varies with the power of the magnet, and consequently with the force induced in the iron, ceteris paribus; and that when the force induced in the iron is a constant quantity, while its distance from a temporary or permanent magnet is variable, the absolute force varies with the distance. This result was not only apparent when the magnetic force was varied by induction, but was also satisfactorily shown when varied by magnets of which the relative powers of induction were previously ascertained.

Mr Harris made a number of nice experiments on the absolute force of attraction and repulsion between two magnetised bodies, which he found to be in the inverse ratio of the square of the distance. When, in the case of attraction, the magnets, however, were nearly approximated in relation to their respective intensities, the increments in the forces began to decline, and in some instances at near approximations the absolute force was in the simple inverse ratio of the distance. In the experiments with the repelling poles, the deviations from the regular force were still more considerable, and, what is curious in this case, the force became less and less, until the polarity of the weaker magnet appeared to be so counteracted by induction, that the repulsion was at length superseded by attraction. Mr Harris next proceeded to determine the law according to which the forces are developed in different points of the longitudinal magnetic axis between the centre and poles of a magnet, and he found that it varied directly as the square of the distance from the magnetic centre; a law which is uniform in bars of steel regularly hardened and magnetised throughout. This law of distribution is exactly the same as that which has been given by Hansteen.

Mr Harris has also published other two memoirs in the Philosophical Transactions for 1831, the first On the Influence of Screens in arresting the progress of Magnetic Action, and the second On the Power of Masses of Iron to control the Attractive Force of a Magnet, of both of which some account will be found in a subsequent section. In his latest paper, On the Investigation of Magnetic Intensity by the Oscillations of the Magnetic Needle, he exposed an oscillating magnetic bar to a bright sunshine; and though he observed the effect observed by Mr Christie, which that philosopher ascribed to the influence of the sun's rays, yet he found that they all disappeared when the needle was made to oscillate in an exhausted receiver.

M. Haldat of Nancy communicated, in 1830, to the society of that city, the results of some interesting researches on the incorciblity of the magnetic fluid, or its power of exerting its influence through all bodies, even the most dense; a property which is not possessed by light, heat, or electricity. In this research he adopted various methods of observation, and interposed a great variety of substances; and from the numerous experiments which he made, he has drawn the following conclusions: 1. That the agent or fluid by which the magnetic phenomena are explained is incorcible in the present state of the science; 2. that iron, considered as presenting an exception to this law, coerces the magnetic influences only by acquiring itself the magnetic state; 3. that incandescence does not give to bodies the power of coercing the magnetic influence. In a previous memoir, M. Haldat had obtained some interesting results on the production of magnetism by friction. He found that all hard bodies may, by means of friction, assist in the decomposition of the magnetic fluid, if their action History. is promoted by the combined action of magnets which, by themselves, are incapable of producing it. If a piece of soft wire, for example, four inches long, and 1-25th of an inch in diameter, is placed horizontally between two bar-magnets, with their opposite poles facing each other, and at such a distance that the wire cannot be magnetised, it will receive distinct magnetism by friction with all hard bodies, such as copper, brass, zinc, glass, hard woods, &c. M. Haldat employed the ingenious process of M. Gay-Lussac, of magnetising soft iron by torsion, in neutralising the wires before they were magnetised. If they are twisted after receiving magnetism, they will preserve the magnetism which they had received before torsion; but if, after being twisted, they are twisted in an opposite direction, they will become perfectly neutral.

M. Haldat likewise made some interesting experiments on the effect of the coercive force in steel on the magnetism produced by rotation, and he found that the force with which a revolving steel disc dragged round a magnetic needle was in the inverse ratio of the coercive force of the steel. When the discs were not hot, they had the same effect as those at the ordinary temperature. We owe also to M. Haldat an interesting paper on magnetic figures. Figures of any kind, when traced by the pole of a magnet on a plate of steel, are rendered visible by sifting upon the invisible tracings filings of steel, which arrange themselves in the most beautiful manner along the outlines of the figure which has been traced.

A series of very interesting experiments have been recently published by M. Quetelet of Brussels, On the successive degrees of Magnetic Force which a Steel Needle receives during the multiple frictions which are employed to magnetise it. These experiments were made principally before 1830, but they were not given to the public till 1833. The following are the general results which were obtained by the author.

1. When a needle or bar that had never been magnetised, is magnetised to saturation by the method of separate contact, the magnetic force acquired is a maximum in relation to the forces which can be given to the same needle or bar by the subsequent reversals of its poles.

2. The magnetic force which a needle can acquire becomes weaker in proportion as the reversal of its poles, has been multiplied. The series of frictions which tend to bring back the poles to their primitive state are more efficacious than the others.

3. This difference between the forces which the needle acquires after the successive reversal of its poles, goes on continually diminishing, and converges towards a limit. It depends in general on the size of the needle in relation to that of the rubbing bars, as well as in its force of coercion.

4. A needle cannot receive all the magnetic force which it can acquire, if the frictions do not take place over all its surface; this becomes particularly sensible in the reversal of the poles.

5. The rubbing bars give (exteris paribus) to bars of the same dimensions as themselves a magnetic force equal to that which they possess, and in bars of different dimensions the forces acquired are as the cubes of their homologous dimensions. The last part of this proposition was long ago established by Coulomb.

6. When we rub magnetic bars with other bars weaker than themselves, the force of the first diminishes in place of increasing; and it appears that the force becomes that which those latter bars would be capable of giving at the first by directly magnetising them.

7. The relation which exists between the forces which a needle or a bar receives by successive frictions, and the number of these frictions, may be expressed by an exponential formula of three constants.

One of these constants appears to change its value with the size of the bars which are magnetised, at least while these bars have a magnitude which does not exceed that of the rubbing bars, and while they are of the same quality of steel.

In this way we know beforehand the successive degrees of force which a bar takes at each friction, if we have previously determined the law of these augmentations for the same rubbing bars, and for any other bar which we get to serve as the modulus. If the bar which is rubbed has been gun to be magnetised, we must calculate first the number of frictions to which this force corresponds, in order to be able to assign the rank of the subsequent frictions, and the magnitude of the corresponding magnetical forces.

8. When the rubbing bars are greater than the bar to be magnetised, from the first complete friction the force of magnetism is very nearly one half of the force which the magnetised bar will finally possess.

After the twelfth complete friction, the magnetic force differs little from that which the rubbing bars can communicate.

We owe also to M. Quetelet two interesting memoirs on the magnetic intensity of different places in Switzerland, Italy, Germany, and the Low Countries.

The influence of the aurora borealis on the magnetic needle, which was observed by Hiorter at Upsal in 1741, and by Wargentin in 1750, had long induced philosophers of the north to regard it as a magnetic phenomenon; and this was greatly confirmed by the fact that the south end of the dipping needle points to that part of the heavens to which the rays of the aurora appear to converge. "The aurora borealis," says Dr Robison, "is observed in Europe to disturb the needle exceedingly, sometimes drawing it several degrees from its position. It is always observed to increase its deviation from the meridian, that is, an aurora borealis makes the needle point more westerly. This disturbance sometimes amounts to six or seven degrees, and is generally observed to be greatest when the aurora borealis is most remarkable.

"This is a very curious phenomenon, and we have not been able to find any connection between this meteor and the position of a magnetic needle. It is to be observed, that a needle of copper or wood, or any substance besides iron, is not affected. We long thought it an electric phenomenon, and that the needle was affected as any other body balanced in the same manner would be; but a copper needle would then be affected. Indeed, it may still be doubted whether the aurora borealis is an electric phenomenon. They are very frequent and remarkable in Sweden, and yet Bergman says that he never observed any electric symptoms about them, though in the mean time the magnetic needle was greatly affected.

"We see the needle frequently disturbed, both from its general annual position, and from the change made on it by the diurnal variations. This is probably the effect of aurora boreales which are invisible, either on account of thick weather or daylight. Van Swinden says he seldom or never failed to observe aurora boreales immediately after any anomalous motion of the needle; and concluded that there had been one at the time, though he could not see it. Since no needle but a magnetic one is affected by the aurora borealis, we may conclude that there is some natural connection between this meteor and magnetism. This should farther incite us to observe the circumstance formerly mentioned, viz. that the south end of the dipping needle points to that part of the heavens where the rays of the aurora appear to converge. We wish that this were diligently observed in places which have very different variations and dips of the mariner's needle."

A valuable series of observations on the influence of the aurora borealis on the magnetic needle was made by Dr Dalton, at Kendal and Keswick, during seven years from May 1786 to May 1793, and has been published in his Meteorological Observations and Essays, which appeared in 1793. During these observations he noticed the effect which they produced on the magnetic needle, and he was thus led to study the phenomena of the aurora, and to establish beyond a doubt the relation of all its phenomena to the magnetic poles and equator. His views and speculations on this subject we shall detail at some length in a future part of this article; but we shall at present give our readers a specimen of the observations which he made on the magnetic needle during the changes of an aurora.

Feb. 12. The aurora appeared at Kendal after 6h. p.m., flaming over two thirds of the hemisphere. The beams converged to a point in the magnetic meridian, about 15° or 20° to the south of the zenith. The following were the changes which he observed in the needle and in the aurora:

| Time | Variation | Observations | |------|-----------|--------------| | 5 o'p.m. | 25° 5' west. | altitude of the clear space south | | 6 35 | 24 49 | 35° altitude of ditto 20°, streamers bright, east. | | 6 42 | 24 55 | streamers bright and active all over the illuminated part. | | 6 50 | 25 0 | disappeared in the west, active east. | | 7 2 | 25 28 | active about the zenith, light faint. | | 7 5 | 25 12 | light faint. | | 7 10 | 24 40 | strong light northward. | | 7 20 | 24 35 | a large uniform still light covering half the hemisphere, with flashes now and then. | | 7 35 | 24 45 | streamers north-west, bright east; clouds. | | 8 0 | 24 45 | the aurora bursting out openly. | | 8 10 | 24 45 | as fine and large a display of streamers as has appeared this evening. | | 8 35 | 24 47 | the light growing fainter and fainter. |

In these observations, the deviation produced by the aurora was 53°. In some cases during the prevalence of auroras Dr Dalton did not observe any perceptible disturbance of the needle.

Professor Hansteen observes, that large extraordinary movements of the needle, in which it traverses frequently with a shivering motion an arc of several degrees on both sides of its usual position, are seldom, perhaps never, exhibited, unless when the aurora borealis is visible; and that this disturbance of the needle seems to operate at the same time in places the most widely separate. "The extent of such extraordinary movements," he adds, "may, in less than twenty-four hours, amount to 5° or 5½°. In most cases, the disturbance is also communicated to the dipping needle; and so soon as the crown of the aurora quits the usual place (the points where the dipping needle produced would meet the sky), the instrument moves several degrees forward, and seems to follow it. After such disorders, the mean variation of the needle is wont to change, and not to recover its previous magnitude till after a new and similar disturbance."

From an extensive series of accurate observations made M. Arago, by M. Arago at Paris since 1818, the needle was almost invariably found to be affected by auroras that were seen in Scotland; and so striking was the connection between the Magnetic two classes of facts, that the existence of the aurora could be inferred from the derangements of the needle. M. Arago of the aurora has likewise discovered, that, early in the morning, often ten or twelve hours before the aurora is developed in a very distant place, its appearance is announced by a particular form of the curve which exhibits the diurnal variation of the needle, that is, by the value of the morning and evening maxima of elongation. From a number of corresponding observations on the hourly declination made by M. Arago and M. Kupffer, who established at Kasan, near the eastern limit of Europe, one of Gambeys compasses, similar to that used at Paris, these philosophers were convinced that, notwithstanding a difference of longitude of above 47°, the disturbances produced upon the needle by the aurora took place at the same instant. It is a curious fact, however, and one yet unexplained, that during the frequent occurrence of the aurora at Port Bowen, Capt. Foster did not observe any peculiar changes in the direction of the needle, although, from his great proximity to the magnetic pole, the diurnal change sometimes amounted to 4° or 5°; and, under such circumstances, the influence of the aurora ought to have been particularly conspicuous.

Mr Christie is of opinion that the direction of the needle may be influenced by the electrical state of the clouds; and he found it to be so in a very distinct experiment which he made for the purpose. Captain Sir Everard Home had observed the same effect produced during thunder storms; and, in two instances, he found that a needle came sooner to rest during a thunder storm than it had done either previous or subsequent to it; the number of oscillations having been reduced in one case from 100 to 40, and in another from 200 to 120.

During the late journey of Captain Back to the polar regions in 1833, 1834, and 1835, he found that the needle was generally affected by the aurora; and on one occasion the deviation which it produced was 8°. "For nearly a month, however" (previous to the 7th January 1834), he remarks, "the needle had not been perceived to be affected by the aurora, which, it may be proper to observe, was always very faint, apparently high, and generally confined to one point of the heavens." Captain Back repeatedly observed, that when the aurora was concentrated in individual beams, the needle was powerfully affected; but that it generally returned to its mean position when the aurora became generally diffused. On several occasions the needle was restless, and exhibited the vibrating action produced by the aurora when this motion was not visible; and Captain Back states that he could not account for this, except by supposing the invisible presence of the aurora in full day.

The only metals which were supposed to have a distinct and decided power, and were therefore called magnetic metals, are iron, nickel, and cobalt. Mr David Lyon has lately endeavoured to show that these metals resemble one another, not only in their principal qualities, but in the numerical values of their qualities; and he adds, that whilst these three magnetic substances have the values above referred to near each other, there are no other substances in which the same values come very near or fall within those of the three magnetic substances. The values to which Mr Lyon alludes are the following:

1. Appendix to Captain Back's Narrative of the Arctic Land Expedition, &c. p. 601. 2. London and Edinburgh Phil. Mag. December 1834, p. 415. 3. M. Pouillet, in his Éléments de Physique, tom. iii. p. 69, refers to some remarkable analogies which he has observed between the distance of the atoms of bodies and their magnetic properties. The preceding speculation, though ingenious, and deserving of attention, has however been overturned by some very recent observations of M. Faraday. "Cobalt and chromium," says he, "are said to be both magnetic metals. I cannot find that either of them is so, in its pure state, at any temperatures. When the property was present in specimens supposed to be pure, I have traced it to iron or nickel."

Mr Faraday has very recently published some interesting observations On the General Magnetic Relations and Characters of the Metals. He is of opinion that all the metals are magnetic, in the same manner as iron, though not at common temperatures, or under ordinary circumstances. He does not allude to a feeble magnetism, uncertain in its existence and source, but to a distinct and decided power, such as that possessed by iron and nickel; and his impression is, that there is a certain temperature for each metal (well known in the case of iron, beneath which it is magnetic, but above which it loses all power), and that there is some relation between this point of temperature and the intensity of magnetic force, which the body, when reduced beneath it, can acquire. Iron and nickel would then be no more exceptions from the metals in regard to magnetism, than mercury is in regard to liquefaction.

In order to investigate this point, Mr Faraday subjected various metals in their pure state to a temperature from 60° to 70° below the zero of Fahrenheit, but he could not detect in them the least indication of magnetism. The metals tried were the following:

- Arsenic - Antimony - Bismuth - Cadmium - Cobalt - Chromium - Copper - Gold - Lead - Mercury - Palladium - Platinum - Silver - Tin - Zinc - Plumbago

Mr Faraday next proceeded to compare iron and nickel with respect to the points of temperature at which they ceased to be magnetic. Iron loses all magnetic properties at an orange heat, and is then to a magnet the same as a piece of copper. Mr Faraday found that the point at which nickel lost its magnetic relations was very much lower than with iron, but equally defined and distinct. If heated and then cooled, it remained unmagnetic long after it had fallen below a heat visible in the dark; and almond oil can bear and give that heat which makes nickel indifferent to a magnet, its demagnetising temperature being about 630° or 640° Fahr.

In order to determine what relation the temperature which took from a magnet its power over soft iron had to that which would take from soft iron or steel its power relative to a magnet, Mr Faraday gradually raised the temperature of a magnet, and found that it lost its polarity rather suddenly when scarcely at the boiling point of almond oil, and then acted with a magnet as cold soft iron. It required to be raised to a full orange heat before it lost its power as soft iron.

"Hence he concludes, the force of the steel to retain that condition of its particles which renders it a permanent magnet, gives way to heat at a far lower temperature than that which is necessary to prevent its particles assuming the same state by the inductive action of a neighbouring magnet. Hence, at one temperature, its particles can of themselves retain a permanent state; whilst, at a higher temperature, that state, though it can be induced from without, will continue only as long as the inductive action lasts, and at a still higher temperature all capability of assuming this condition is lost. The temperature at which polarity was destroyed appeared to vary with the hardness and condition of the steel. Fragments of loadstone of very high power were then experimented with. These preserved their polarity at higher temperatures than the steel magnet; the heat of boiling oil was not sufficient to injure it. Just below visible ignition in the dark they lost their polarity, but from that to a temperature a little higher, being very dull ignition, they acted as soft iron would do, and then suddenly lost that power also. Thus the loadstone retained its polarity longer than the steel magnet, but lost its capability of becoming a magnet by induction much sooner. When magnetic polarity was given to it with a magnet, it retained this power up to the same degree of temperature as that at which it held its first and natural magnetism."

Some of the results observed by M. Pouillet stand in opposition to some of the preceding statements. M. Pouillet considers it as certain that there are five simple magnetic bodies, viz:

- Iron - Chrome - Manganese - Nickel - Cobalt

and in consequence of having observed some remarkable analogies between the distance of the atoms of bodies and their magnetic properties, he was led to suppose that the magnetic limit of different bodies ought to be found at very different temperatures. "I have indeed," says he, "demonstrated by experiment, 1. that cobalt never ceases to be magnetic, or rather that its magnetic limit is at a temperature higher than the brightest white heat; 2. that chrome has its magnetic limit a little below the temperature of dark blood-red heat; 3. that nickel has its magnetic limit about 350° centigrade nearly at the melting point of zinc; and, 4. that manganese has its magnetic limit at the temperature of from 20° to 25° below zero." Experiments, continues he, "on these five magnetic bodies seem to prove, 1st, that heat acts upon magnetism only in consequence of the greater or less distance which it occasions between the atoms of bodies; and, 2d, that all bodies would become magnetic if we could by any action whatever make their atoms approach within a suitable distance."

Among the latest researches on magnetism are those of Professor Gauss of Göttingen, who has published an account of them in a treatise entitled Intensitas vis Magneticae Terrestris ad absolutum mensuram revocata. His object is to impart to magnetical observations the accuracy of astronomical ones. By observing the oscillations of a magnetised bar, he finds the product of the horizontal intensity of the earth's magnetism, and the static momentum of the free magnetism of the bar; and by eliminating the latter from the two equations, he obtains an absolute measure of the former, independent of the magnetism of the bar. The horizontal intensity thus found is then to be multiplied by the secant of the dip of the needle, in order to give the absolute intensity. In this inquiry Professor Gauss found it necessary to deduce from observation the true law of magnetic action, which, from a number of consistent and carefully made experiments, he found to be in the inverse ratio of the square of the distance. From a series of accurate experiments,

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1 London and Edinburgh Phil. Mag. March 1836, p. 178. 2 Ibid. p. 157. 3 Éléments de Physique, 2d edit. tom. iii. p. 89, Paris, 1832. 4 M. Pouillet remarks elsewhere, that manganese does not become magnetic till it is cooled down to 15° or 20° below zero. (El. Phys. iii. p. 18.) Professor Gauss found the horizontal intensity at Göttingen, on the 18th September 1832, to be 1.7821; and taking the exponent of gravity in moving bodies at the place of observation as the unit of force, and using the Paris line and the Berlin pound, he found the absolute horizontal intensity to be 0.0039131; and as he found the dip at Göttingen on the 23rd June 1832 to be 68° 22' 52", the absolute intensity of terrestrial magnetism will be Sec. 68° 22' 52" × 0.0039131.

Professor Gauss has proposed, and put in practice, a very accurate method of observing the daily variation of the needle, and of determining the time of vibration of a needle or magnetised bar. He fixes a plane mirror on the end of the bar, and perpendicularly to its axis, and by observing the reflected image of the divisions of a scale, by the aid of a theodolite placed at a distance, he is able to observe and to measure the minutest changes.

The magnetised bar employed by Gauss is of much larger dimensions than the bar of Prony's magnetic telescope; the small ones, which he uses as magnetometers, being four pounds weight, and the large ones twenty-five pounds; two of which, when fastened together, form the apparatus or multiplier of induction for rendering sensible and measuring the oscillatory movements predicted by a theory founded on Mr Faraday's great discovery. By this valuable invention of Professor Gauss, the observer is not under the necessity of approaching the magnetised bar, so that no disturbance is occasioned by the currents of air produced by the proximity of the observer's body, so that observations may be made in the smallest intervals of time.

With apparatus similar to that of Professor Gauss, simultaneous observations have been made in 1834 and 1836, at intervals of five or ten minutes, at Göttingen, Copenhagen, Altona, Brunswick, Leipzig, Berlin, Milan, and Rome. It appears, from the graphic representation of the results, that the smallest inflexions of the horary curves are parallel, and consequently the disturbing causes which produce them simultaneous at Milan and Copenhagen, two of the places of observation, which have a difference of latitude of 10° 13'.

In giving an account of Professor Hansteen's labours, we have briefly noticed his journey in Siberia, and the erection of magnetic observatories by the emperor of Russia, on the recommendation of Baron Humboldt; and we have also referred to the early researches of this distinguished philosopher. When travelling in the equinoctial regions of America during the years 1799-1804, Baron Humboldt had devoted much attention to the determination of the intensity of the magnetic forces, and of the dip and variation of the needle. Upon his arrival in Europe, he conceived the design of examining the progress of the horary changes of the variation, and the perturbations to which it is subject, by employing a method which had never been adopted on an extended scale. In a large garden at Berlin, he measured, particularly at the period of the equinoxes in 1806 and 1807, the angular alterations of the magnetic meridian, at intervals of an hour, often of half an hour, without interruption, during four, five, or six days, and as many nights. The instrument employed was Prony's magnetic telescope, suspended according to the method of Coulomb, and capable of being reversed upon its axis. It was placed in a glass frame, and directed towards a very distant meridian mark, the illuminated divisions of which indicated six or seven seconds of hourly variation. In these researches Baron Humboldt was struck with the frequency of oscillations whose amplitude extended beyond all the divisions of the scale, and which repeatedly took place at the same hours before sunrise. "These vagaries of the needle," says the baron, "the almost periodical return of which has recently been confirmed by M. Kupffer, in the account of his travels in the Caucasus, appeared to me the effect of a re-action of the interior of the earth towards the surface; I should venture to say, of magnetic storms, which indicate a rapid change of tension." With the view of investigating the causes of these disturbances, Baron Humboldt proposed to erect similar apparatus on both sides of the meridian of Berlin; but the political tempest of Germany, and his mission to France by the government, delayed the execution of his plan. M. Arago, however, as we have already seen, began and prosecuted his inquiry with singular success.

When Baron Humboldt again fixed his residence in Germany in 1837, he erected one of Gambeys compasses in a magnetic pavilion, without any iron, in the middle of a garden, and began a series of regular observations in the autumn of 1828. At his request, the Imperial Academy and the curator of the university of Kazan erected magnetic observatories at St Petersburg and Kazan; and the imperial department for mines has established similar stations at Moscow, Barnaul, and Nertschinsk. The academy, too, has sent Mr George Fuss to Pekin, where he has procured the erection of a magnetic pavilion in the convent garden of the monks of the Greek church. Since Mr G. Fuss's return, M. Kowanko, a young officer of mines, continues the horary observations corresponding to those made in Germany and Russia. Admiral Greig has established one of Gambeys compasses in the Crimea at Nicolaieff. Baron Humboldt has procured the establishment of a magnetic apparatus at the depth of thirty-five fathoms, in an adit in the mines of Freiberg in Saxony. Baron Von Wrangel has been provided with one of Gambeys compasses at Sitka, in one of the Russian settlements. M. Arago has caused to be erected, at his own expense, one of Gambeys compasses in the interior of Mexico, where the soil is 6000 feet above the sea. The French minister of marine has established a magnetic station in Iceland, and the necessary instruments will be sent this summer (1836) to Reikavig; and Baron Humboldt, at the desire of Admiral de Laborde, has sent instruments to the Havannah in Cuba, to furnish a magnetic observatory under the tropic of Cancer.

Some years ago, the writer of this article urged a distinguished and influential member of the British government to establish magnetic observatories in England and the colonies, but no steps were taken in consequence of this application. Baron Humboldt has, however, addressed an interesting letter to his Royal Highness the Duke of Sussex, as president of the Royal Society, soliciting this body to extend, in the colonies of Great Britain, the line of simultaneous observations, and to establish permanent magnetic stations, either in the tropical regions on each side of the magnetic equator, or in the high latitudes of the southern hemisphere, and in Canada. We hope that the Royal Society will use its influence with the British government to have this proposal carried into effect, and that this country shall not be exposed to the humiliation of being indifferent to the progress of those branches of scientific inquiry which its extensive influence in various regions of the globe enables it effectually to advance.

CHAP. II.—ON THE GENERAL PHENOMENA AND PROPERTIES OF MAGNETIC BODIES.

A body is said to be magnetic when it has the power of attracting soft iron, either in the subdivided state of iron properties filings, or in large portions; or of attracting and repelling other magnetic bodies like itself; of taking a particular position when freely suspended, or moving on a pivot; and of communicating magnetism either temporarily to soft or permanently to hard iron in the form of steel. Hence we may arrange the general properties of magnetic bodies under the following heads. 1. On the attractive power of magnetic bodies upon soft iron.

2. On the attractive and repulsive power of magnets over each other, or over iron either temporarily or permanently magnetised.

3. On the effect of masses of iron on the attractive force of a magnet.

4. On the polarity of magnetic bodies.

5. On the power of magnets to communicate magnetism to other bodies.

6. On the distribution of magnetism in artificial magnets.

7. On the effect of division and fracture on the distribution of magnetism.

8. On magnetic figures.

Sect. I.—On the Attractive Power of Magnetic Bodies upon Soft Iron.

The natural magnet or loadstone was for a long time the only body considered as possessing magnetic properties. It is an ore of iron, of a grey colour, and a dark metallic lustre. Its specific gravity is about four and a half times that of water. It crystallizes in the form of the regular octahedron, and it consists of from 85 to 75 parts of iron, and from 15 to 25 parts of oxygen. It is found in almost every part of the world, and often forms rocks of considerable magnitude; but different specimens of it possess very different powers of attraction.

Loadstone. The smallest loadstones generally have a greater attractive power in proportion to their size than larger ones. They have been found of such strength, that though weighing only about twenty-five grains, they could lift a piece of iron about forty-five times heavier than themselves. A small magnet set in a ring, and worn by Sir Isaac Newton, is said to have been capable of lifting 746 grains, or 250 times its own weight; and it is stated by Cavallo, that he has seen a loadstone which weighed only about six and a half grains, which lifted a weight of 300 grains.

Natural loadstones often possess unequal powers of attraction in different parts of their mass, in consequence of want of homogeneity of structure and composition; and hence a portion has often been cut from a large loadstone which could lift a greater weight of iron than the large one itself, the portion detached having possessed the most suitable structure, and the other part having weakened the action of the powerful part by keeping the body to be lifted at a greater distance from those points where the magnetism was strongest. It is no doubt from a similar cause that small magnets have a greater proportional power than large ones, or that those of two pounds weight have seldom been found capable of lifting more than ten times their own weight of iron.

If we now take a natural loadstone L, however shapeless, and, after rolling it in a quantity of iron filings, afterwards withdraw it, we shall find that the filings are accumulated more abundantly in two opposite points A, B, than in any other, as shown in fig. 1.

These two points A, B are called the poles of the magnet, and are the points of greatest attraction. When either of these poles is held at a distance from the iron filings, the filings will be attracted to it, and will adhere with such force that it is difficult to brush them off.

If we suspend a small needle of iron or steel by a fine linen or silken fibre, or balance it on a pivot, and bring the poles of the loadstone L near it, it will be attracted to it in the first case, or will oscillate on its pivot in the second case.

If we make the needle float on water in a glass tumbler, and bring any pole of L on the outside of the tumbler, the needle will be attracted towards the pole, notwithstanding the interposition of the glass; and by using the needle upon a pivot, it will be found that the attractive force of the loadstone is in no respect diminished by the interposition of any substance whatever, except iron, conductors and non-conductors of electricity having no effect whatever in stopping or diminishing the action of the loadstone, unless the interposed body be iron, or contains iron in any of its metallic states.

While the loadstone thus attracts iron, and all bodies containing it in a metallic state, these same bodies exercise a reciprocal attraction upon the loadstone, action and re-action being equal and opposite. The truth of this may be exhibited by suspending a magnet, and bringing into the vicinity of its poles a piece of soft iron. The magnet will be gradually attracted by the iron, in the same manner as if the iron had been suspended and a pole of the magnet held near it.

Sect. II.—On the Attractive and Repulsive Power of Magnets over each other, or over Iron either temporarily or permanently magnetised.

If we suspend near each other two loadstones, AB, A'B', like that shown in fig. 1, by two threads T, T, we shall find, by changing the relative position of their poles, A B, A' B', that there are certain positions in which these poles attract each other, and others in which they are repelled. By marking the poles which attract each other, such as A, B', and A', B, we shall find that the poles which repel each other are A, A' and B, B', and that this mutual attraction and repulsion takes place under every change of circumstances.

If we suspend a piece of soft iron ab from a loadstone AB, we shall find that the end b of the iron exercises the same attractive and repulsive power upon the poles A' B' (fig. 2) of a suspended magnet that B did; and in like manner, if the piece of iron ab is suspended from the pole A', the end a' will exercise the same attraction and repulsion upon the poles of a suspended magnet that A did.

Sect. III.—On the effect of Masses of Iron on the Attractive Force of a Magnet.

If we suspend a piece of iron C from the arm of a balance, it will be attracted by the pole P of a magnet A, and will descend towards P in virtue of this attraction. If we now place a mass of iron I close to A, the suspended iron C will rise, as if the attractive force of P were diminished. This power of the mass of iron I seems only to extend to a given point within the magnet A, the distance between the magnet and the iron remaining the same:

According to Norman, the best loadstones were those brought from China and Bengal. for if the iron C is suspended above a point x at some distance from P, the action of I will not be felt at the point x except by diminishing the distance between P and C, or by increasing the neutralising power of the mass I.

Mr Snow Harris, to whom we owe this experiment, has shown that a similar effect is produced when the iron I is placed between the magnet PA and the suspended iron C, and also when I is placed below P. In the first of these cases I stops the attraction of P upon C, and acts as a screen.

Sect. IV.—On the Polarity of Magnetic Bodies.

If we suspend a loadstone, as in fig. 2, or make it float upon water or mercury, by placing it on a thin plate of cork or wood, it will gradually change its place till it rests in a position where a line joining the poles A, B is nearly north and south. This is, generally speaking, the case in Europe, the end A, which points northward, deviating in some places from the meridian to the west, in some places to the east; while in other parts of the globe it points exactly to the north. The deviation of the loadstone from the meridian is called its declination or variation. This property of the magnet is called its polarity, or directive power; and the pole A, which turns to the north, is called its north pole; and the pole B, which turns to the south, its south pole. It will now be found that the poles and magnets A, A', or B, B', which repel each other, are either both north or both south poles; and that the north and south poles attract each other. Hence there is in magnetism, as there is in electricity, two opposite powers or principles, namely, the northern and the southern, or boreal and austral magnetism; and, as in electricity, a repulsion takes place between the two powers of the same name, and attraction between the two powers of an opposite name.

The magnetism from which loadstones derive their polarity, or their tendency to direct themselves to particular points of the compass, is obviously derived in some way or other from the earth or its atmosphere; and hence it is called the Magnetism of the Earth, or Terrestrial Magnetism, which will be treated more fully in a future part of this article.

Sect. V.—On the Power of Magnets to communicate Magnetism to other Bodies.

We have already seen, that if a piece of soft iron is suspended to a magnet by the attraction of one of its poles, the iron becomes magnetic, but only during the time that it is in contact with the loadstone. But if we use a piece of hardened iron, or steel, a b, and suspend it as in fig. 3, it will be found to have acquired a permanent magnetism, the strength of which will depend on the power of the natural magnet A B, and on the time which the steel bar has been suspended. The pole a will be a north pole similar to A, and the pole b a south pole similar to B; and the little magnet a b will possess all the properties of the natural magnet, such as attraction for soft iron, and polarity; and its action upon another little steel magnet a' b', made in a similar manner, will be the same as the action of two natural magnets upon each other. A steel magnet thus made is called an artificial magnet; and we shall in the sequel consider the magnets of which we speak as steel bars rendered permanently magnetic.

A little magnet a b has been made by a very simple process, namely, that of contact with the pole of a natural magnet; but there are more complex and efficacious methods, by which a very high degree of permanent magnetism can be communicated to steel, which will be fully explained in the practical part of this treatise.

In order to communicate magnetism from a natural or artificial magnet to unmagnetised iron or steel, it is not necessary that the two bodies be in contact. The communication is effected as perfectly, though more feebly, when the bodies are separated by space.

If the north pole N of an artificial steel magnet A is placed near the extremity s of a piece of soft iron B, the end s will instantly acquire the properties of a south pole, and the opposite end n those of a north pole. The opposite poles would have been produced at n and s if the south pole S of the magnet A had been placed near the iron B.

In like manner, the iron B, though only temporarily magnetic, will render another piece of iron C, and this again another piece D, temporarily magnetic, north and south poles being produced at n', s', and n" s".

The magnetism inherent in B, C, and D, is said to be induced by the presence of the real magnet A, and the phenomena are exactly analogous to the communication of electricity to unelectrified bodies by induction, the positive state inducing the negative, and the negative the positive, in the parts of a conductor placed in a state of insulation near an electrified body.

In order to show by simple experiments that soft iron is itself a magnet while placed near a magnet, let A be a magnet, and K a key held near its lower edge. A nail N will remain suspended by virtue of its induced magnetism; but if A is withdrawn, or K removed from A, the nail N will instantly fall, the induced magnetism diminishing with the distance.

If we hold the key K above a portion of iron filings, they will not be attracted by it; but if we then bring the magnet A near the ring of the key, as in the figure, the iron filings will instantly start up, and be attracted by the key.

We have already noticed, in Sect. I., that the iron attracts a magnet re-acts upon the magnet, and attracts of iron on it in return. The same is the case with a bar of iron on which magnetism is induced. It re-acts on the magnet which induces its magnetism, and increases its magnetic intensity. Hence we derive a distinct explanation of the remarkable facts, that a magnet has its power increased by having a bar of iron placed in contact with one of its poles, and that we can gradually add more weight to that which is carried by a magnet, provided we make the addition slowly and in small quantities, the power of the magnet being increased by the re-action of each separate piece of iron that it is made to carry.

If the bar of iron on which magnetism is induced is Consecutive, and the strength of the magnet great, a succession of five poles is produced along its length, a north pole always following a south pole, and vice versa.

These facts enable us to explain the phenomena of Magnetic attraction and repulsion, which are necessary attraction consequences of magnetic induction. The magnet attracts and repels a piece of iron by inducing an opposite polarity at the end in contact with it, and the two opposite principles attract each other. In like manner, the north pole of one magnet attracts the south pole of another, and the north and south poles repel each other, in consequence of the attraction and repulsion of the opposite or similar principles. The attraction of iron filings is explained in the same manner. The particle of iron next the magnet has magnetism induced upon it, and it becomes a minute mag- net, like B in fig. 5. This particle again makes the next particle a magnet, like C, and so on, the opposite polarities in each particle of the filings attracting one another, as if they were real magnets.

In comparing the amount of the attractive force of two dissimilar poles of two magnets, with the amount of the repulsive force of the two similar poles, it has been found that the former force is considerably greater than the latter. This result is a necessary consequence of the inductive process above described. When the two attracting poles are in contact, each magnet tends to increase the power of the other, by developing the opposite magnetisms in the adjacent halves, and thus increasing their mutual attraction. But when the two repelling poles are brought into contact, the action of each half brought into contact has a tendency to develop in that half a magnetism opposite to that which it really possesses, and thus to diminish the two similar principles, and weaken their repulsive power. This injurious influence of opposite poles upon the repulsive power of the magnets in action, is finely exhibited when one of the magnets is very powerful, and the other very weak. When the two similar poles are held at a moderate distance, a repulsion is distinctly exhibited; but when they are brought into contact, the stronger attracts the weaker magnet, an effect which is produced by its actually destroying the similar weak magnetism in the half next it, and inducing in that half the opposite magnetism, which, of course, occasions attraction.

When the magnet A and the piece of iron B are placed in the same straight line, as in fig. 5, the pole N acts favourably in inducing south polar magnetism at n, and north polar at s; but it is evident that the remote pole S must tend to weaken the inductive force of N, by inducing, though in a feeble degree, north polar magnetism at n and south polar at s. If the soft iron B is placed as in fig. 7, the induced magnetism will be nearly as strong as before, the greater proximity of N tending to produce south polar magnetism in n, being compensated by the increased proximity of S tending to produce north polar magnetism in n. In the inclined position C the induced magnetism is still stronger, as S acts more powerfully upon n; and when the two are parallel, as in fig. 8, the two bars or magnets are in the position most favourable for developing and sustaining the magnetism which they receive or possess.

Hitherto we have considered the natural and artificial magnet as producing magnetism in soft or hard iron, distributed in the same manner as in the inducing magnet; but by the action of one or more magnets, we can distribute the magnetism in various ways, as follows:

In the case of bars, we may have a north pole in the middle of it, and a south pole at each extremity. Thus, in fig. 9, if the magnet NS has its north pole N placed opposite the middle of the soft iron bar mn, this bar will have a south pole at s and north poles at n, n. The very same effect will be produced if, as in fig. 10, we place the soft iron bar B between two magnets A, C whose north poles N, N are nearest the bar. These north poles N, N tend to produce south poles at s, s, and consequently northern polarity in the middle at n. In the preceding case a south pole may be produced in the middle, and north poles at the ends of the bar, by placing the south poles of the magnets where the north poles are placed.

In like manner, a piece of soft iron ss, ss, of the form of a cross, will have south poles at s, s, s, s, if the south pole S of a magnet A is placed on or near its centre, as in fig. 11, as it may be conceived to consist of two bars ss, ss. For the same reason, if a circular plate of soft iron is substituted in place of the cross ss, ss, and the south pole S of the magnet placed upon or near its centre, that centre will be a north pole, and every point of the circumference of the plate will be a south pole.

A very instructive experiment, founded on magnetic induction, is exhibited in fig. 12, where several soft iron wires or slender bars sn, sn, sn are suspended at the north pole N of a magnet N. Each of the ends s, s becomes a south pole by induction from the action of the north, and consequently the lower ends n, n, n north poles. The south poles s, s, s have a tendency to repel each other, but are prevented from yielding to their repulsive forces in consequence of their strong adhesion to the north pole N. The north poles n, n, n, however, are free from this restraint, and exhibit their mutual repulsion by their diverging, as shown in the figure. Hence we see the reason why rows of iron filings adhering to each other, when attracted by a magnet, keep separate from each other by the repulsive forces of the similar poles.

In the following form of the experiment given by Cavallo, the repulsion of both poles is well illustrated. If we suspend two short pieces of soft iron wire ns, ns by threads, they will hang in contact in a vertical position. If we now bring the north pole N of a magnet A to a moderate distance from the wires, they will recede from each other, as in fig. 13. The ends s, s, being made south poles by induction from the north pole N, will repel each other, and so will the north poles n, n. This separation of the wires will increase as the magnet A approaches nearer them; but there will be a particular distance at which the attractive force of N overcomes the repulsive force of the poles s, s, and causes the wires to converge, as in fig. 14, the north poles n, n still exhibiting their mutual repulsion.

The neutralisation or destruction of induced magnetism, by two equal and opposite magnetic actions, is shown in the following experiment, given by Dr Robison. If we take a forked piece of soft iron CDE, and suspend it by the branch D from the north pole of a magnet B, it will be magnetised by induction, and will carry a key at its lower end E, which will be a north pole. If we now apply to the other branch C the south pole S of another and equal magnet A, the key will instantly drop off. This obviously arises from the south pole S inducing a south pole at E, which either destroys or neutralizes the north polar magnetism previously induced by N. It is very obvious, from the preceding experiments, that in regular magnets, with a north pole at one end and a south pole at the other, the two kinds of magnetism, north polar and south polar, are equally and regularly distributed, the one occupying one half of the magnet, and the other the other half. It is obvious also that each kind of magnetism has no intensity at the centre of the magnet, or its middle part, and that it increases, according to some regular law, from that point towards the two poles at the extremities of the magnet.

The first person who determined the law of distribution which we have now mentioned was M. Coulomb. The magnet which he employed for this purpose was a cylinder two lines in diameter, twenty-seven inches long, and its weight 1948 grains; and he ascertained the intensity of magnetism at each point, from its middle to its extremity, by observing the number of oscillations which a small magnetic needle performed in a minute, when it was made to oscillate before different points of the wire. He had previously observed the number of oscillations which the same needle performed out of the sphere of the magnet, and he considered the magnetic intensity as proportional to the difference of the squares of those two numbers of oscillations. The first needle which he employed was three lines in diameter and six lines long, and it was made of such a size, and of such hardness, that its magnetism should not be perceptibly altered by the action of the wire during the experiments; for if any change did take place, the results obtained at different points of the magnet could not be compared. The great length of twenty-seven inches was given to the magnet, in order that its remoter pole might be so distant from the needle that it would be unnecessary to make any allowance for its action upon the oscillations of the needle. In this way Coulomb obtained the following results:

| Distances from the North Pole of the Magnet | Observed Intensity of the Magnetism at these distances | |--------------------------------------------|--------------------------------------------------------| | 0 | 165 | | 1 | 90 | | 2 | 48 | | 3 | 23 | | 4, 5 | 9 | | 6 | 6 |

The distribution of the magnetism is exhibited in fig. 16, where AN is half of the magnet, and N its north pole; and the ordinates to the curves represent the intensities in the preceding table.

These experiments were repeated by Coulomb, with magnets of the same shape and diameter, but of a less length, all other circumstances being unchanged, and he obtained nearly the same results for the three inches of the magnet nearest N; and hence he concluded, that whatever was the length of the magnet, provided it was greater than six or seven inches, the three inches at both its north and south poles gave always the same results as the twenty-seven inch magnet. From this point towards the centre the magnetism became weak and insensible in all of them; and in very long magnets he even found that the ordinates sometimes passed from positive to negative.

M. Biot has remarked that the curve of intensity, as determined by Coulomb, results from the combination of two logarithmic curves ACB', A'CB, which, setting out from each pole A, B of the magnet AB, would have their ordinates equal and in an opposite direction, as shown in fig. 17. The intensities calculated upon this supposition agree exactly with the observed results.

As Coulomb had examined the distribution of magnetism only in magnets of considerable size, M. Becquerel was desirous of ascertaining if the law was observed in steel wires of a small diameter, such as 1/30th of a millimetre, or 1/300th of an inch. In order to procure such wires, he encased a steel wire one millimetre in diameter in a cylinder of silver, and having drawn out the whole into a wire, the silver was removed by means of boiling mercury. He employed the method used by Coulomb in determining the law of distribution; but, on account of the fineness of the wires, and the weakness of the magnetism which they acquired, he was obliged to make some changes in the method. He obtained, however, the very same results as those given by Coulomb.

A number of interesting experiments on the distribution of magnetism have been made by M. Kupffer of Kasan, by means of the method of Coulomb. He employed a flat and very narrow needle, twelve millimetres long, and he placed it at a horizontal distance of three decimetres from a cylindrical bar magnet of cast steel not tempered, 607 millimetres long and 12½ millimetres thick. He began his experiments with magnets that possess a weak degree of magnetism. In magnetising them, he rubbed the steel bar perpendicularly on the north pole of a very strong artificial magnet, and he replaced the bar vertically before the needle, the north pole of the former being uppermost. He found that the south pole was stronger than the north pole, and that the point of indifference, or the neutral point, was nearer the stronger pole than the other. Upon reversing the magnet, the magnetic intensities of its different points increased, and the neutral point approached the middle of the magnet. These changes were produced successively, and the magnet did not attain its final state till it had remained some time in the same position. Kupffer observed, that whenever the magnetic intensities of the bar increased, the neutral point slowly approached the middle point; that this point was always nearer the stronger pole; that a bar magnetised vertically was always more powerful when its north pole was downwards; and that a bar magnetised by the method mentioned above was always strongest in the pole immediately produced by that of the magnet.

After detailing his observations with a bar magnetised to saturation, he proceeds to determine the influence exercised by the form of the extremities of the bar on the magnetic intensity, and on the position of the neutral point. A cylindrical bar of steel, cast but not tempered, having been rounded at one of its ends, and magnetised to saturation, was placed fourteen centimetres from a magnetic needle, and in the line of its direction. When its General Properties of Magnetic Bodies.

The rounded north pole was directed to the south; the force of the rounded north pole was 2:0319, and that of the south pole was 2:1558. In the opposite position of the bar, the magnetic force of the north pole was 2:2198, and that of the south pole 2:3006, the neutral point being in the middle.

The rounded end of the bar was now filed to a point, and made sharper and sharper in every successive experiment, after being each time magnetised to saturation. The force of the sharpened pole diminished with its acuteness. The neutral point receded always from this extremity.

In order to ascertain the distribution of magnetism in the interior of magnets, Coulomb formed sixteen rectangular magnets out of the same piece of steel. Each was six inches long, nine and a half lines wide, and 382 grains in weight. They were annealed at a white heat, without being tempered, in order that he might be certain of having them always in the same state. He magnetised them all to saturation, and formed bundles with a certain number of them, similar poles being placed together. The magnets in each bundle were bound tightly together with a strong silk thread. Each bundle was then placed in a torsion balance, and placed 30° out of the magnetic meridian. The force of torsion necessary to retain it in this position was a measure of its magnetic intensity. The following were the forces or degrees of torsion necessary to keep the different bundles at rest:

| Magnets | Degrees of Torsion | |---------|--------------------| | 1 | 82° | | 2 | 125° | | 4 | 150° | | 6 | 172° | | 8 | 188° | | 12 | 205° | | 16 | 229° |

Hence it follows that the magnetic force of each bundle increases in a ratio much less than that of the number of plates.

Coulomb next determined the magnetic state of each of the magnets composing the bundles of eight and sixteen magnets; and he found that the two outermost magnets, those which formed the surface of the bundles, had a much greater force than the rest.

The first had a force which measured 46°.

The second 48°.

And the mean force of all the rest was 30°.

A single magnet had its directing force 82°, while for sixteen of them united the mean directing force of each was only 14°-3, that is, about the sixth part of the other.

In examining the bundle of eight magnets by the method of oscillation, he found that the two outermost performed twenty oscillations in 90½ minutes, while all the rest performed the same number in from 211 to 278 nearly, showing the weakness of their magnetism. It is curious that the outermost but one had its poles reversed.

Coulomb also found that a bundle of magnets will take nearly the same degree of magnetism as a single magnet of the same shape and weight; which leads us to believe that, in magnets of one piece, the magnetism diminishes from the surface to the centre, as in the preceding bundles of magnets.

Sect. VII.—On the Effect of Division and Fracture in the Distribution of Magnetism.

As no natural or artificial magnet has ever been seen with only one pole, or one kind of magnetism, it became interesting to determine experimentally the distribution of magnetism in a part of a magnet cut from its north or south extremity. This experiment has been often made, both by cutting it through at the middle or neutral point, or by cutting or breaking off a portion from the end of it. If NS, for example, is a magnet, N its north and S its south pole, and ACB the curve representing the intensity of its magnetism; then, if we cut it through the middle C, each half ns, n's will be a complete magnet, with a north pole at n, and a south one at s, and their neutral points at e, e'; the curves at acb, a'e'b, representing the distribution of their north and south polar magnetism, being similar to the curve ACB of the large magnet of which they are the halves.

When Epinus made this curious experiment, he did not divide the magnet in two, but he set two steel bars end to end, and magnetised them as one magnet, so that this compound magnet had its magnetism distributed as in a single bar, like NS, fig. 18. He then separated them, and found that each bar was a perfect magnet, with two poles. Dr. Robison repeated this experiment successfully on some occasions; but he sometimes found indications of the compound magnet acting as two magnets. We are persuaded that this arose from an imperfect union of the two bars, and not from any defect in Epinus's experiment. The united ends of the bars should be ground together, so as to be kept in perfect contact, and preserved in this state by a powerful pressure during the time that they are magnetised. If this be done, we have no doubt that they will act on iron fillings, and throw them into curves, as if they were a single bar, and will, by examination with a fine needle, exhibit the same regular distribution of magnetism which takes place in the most perfect magnet.

Upon the separation of the magnets thus united, Epinus found that two poles were instantly developed in each half, but that the neutral points c, c', fig. 18, were nearer the interior poles s, s', or, what is the same thing, nearer the original neutral point C, than to n and s'. In the space of about a quarter of an hour it had, however, advanced nearer to the middle points c, c', and continued for some hours, and sometimes for days, to advance to these points, which it finally reached, thus completing the regular distribution of the two opposite magnetisms.

Some observations, but not very accurate ones, have been made on the division of magnets in the direction of their lengths. According to Dr. Derham, the two portions sometimes have contrary, and sometimes the same poles, as when they were united. When one portion was much thinner than the other, the thinner portion had generally its poles reversed. This experiment does not possess much interest; for it can scarcely be doubted that, if we could divide a magnet in the direction of its length without any violence or concussion, each portion, whether thinner or thicker, would have, when separate, the same polarities as when combined. The experiment would be easily made by pressing two equal steel bars into close contact, magnetising them in this state, and then separating them.

A very remarkable analogy has been pointed out by Sir David Brewster, between the preceding results and those which he has obtained with parallelopipeds of glass which received the doubly refracting structure by being quickly cooled on all their surfaces from a state of red heat. This change is analogous to that of temper in a magnet; and the effect of it is to produce a certain development of positive and negative double refraction throughout the whole of the parallelopiped of glass. These phenomena will be minutely explained in our article on Optics; but we may state at present, that the structure of the glass modifies the action of the ether which it contains, just as the structure of the tempered steel keeps the two magnetisms in an uncombined state. This is shown in fig. 19, where

Fig. 19.

AB is a thick plate of glass quickly cooled. The middle portion of it P has positive, and the external portions N, N, negative double refraction. The density of the ether in each of these portions varies according to a regular law; and the intensity of the doubly refracting force, at different points both of the positive and negative structures, is represented by a curve formed by the superposition of a straight line and a parabola. If we now cut the parallelopiped of glass into two halves, through the dotted line AB, fig. 19, each half will have the same structure as the whole, as shown in fig. 20; the parts that were formerly positive being now negative, and vice versa; and the intensity of the doubly refracting force in each half will be represented by the ordinates of a curve formed by the superposition of a straight line and a parabola. This fact is in perfect analogy with the magnetic one, and there are many other remarkable points of resemblance which we have not space to describe at present.

Sect. VIII.—On Magnetic Figures.

In our article on Electricity we have given an account of the beautiful electrical figures discovered by M. Lichtenberg, and which form one of the most interesting popular experiments in that science. We are indebted to M. Haldat of Nancy for the analogous discovery of magnetic figures, which may be easily produced. For this purpose he employs plates of steel from eight to twelve inches square, and from one twentieth to one eighth of an inch thick. The plates which he used were of that kind of steel which is used for the manufacture of cuirasses, so that it did not require to be tempered, being sufficiently hard to preserve the magnetism communicated to it. Figures of any kind may be traced on the surface of the steel plate, either by one magnet or by several combined; and the best form for this purpose is that in which the poles are rounded. In this way we may write upon a steel plate the name of a friend, or sketch a flower or a figure, with the extremity of a magnet. If it is a south pole that we use, all the traces which it makes will have north polar magnetism; and if we shake steel filings upon the plate out of a gauze bag, the filings will arrange themselves in the empty spaces between the lines traced by the pole of the magnet, and thus represent in vacant steel the name which has been written, or the flower or figure which has been sketched. "These figures," says M. Haldat, "have a perfect resemblance to those which are formed on the surface of non-magnetic plates, viz. wood, card, glass, or paper, under which a magnet is placed. The resemblance between the two sorts of figures, when the magnets and the parts magnetised have the same form, is not only exact in the whole figure, but even in the smallest details. The filings collect at the parts where the magnetism is most intense, they arrange themselves in pencils and radii, and form the same curves which we have represented in fig. 1, page 702. These curves, and pencils, and rays, so similar at the two figures, poles of the same magnet, have such a resemblance that they do not allow us to distinguish the two parts from one another."

M. Haldat has likewise produced these curves by interposing between the tracing magnets and the steel plates solid non-magnetic bodies, such as cards, glass, and even metallic plates that are not ferruginous. This method of producing magnetism in the steel plate by induction gives the same figures; but, in order to be efficacious, the magnet must have its pole carried parallel to and at a small distance from the plate of steel, and must repeat its traces, in order that the magnetism may be sufficiently developed. For rectilineal figures, M. Haldat employs rules with grooves, which keep the motion and distance of the bar invariable; for curvilineal figures he interposes some thin and uniform plate, and he can vary the distinctness of the figures by varying the distance of the tracing pole of the magnet.

In sitting the iron fillings upon the steel plate, a gentle vibration of the plate, by tapping its edge with the ring of a small key, will assist the fillings in taking their proper places; but we must avoid such vibrations as will produce regular acoustic figures, unless we wish, as M. Haldat has found to be practicable, to unite the magnetic with the acoustic figures, which produces very interesting and varied forms.

M. Haldat has found that the magnetic figures will continue for six months. In order to remove the magnetism which produces them, he recommends the heating of the plate upon red-hot charcoal, till it is brought to the straw-yellow temperature. In order to render the repolishing of the plate unnecessary, M. Haldat tins it, and the temperature at which the tin melts, when it is required to efface the magnetism, indicates the necessary heat. M. Haldat employs also another method, which is perhaps the best. He places the steel plate upon a block of wood, and by repeated and violent blows of a wooden hammer he removes the magnetism of the plate, the figures gradually becoming weaker and weaker when the experiment is tried with it in different stages. The effect is often produced in three or four minutes.

As the figures traced on the steel are nothing more than magnets of different forms, and are surrounded on all sides with a substance capable of acquiring the magnetism which may be developed by communication, we might expect, as M. Haldat remarks, that this means of communication between the opposite poles of the magnets would bring them into a neutral state. This, however, is not the case, and the portion of the metal which surrounds the magnetic figure performs the part of the armature of a loadstone, and the magnetism is thus kept up. If the figure be a simple rectangle, like that of a bar-magnet, the state of the plate, examined with a small needle, is exactly the same as a bar-magnet, and the parts which surround this magnetic portion are in a neutral state, as if unconnected with the rectangular space; from which it follows that the magnetic virtue, which communicates itself so easily by influence, ceases to communicate itself between the continuous parts of a magnetisable body, of which one portion is magnetic, and the rest in a neutral state.

In carrying into effect the preceding method of making improved magnetic figures, a very great difficulty must be experienced in recollecting the invisible traces made by the pole of the magnet, so as to complete a regular figure or drawing. When the figures are made immediately, as M. Haldat ex- presses it, that is, by the actual contact of the pole of the magnet, without any intermediate body, the best method would be to cover the plate of steel with the slightest coating of grease, and sift upon the surface, through a linen bag, some of the finest flour. The pole of the magnet, while tracing the figures on the steel, will remove the flour, and thus exhibit to the eye an accurate picture of what it has traced; and it will thus be easy to make the magnetic figures more distinct by repeating the traces with the magnet. The same thing may be done by putting an etching ground upon the steel plate, and tracing the figure as before. When the figure is completed, the coating of grease and flour, or the etching ground, must be removed previous to the application of the iron filings.

When the figures are to be produced immediately, or by the intervention of a non-magnetic substance, such as paper, card, wood, or glass, a fine dust may in like manner be laid upon the surface; but when the interposed substance will receive the mark of a pencil or sharp point, it would be preferable to attach to the cylindrical pole of the tracing magnet a very short point of a non-magnetic substance, which would make a visible mark on the paper, card, or wood, without strewing any fine dust on their surfaces. By the use of such a point, indeed, we may dispense altogether with the interposed substance, and communicate the magnetism by induction to the steel plate, in the very same way as if it had been done by the intervention of a non-magnetic plate whose thickness is equal to the length of the short point or tracer affixed to the pole of the magnet.

The magnetic figures might be rendered permanent by covering the steel plate either with a gummy or balsamic solution, which will indurate by exposure to the air; or with a coating of some easily melted substance, which becomes fixed at ordinary temperatures. If we sift the iron filings on the steel plate when covered with such a fluid, the filings will take their magnetic position round the traced lines, and will become fixed by the induration or solidification of the fluid coating.

**CHAP. III.—ON THE MAGNETISM OF BODIES NOT FERRUGINOUS.**

**Sect. I.—On the Magnetism of Metals, Minerals, and other Bodies.**

Iron was long regarded as the only body endowed with the property of acting and of being acted upon as a magnet; and though other metals and substances have been recently found to possess the same property, and though all substances whatever have been found by Coulomb to obey the power of a strong magnet, yet it is still a matter of doubt whether the magnetic effects thus produced are owing to a magnetism residing in the proper substance of the body, or are owing to a minute quantity of iron which enters into their composition.

The most magnetic metal next to iron is nickel. It receives and retains communicated magnetism longer than any other metal, and needles of nickel have a distinct polarity. These properties have been found in nickel after it has been repeatedly purified, though some authors have stated that they could not detect this property in certain specimens. A very decisive and instructive experiment on the magnetic qualities of nickel was made by M. Biot. He possessed a needle of nickel which had been purified by M. Thénard. It was 212 millimetres long, six broad, and 5-178 grains in weight. Having made a needle of steel of exactly the same dimensions, and which weighed 4-586

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1 *Traité de Physique*, tom. iii. p. 126. being made red hot, excepting indeed when some pieces of iron are concealed in them, which sometimes occurs; but in this case the piece of brass, after having been made red hot and cooled, will attract the needle more forcibly with one part of its surface than with the rest of it; and hence, by turning the piece of brass about, and presenting every part of it successively to the suspended magnetic needle, one may easily discover in what part of it the iron is lodged.

8th. In the course of my experiments on the magnetism of brass, I have twice observed the following remarkable circumstance: A piece of brass which had the property of becoming magnetic by hammering, and of losing the magnetism by softening, having been left in the fire till it was partially melted, I found upon trial that it had lost the property of becoming magnetic by hammering; but having been afterwards fairly fused in a crucible, it thereby acquired the property it had originally, viz. that of becoming magnetic by hammering.

9th. I have likewise often observed, that a long continuance of a fire so strong as to be little short of melting hot, generally diminishes, and sometimes quite destroys, the property of becoming magnetic in brass. At the same time the texture of the metal is considerably altered, becoming what some workmen call rotten. From this it appears, that the property of becoming magnetic in brass by hammering, is rather owing to some particular configuration of its parts, than to the admixture of any iron; which is confirmed still farther by observing that Dutch plate brass (which is made, not by melting the copper, but by keeping it in a strong degree of heat whilst surrounded by opus calcinarius) also possesses that property, at least all the pieces of it which I have tried have that property.

From these observations it follows, that when brass is to be used for the construction of instruments wherein a magnetic needle is concerned, as dipping needles, variation compasses, &c. &c., the brass should be either left quite soft, or it should be chosen of such a sort as will not be made magnetic by hammering, which sort, however, does not occur very frequently.

These judicious suggestions of M. Cavallo respecting the condition of the brass parts of azimuth compasses were not attended to as they ought, and we have no doubt that various grave errors have arisen from their neglect. Many examples have recently occurred, in which the errors were detected; and it is now the invariable practice of well-informed instrument-makers to reject hammered brass bowls for compasses, and to use those which are cast and turned for the purpose.

M. Cavallo and others have observed, that cobalt, zinc, copper, and bismuth, as well as their ores, are attracted by the magnet and antimony when gently heated. Minerals which are not metallic are almost all acted upon by the magnet, particularly where they have experienced the action of fire. The pure earths, and particularly silex, are found to have the same property. Among minerals, the following table shows those which are attracted and those which are not attracted by the magnet; but we place little faith in their accuracy.

| Minerals not attracted. | Minerals attracted. | |------------------------|---------------------| | Diamond | Oriental ruby | | Pellucid crystals | Chrysolite | | Amethyst | Tourmaline | | Topaz | Emerald | | Caledony, and other | Garnet | | crystals whose colouring matter is expelled by heat. | Several micas containing iron. |

Some accurate experiments have been made on mica by M. Biot. The chemical composition and optical structure of different varieties of this mineral vary greatly. M. Biot examined particularly mica from Siberia and mica from Zinwald in Bohemia. Though both were highly pellucid, yet chemical re-agents indicated in each the existence of oxide of iron. In the Bohemian mica it was greatest, and, according to an accurate analysis by Vauquelin, amounted to 20 per cent. Before the Siberian mica was analysed, M. Biot tried their magnetic properties. He cut out of each, thin rectangular plates of the same form, which he subdivided into smaller similar pieces, and having united them in a bundle, he suspended each bundle by a silk fibre, and caused each bundle to oscillate in succession between the poles of two strong magnets. The bundle of Zinwald mica performed twelve oscillations in fifty-five seconds, and that of the Siberian mica only seven in the same time. Hence the magnetic powers of the two micas were as 6:8 to 20, the ratio of 4:9 and 1:4 to the squares of the number of oscillations. If the oxide of iron, then, be the cause of their magnetic virtue, it should exist in the above proportions of 6:8 to 20; and as it was found to be 20 per cent. in the Zinwald mica, it ought to be 6:8 in the Siberian. It is very remarkable that the result of Vauquelin's analysis gave exactly this percentage of the oxide of iron, though it was not known to M. Biot till his experiment had been made.

The existence of magnetism in brass, while there was not the least trace of it either in the copper or zinc of which it is composed, led philosophers to investigate the effects produced by the union of different metals, or by their combination with other substances. Iron itself is a simple chemical body. Steel is a combination of iron and carbon. The loadstone is a combination of iron and oxygen; and as no magnetism is found either in carbon or oxygen, we are naturally led to believe, as M. Pouillet has remarked, that the magnetic fluid resides in the substance of the iron, and that it is carried with the atoms of that metal into all the chemical combinations which they form. We may therefore expect to find magnetic properties more or less developed in all ferruginous bodies, whether the iron be an accidental or an essential ingredient; and indeed cast iron, plumbago, and the oxides and sulphures of iron, exert a sensible action on the magnetic needle.

These views, however, are not in unison with facts which seem to have been well ascertained. Dr Matthew Young found, that the smallest admixture of antimony was capable of destroying the polarity of iron; and M. Seebeck states, that an alloy of one part of iron and four parts of antimony was so completely destitute of magnetic action, that, even when it was put into rotation, it exerted no power over the magnetic needle. The magnetic qualities of nickel also are destroyed by a mixture with it of other metals. Cheney found that a very small proportion of arsenic deprived a mass of nickel which had previously exhibited a strong magnetic power, of the whole of its magnetism; and Dr Seebeck found that an alloy of two parts of copper with one of nickel was entirely devoid of magnetism, and on this account he recommends it as well suited for the manufacture of compass boxes. On the other hand, Mr Hatchet ascertained, that when a large proportion of carbon, or sulphur, or phosphorus, was combined with iron, the iron was enabled fully to receive and retain its magnetic properties; but he at the same time found that there was a limit beyond which an excess of any of these three substances rendered the compound wholly incapable of receiving magnetism.

Animal and vegetable substances, after combination, are said to be attracted by the magnet. The flesh, and particularly blood, are acted upon more powerfully than other parts, and bone less powerfully. Burned vegetables have the same property, and also soot and atmospheric dust; and M. Cavallo has maintained, that brisk chemical effervescence acted upon the magnetic needle. Sect. II.—Account of the Experiments of Coulomb, Becquerel, Arago, and Seebeck, on the Existence of Universal Magnetism.

These various experiments on the magnetic power of so many classes of bodies, differing essentially in their composition, and in many of which it could not be reasonably supposed that iron existed, led some philosophers to believe that almost all substances gave indications of magnetism. M. Cavalli announced this opinion, but Mr. Benet questioned the accuracy of the experiments, and ascribed the movements observed in the needle to the agitation of the air in the receiver, arising from changes of temperature produced by the proximity of the observer's body, or from other causes.

It was not therefore till 1802, that the supposition of universal magnetism was put to the test of rigorous experiment. The apparatus which Coulomb employed for this purpose is shown in fig. 21, where AA is a glass receiver perforated at its top, and having a tube A'B, with a cork B, which could be raised and lowered with facility. Through this cork passed a rod tt of wood or metal, to which was attached a silk fibre, which suspended a ring of very fine paper, on which the small needle ns (about the third of an inch long and 3/5th thick) was placed. The receiver was then placed so as to enclose the opposite poles N, S of the powerful magnets M, M, each formed of four bars of steel tempered to a white heat. Each bar was seventeen inches long, three fifths of an inch wide, and one sixth of an inch thick, each bundle of four bars being one and three eighths of an inch wide, and one third of an inch thick. The distance N S of their poles was eight tenths of an inch. In making the experiments, the rod tt was turned till the needle ns was removed from the influence of the magnets; and after the number of its oscillations was observed, the rod tt was turned till the needle descended between the poles N, S of the magnets, when the number of oscillations of the needle was again counted, or the time in which a given number of oscillations was performed. If the needle performed the same number of oscillations in the same time, whether it oscillated between the poles N, S, or beyond their influence, it is obvious that the magnets exercised no power over them; but this was never the case, and Coulomb found that all substances whatever, when formed into small needles, turned themselves in the direction of the poles N, S, and, after a few oscillations, finally settled in that position. When these bodies were moved a very little way out of their position of equilibrium, they immediately began to oscillate round it, the oscillations being always performed more rapidly in the presence of the magnets than when they were removed out of their influence. Gold, silver, glass, wood, and all substances, whether organic or inorganic, thus obey the power of the magnets. Hence we cannot avoid the conclusion, either that all bodies are susceptible of magnetism, or that they contain minute quantities of iron, or other magnetic metals, which give them that susceptibility. M. Biot does not consider this alternative so inevitable as it appears, and throws out the conjecture, that the action may not be magnetic, but may be owing to some small force similar or analogous to the electrical forces developed by the simple contact of heterogeneous bodies. This no doubt might be, if there was any contact; and, in the absence of any reasons whatever for ascribing the observed effects to another cause, we cannot but rest between the alternative opinions above mentioned, giving a preference to that which ascribes the phenomenon to the existence in all bodies of a slight susceptibility to magnetic action.

This opinion derives considerable support from the experiments made by Coulomb on the comparative magnetic susceptibilities of cylindrical needles of gold, silver, lead, copper, and tin, which had been purified with the greatest care by MM. Sage and Guyton, and the results of which we have already given in our history of magnetism. M. Coulomb made a number of experiments on the effects experienced by needles of white wax, containing different proportions of iron filings, and he found that the intensities of the action which they experienced when oscillating between two magnets, was proportional to the absolute quantities of iron which they contained, the distribution and chemical state of the ferruginous particles being the same.

Since the time of Coulomb, methods different from his have been employed in developing magnetism in all bodies whatever. In order to detect small quantities of iron in minerals, M. Hauy employed the process of what he calls double magnetism. For this purpose, he placed a small bar-magnet in the direction of the needle, and in the same horizontal plane, the two similar poles being placed towards each other. The magnet being now brought slowly towards the needle, the latter deviates from the direction of the magnetic meridian, and takes a position perpendicular to it; an effect arising from the combined action of the poles of the magnet and the earth upon the magnetism of the needle. In this position, a very feeble magnetic action is sufficient to make the needle turn round and place its south pole opposite the north pole of the needle.

When the magnet is above the plane of the needle, and their opposite poles placed near each other, the needle does not change its direction while the point of suspension is beyond the bar, and at a suitable distance; but it is not so when the distance changes, for it tends continually to place itself perpendicular to the line of the poles.

This important subject has been investigated by M. Becquerel, who obtained the following results. His bar of iron magnet consisted of six united bars, each eight decimetres long and two centimetres broad. The needle was placed at different heights within and without the bar, and he sought to determine for each height the horizontal distance from the point of suspension (which is always in the line of the poles) to the nearest extremity of the needle, in order that its direction might be perpendicular to that line. The results were as follow:

| Vertical Distances from the Centre of Suspension to the Bar | Horizontal Distances of the Center of Suspension to the Extremity, in order that the Needle might take a perpendicular position | |-------------------------------------------------------------|------------------------------------------------------------------| | Millimetres | Millimetres | | 100 | 60 within | | 150 | 55 | | 200 | 46 | | 250 | 33 | | 300 | 12 | | 350 | 45 without | | 400 | 82 |

Hence it appears, that when the centre of suspension is... When M. Becquerel substituted for his magnetic needle a needle of soft iron, the results were exactly the same, differing only in their intensity. We come now to the original part of M. Becquerel's inquiry. Instead of a needle he used a small paper case filled with deutoxide of iron, or a mixture of deutoxide and tritoxide. With the former the effects were the same as with the steel needle; but it was different with the latter, in which one part of deutoxide was mixed with thirty parts of tritoxide.

If the centre of suspension be placed as near as possible to the north pole of the bar-magnet, and in the line of the poles, the paper case will take immediately a direction perpendicular to this line, instead of one coincident with it, as a soft iron needle would have done. If we put it out of this direction, it will return to it by a series of oscillations, whose velocity depends on the quantity of the deutoxide. From this it follows, that all the south polar magnetism of the paper case is situated on the side of it next the bar-magnet, while the north polar magnetism is on the other side, as may be exhibited by carrying a small magnetic needle along the paper case. Such a distribution of magnetism is impossible in soft iron or tempered steel.

If the centre of suspension be above the bar, the paper case will deviate from the position which it had at first, and tend to place itself in the direction of the line of the poles; an effect quite opposite to that produced by a steel or iron needle. The following were the experimental results:

| Vertical Distances from the Centre of Suspension from the Bar | Horizontal Distances of the same Centre to one of the Extremities of the Bar | Deviations of the Paper Case from the Direction perpendicular to the Line of the Poles | |---------------------------------------------------------------|-------------------------------------------------|--------------------------------------------------| | 10 millimetres | | | | 5 | | 24° | | 10 | | 44 | | 15 | | 60 | | 20 | | 78 | | 25 | | 84 | | 30 | | | | 5 | | 50 | | 10 | | 65 | | 15 | | 78 | | 20 | | 77 | | 30 | | 32 | | 5 | | 70 | | 10 | | 76 | | 15 | | 82 | | 20 | | | | 30 | | |

The transverse magnetism acquired by the paper case is permanent for some time, however small may be the proportion of deutoxide which it contains.

M. Becquerel next filled the paper case with very pure tritoxide, obtained by calcining nitrate of iron. The effect was much weaker than before. When the point of suspension was very near one of the extremities of the bar, the paper case still placed itself in a position perpendicular to the line of the poles; but if this point was placed above or below the bar, changing at the same time the vertical distance, the paper case deviated from its primitive direction, without, however, taking a direction perpendicular to that which it commonly takes when the centre of suspension is very near the extremity. It might be possible, M. Becquerel thinks, to attain the perpendicular direction by employing much stronger magnets. The following were the experimental results:

| Vertical Distances from the Point of Suspension to the Bar | Horizontal Distances of the same Point from the end of the Bar | Deviations from the Direction perpendicular to the Line of the Poles | |-------------------------------------------------------------|---------------------------------------------------------------|-------------------------------------------------------------------| | Without the Bar | | | | 5 millimetres | | | | 5 | | 25° | | 10 | | 34 | | 15 | | 48 | | 20 | | 55 | | 25 | | 70 | | 5 millimetres | | | | 5 | | 32 | | 10 | | 37 | | 15 | | 43 | | 20 | | 46 | | 25 | | 40 | | 10 millimetres | | | | 10 | | 26 | | 15 | | | | 20 | | 45 | | 25 | | 51 | | 10 millimetres | | | | 10 | | 20 | | 15 | | 30 | | 20 | | 45 | | 25 | | 50 |

Whenever the tritoxide contains the smallest quantity of the deutoxide, the velocity of the oscillations increases very powerfully. If, for example, we take two paper cases, one filled with tritoxide, and the other with tritoxide mixed with one thirtieth of the deutoxide, the first will perform twelve oscillations in thirty seconds, round a direction perpendicular to the line of the poles, while the other will execute twenty-five in the same time. Hence we may by this means readily determine the quantity of the deutoxide of iron contained in the tritoxide.

M. Becquerel next employed needles of wood, gum-lac, with and other substances, which have still a feebler magnetism than the tritoxide of iron. He placed a needle of wood, &c., white wood, &c., four centimetres long and two millimetres in diameter, above the interval between the opposite poles of two bar-magnets, as in fig. 21, the distance between N and S being three or four millimetres. The point of suspension was as near as possible to N S. The needle placed itself perpendicular to the line of the poles N S, in place of the position observed by Coulomb, coincident with N S. It comports itself therefore like the mixture of deutoxide and tritoxide of iron, or like the tritoxide alone. But if we separate gradually the extremities N, S of the bars, the wooden needle will place itself in the line N S, joining the poles, as shown in the figure. The deviations were as follow:

| Distances of N, S | Deviations of the Wooden Needle from the perpendicular position | |------------------|---------------------------------------------------------------| | 3 or 4 millimetres | 0° | | 10 | 18 | | 20 | 36 | | 30 | 56 |

When the bars are very close, and the needle in the perpendicular position, if we draw it out of this position, and keep it some instants in the direction of this line, it will remain there; but the smallest motion will cause it to return into its primitive direction, which it takes in preference to any other.

If we use only one bar-magnet, and place the wooden needle precisely opposite one of its poles, and as near as possible to the end of the bar, it will still direct itself perpendicularly to it; but if, while the point of suspension remains always in this line, we advance it within the bar, the needle will deviate from its direction, without, however, reaching the position of 90°, as will be seen from the following results: Magnetism.

Distances of the Centre of Suspension from the Extremity of the Bar.

| Distances | Deviations of the Wooden Needle | |-----------|--------------------------------| | 5 millimetres | 12° | | 10 | 18 |

Beyond ten millimetres the deviations increase insensibly and irregularly, so that they cannot be measured.

General conclusion. From these interesting experiments, M. Becquerel concludes that the magnetic effects produced by a strong bar-magnet upon a magnetic needle, or one of soft iron, differ essentially from those which take place in all bodies where the magnetism is very weak. In the former, whatever be their positions and directions, the magnetism is always distributed in the direction of their length, to the exclusion of every other direction; whereas in the tritoxide of iron, wood, and gum-lac, it is distributed in a direction which depends on the distance of the body from the poles of the magnet, so that the distribution varies with the direction which the magnet causes these needles to take, in virtue of the action which it exercises over them.

The universal prevalence of magnetism in all bodies whatever has been established by a beautiful discovery of M. Arago. This distinguished philosopher conceived the idea of studying the oscillations of a magnetic needle when placed above or near any body whatever. Having suspended a magnetic needle above metal, or even water, and caused it to deviate a certain number of degrees from its position, it began, when left to itself, to oscillate in arcs of less and less amplitude, as if it had been placed in a resisting medium; and, what was peculiarly curious in these experiments, this diminution in the amplitude of the oscillations did not alter the number of oscillations which were performed in a given time. The following were some of M. Arago's experiments with water, ice, and glass, the semiamplitude of the oscillations being at the instant 43°.

The distance of the water from the needle was 0-65 millim. The amplitude lost 10° in 30 oscillations. When the distance was 52-2 millim. A loss of 10° of amplitude required 60 oscillations. That is, the number of oscillations required to diminish the amplitude 10° was twice as great when the distance of the needle from the water was 52-2 millimetres, as when it was 0-65.

By placing the same needle upon ice, M. Arago obtained the following results:

| Distances from the Needle from the Ice. | Diminution of the Amplitude. | Number of Oscillations by which this diminution was effected. | |----------------------------------------|-----------------------------|---------------------------------------------------------------| | Millimetres | | | | 0-70 | From 53° to 43° | 26 oscillations | | 1-26 | From 53 to 43 | 34 | | 30-50 | From 53 to 43 | 56 | | 52-20 | From 53 to 43 | 60 |

By placing another needle near a plate of crown glass, he obtained the following results:

| Distances | Diminution of the Amplitude. | Number of Oscillations | |-----------|-----------------------------|------------------------| | 0-91 | From 90° to 41° | 122 | | 0-99 | From 90 to 41 | 180 | | 3-04 | From 90 to 41 | 208 | | 4-01 | From 90 to 41 | 221 |

Plates of metal afforded M. Arago similar results; but he nevertheless observed that those metals which act with more energy than glass, wood, &c., have a mode of action different from that of these substances. From all these results, it is manifest that all bodies, when placed near a magnetic needle in a state of oscillation, exercise over it an action, the effect of which is to diminish the amplitude of its oscillations, without altering their number; and hence the doctrine of the universal prevalence of magnetism in all bodies derives a new confirmation.

When Dr Seebeck of Berlin heard of the discovery of M. Arago, he made a magnetic needle two and an eighth inches long oscillate at a distance of three lines above plates of various bodies, and counted the number of oscillations which were required in each case to reduce the amplitude from 45° to 10°.

Substances employed. Thickness of the Plates. Number of Oscillations of the Needle.

| Substance | Thickness | Plates | Oscillations | |-------------|-----------|--------|--------------| | Marble | 0-0 line | 116 | | | Mercury | 2-0 | | 112 | | Bismuth | 2-0 | | 106 | | Platinum | 0-04 | | 94 | | Antimony | 2-0 | | 90 | | Lead | 0-75 | | 89 | | Gold | 0-2 | | 89 | | Zinc | 0-5 | | 71 | | Tin | 1-0 | | 68 | | Brass | 2-0 | | 62 | | Copper | 0-3 | | 62 | | Silver | 0-3 | | 55 | | Iron | 0-4 | | 6 |

Dr Seebeck found, that in alloying magnetic with non-magnetic substances, he formed compounds which exercised no action on the magnetic needle. The alloys which had particularly this singular property were those consisting of four parts of antimony and one of iron, or two parts of copper and one of nickel. In these cases the magnetism of the two ingredients must have been neutralized by their opposite actions.

CHAP. IV.—ON THE DEVELOPMENT OF MAGNETISM IN ALL BODIES BY ROTATION.

When M. Arago was engaged in the experiments described in the preceding chapter, the idea occurred to him of trying if the magnetic needle would be dragged along by the rotatory plates which had the power of diminishing the amplitude of its oscillation. This happy conjecture was immediately confirmed by experiment, and one of the most beautiful discoveries added to the science of magnetism.

The apparatus which he used for this purpose is shown in Plate CCCXXXV. fig. 3, where H is a clock made of copper, with the exception of two or three pivots, which are of steel. It is supported on a tripod stand TT, which can be levelled by screws S, S at the end of its three feet; and the object of it is to give a rapid rotatory motion by a vertical axis, on which is fitted a piece a, b, c, fig. 4, with three branches, upon which the revolving discs are to be placed. These discs are perforated at their centre by a small hole which receives the prolongation of the axis of rotation, and they are kept upon the branches a, b, c, by the pressure of a screw. Wings, w, &c. (fig. 4), which can be inclined at any angle, are applied for the purpose of retarding the velocity of the discs. A plate PP, with an opening in its centre a little larger than the diameter of the discs, rests upon the table TT, and a sheet of paper f, f, shown in fig. 22 (which is an enlarged view of that part of the apparatus). When a disc of copper was placed on the support \(a, b, c\), Plate CCCXXV. fig. 4, as shown at PP, fig. 22, and the copper made to revolve beneath the needle \(ad'\) with the sheet of paper \(ff\) intervening, the needle \(ad'\) is drawn out of the magnetic meridian the instant that the copper begins to revolve, and with a degree of force proportional to the velocity of rotation. As the force with which the needle is dragged from its place is opposed to the magnetic action of the earth, which tends to keep the needle in the magnetic meridian, the needle will take a position of equilibrium depending on the ratio of these forces. When the motion of the copper disc, however, is very rapid, the magnetism of the earth is overpowered by that of the revolving plate, and the needle does not stop, but continues to turn. The action of the revolving disc decreases in proportion as the distance of the needle from the plate \(PP\) is increased, the velocity being the same; so that if the motion of the needle be continuous when the two bodies are separated only by a sheet of paper, the needle will take a fixed position by increasing its distance from the plate; and its deviation from the magnetic meridian becomes less and less as it is removed to a greater height above the disc. When the plates have portions cut out in the direction of their radii, their action on the needle is diminished.

In trying plates of various metals, M. Arago found the results so dependent on the alloy which the metals contained, that he did not publish the results which he obtained. He devoted his attention to the determination of the directions of the force which is developed in the revolving discs, and for this purpose he sought the components of this force in the direction of three lines parallel to three co-ordinate planes perpendicular to each other. The component perpendicular to the plate he found to be an attractive force, which may be rendered sensible by means of a very long magnet suspended by a thread vertically to the extremity of the arms of a balance kept in equilibrium by a weight at the other extremity. The moment that the plate begins to revolve, the magnet is repelled, and the beam of the balance inclines to the other side. The second component is horizontal and perpendicular to a vertical plane which contains the radius abutting against the projection of the pole of the needle. This is the force which gives a motion of rotation to the needle, and it acts in the direction of a tangent to the circle. The third component is parallel to the radius which abuts against the projection of the pole of the needle. It may be determined with a dipping needle placed vertically, so that its axis of rotation is continued in a plane perpendicular to one of the radii of the disc. A similar needle placed at the centre of the disc experiences no action. There is also a second point, nearer the margin than the centre, where a needle experiences no change in its position; but between these points the lower pole is constantly attracted towards the centre, while it is repelled beyond that point.

No sooner were M. Arago's experiments announced to the Institute, which was done at the sitting of the 7th March 1825, than philosophers in every part of Europe repeated them, and succeeded in adding several important facts to those discovered by M. Arago. MM. Babbage, Herschel, Barlow, Nobili, Baccelli, Christie, and MM. Prevost and Colladon, took a prominent part in these researches. The results obtained by Messrs Babbage and Herschel were the most important, and the experiments were made in a manner different from those of M. Arago. A horse-shoe magnet, which lifted twenty pounds, was made to revolve rapidly round its axis of symmetry, placed vertically, with its poles uppermost. A circular disc of copper, six inches in diameter and \(\frac{1}{3}\) th of an inch thick, was suspended above the revolving magnet. As soon as the rotation of the magnet commenced, the copper began to turn in the same direction, at first slowly, but afterwards with an increasing velocity. When the magnet was made to turn in the opposite direction, the disc of copper changed the direction of its motion also, and exhibited the same phenomena. Metallic plates, ten inches in diameter and half an inch thick, when interposed between the magnet and the copper disc, did not sensibly modify the results, as M. Arago had observed. Glass produced no effect, but a sheet of tin-plate iron diminished greatly the influence of the magnet, while two such plates almost destroyed it. They also found that a disc of copper ten inches in diameter, and half an inch thick, and revolving with a velocity of seven revolutions in a second, did not communicate any motion to a similar disc freely suspended above it.

In comparing the influence of different metals, each disc had the same diameter and the same velocity; and the following were the results which were obtained by this and another method of observation.

| Ratio of the Force to that of Copper | Ratio by another Method | |--------------------------------------|------------------------| | Copper | 1:00 | | Zinc | 0:90 | | Tin | 0:47 | | Lead | 0:25 | | Antimony | 0:11 | | Mercury | 0:00 | | Bismuth | 0:01 | | Wood | 0:00 |

The second method of observation by which the results in the last column were obtained was more expeditious than the first. Portions of different bodies of the same form and dimensions were suspended above a revolving magnet, and the time of successive oscillations and the points of equilibrium were observed.

Our authors next sought to determine the effect produced by a solution of continuity in the metallic disc upon which the revolving magnet acted. For this purpose a disc of lead twelve inches in diameter and one tenth of an inch thick was suspended at a given distance from a horseshoe magnet revolving with the ordinary rapidity, first in its entire state, and afterwards in the state shown in the annexed figures, the black lines in the direction of the radii be... ing the planes where the lead was cut through. The accelerating forces, represented by \( \frac{s}{t} \), where \( s \) is the number of revolutions, and \( t \) the time employed, are as follow:

| Uncut Disc as in fig. 23 | Disc fig. 24 | Disc fig. 25 | Disc fig. 26 | Disc fig. 27 | |--------------------------|--------------|--------------|--------------|--------------| | 1258 | 1047 | 913 | 564 | 432 | | | | | | 324 |

Effects similar, but differing in degree, were obtained with other metals. With soft tinned iron the cutting produced a very slight diminution of effect, whilst in copper the same operation reduced the accelerating force in the ratio of five to one.

Messrs Babbage and Herschel next tried the effect of filling up the cuts with other metals. A light upper disc, suspended at a given distance above a revolving magnet, performed six revolutions in 54°8'. When it was cut as in fig. 27, its magnetic action was so weakened that it took 121°3' to perform six revolutions. When the eight open radial spaces were filled up with tin, its magnetic action was restored to such a degree that it made six revolutions in 57°3'. This fact is very interesting, as tin has less than half the energy of copper. The following results were obtained from other experiments, the numbers representing the accelerating forces or the magnetic energies developed in the plates:

- Brass not cut: 1·00 - Brass cut: 0·24 - Brass soldered with bismuth: 0·53 - Brass soldered with tin: 0·88 - Copper not cut: 1·00 - Copper cut: 0·20 - Copper soldered with tin: 0·91

In determining the law of the force in relation to the distance, Messrs Babbage and Herschel found it to vary between the ratio of the square and the cube of the distance. Mr Christie found, that when the revolving disc was thick and the needle delicate, the force which produced the deviation of the needle increased directly as the velocity of rotation, and inversely as the fourth power of the distance. MM. Prevost and Colladon found that the angles of deviation, and not their sines, increased in the direct ratio of the velocity, at least within certain limits; and that the sines of the angles of deviation were in the inverse ratio of the two and a half power of the distance.

M. Haldat made some interesting experiments on this subject. He found that every needle, however weak was its magnetism, obeyed the action of the revolving disc; but that this action disappeared entirely when its polarity disappeared. He found it impossible to magnetise needles by the action of the revolving disc, however rapid; and, in consequence of ascribing this effect to the want of coercive power, he employed discs of iron and steel, both soft and hardened.

A disc of soft iron acted with more energy than one of copper, and with the same velocity it dragged the needle twice the distance that a disc of brass did. Iron strongly hammered acted like soft iron, and was unable to give polarity to a steel needle. But a disc of untempered steel one twenty-fifth of an inch thick did not produce any appreciable effect on the magnetic needle, which, after a few irregular oscillations, maintained its ordinary position of equilibrium. Hence our author concluded that the force which acted upon it was in the inverse ratio of the coercive force. M. Haldat also found that discs in a state of incandescence exercised the same action as those at the ordinary temperature.

We have already seen, in our historical detail, that about six months previous to the announcement of M. Arago's discoveries, Mr Barlow had announced to the Royal Society of London the result of a series of experiments on the magnetic effects produced by iron in rotation. Having found that an iron ball performing 640 revolutions in a minute caused a magnetic needle to deviate several degrees, and to take a fixed position during the continuance of the motion; that the needle deviated in an opposite direction when the motion of the ball was reversed; that there were certain positions in which a bomb twelve inches in diameter, moved by a steam-engine, occasioned no deviation in the needle; that in some positions the deviation was in one direction, and in other positions in another; and that the deviation varied between 0° and 80°; he constructed a regular apparatus for determining the laws of these phenomena, and in which the iron which formed part of it should not influence the results. This apparatus is shown in Plate CCCXXV., fig. 2, where \( S \) is an iron sphere, made to revolve on a horizontal axis \( AB \); by means of two wheels, like an electrical machine, their diameters being as six to one, so as to perform 720 revolutions in a minute. A table \( LM \) was placed near the sphere, for holding the needle, so that the needle could be placed in any position, either above or below the sphere. The table \( LM \) being brought to the height of the axis \( AB \), the needle was placed successively in different positions round the sphere. The influence of the earth's magnetism on the needle being destroyed or neutralised by the action of a magnet properly placed for this purpose, and shown at NS standing vertically, Mr Barlow found, that whatever was the azimuth of the needle, its north pole approached the sphere \( S \) when the upper part of the sphere was moving towards the needle, and that its south pole approached the sphere when the upper part moved from the needle.

Having placed the axis of rotation sometimes in the magnetic meridian, sometimes in the direction of east and west, and sometimes in intermediate positions, he found, that whatever was the direction of the axis of rotation, the needle being always a tangent to the sphere, the north end of the needle was attracted when the sphere moved towards the needle, and repelled when its motion was from the needle. When the needle was carried round the revolving sphere in the semicircle, where the motion was directed towards the needle, its north extremity approached the sphere, and in the other semicircle it receded from it. The points where the sphere exercised upon it no action were at the two extremities of the axis, and those where the effect was a maximum were at the two extremities of an axis at right angles to this. In this case the direction of the needle was towards the centre of the ball.

The different positions of the needle are shown in fig. 28, where \( s \) is the sphere, \( ab \) its axis of rotation, and

---

1 Bill. Univers. tome xxix. p. 316. The lines \( ns, ns', \) &c. show the primitive position of the needle, and the dotted lines \( n's, n's', \) &c., those assumed by it when the motion is made from \( c \) to \( d \). The effects are reversed when the motion is made from \( d \) to \( c \).

If we carry the needle, when perfectly neutralised, round the sphere, and parallel to its axis, it has a tendency to place itself at right angles to the axis, and takes opposite directions at certain parts of the circle. If, for example, the axis be in the magnetic meridian, and the motion directed from the west to the east point of the horizon, the needle will direct itself to the west, and will do the same at all points between the horizon and an altitude of 60°. Beyond this the north end will direct itself to the east till it has passed the zenith 30° to the west; and then from his point to the west horizon, the north extremity will direct itself to the west, and similar changes will take place under the sphere. The same effects are produced, whatever be the direction of the axis and that of motion.

When a magnetic needle not neutralised is placed in different positions round the sphere whose axis is in the magnetic direction, the effects produced are as shown in fig. 29, where \( AB \) is the axis of rotation, the black lines representing the natural deviations of the needle, and the dotted ones those which it assumes when the sphere is in motion. Beginning at the point \( A \), if the motion be from left to right, that is, from west to east, the needle moves from \( n \) to \( n' \) in the same direction till it arrives at 30°. It then remains in its natural direction. The needle moves in a contrary direction from right to left at 60°, 75°, and at 90°.

Mr Barlow was next desirous of ascertaining the different effects produced by a solid and a hollow ball of iron, and with this view he put in motion a solid ball 7-87 inches in diameter, and weighing sixty-eight pounds, and also a hollow sphere of iron, weighing only about thirty-four pounds. Both of them performed 640 revolutions in a minute, and the following were the average results:

| Weight | Mean deviation of the solid ball | Mean deviation of the hollow ball | |--------|---------------------------------|----------------------------------| | | 28° 24' 68 lbs | 15° 5' 34 |

When the two balls were at rest, the difference of their action was nothing.

Mr Barlow's paper on rotation was communicated to the Royal Society on the 14th April 1825, and on the 20th Mr Christie communicated one On the Magnetism of Iron arising from its Rotation. Mr Christie's experiments were made with circular plates of iron put in motion by an ingenious piece of machinery, by which he could make the plate revolve in every possible plane in reference to the magnetic meridian. From a great body of well-designed experiments, he obtained the following general law of the deviation due to rotation, so that the direction of the rotation being given, he could tell the direction of the deviation. This law we must give in his own words.

"I refer the deviations of the horizontal needle to the deviations of magnetic particles in the direction of the dip, or to those of a dipping needle passing through its centre; so that, in whatever direction this imaginary dipping needle would deviate by the action of the iron, the horizontal needle would deviate in such a manner as to be in the same vertical plane with it: thus, when the north end of the horizontal needle deviates towards the west, and consequently the south end towards the east, I consider that it has obeyed the deviation of the axis of the imaginary dipping needle, whose northern extremity has deviated towards the west, and its southern towards the east; so that the western side of the equator of this dipping needle has deviated towards the south pole of the sphere, and its eastern side towards the north pole. It would follow from this, that if the north and south sides of the equator of the dipping needle (referring to these points in the horizon) deviated towards the poles, no corresponding deviations would be observed in the horizontal needle; the effect, in this case, taking place in the meridian, would only be observable in the angle which the dipping needle made with the horizon. As it is not my intention at present to advance any hypothesis on the subject, I wish this to be considered only as a method of connecting all the phenomena under one general view. Assuming it then for this purpose, it will be found that the deviations of the horizontal needle due to rotation are always such as would be produced by the sides of the equator of this imaginary dipping needle deviating in directions contrary to the directions in which the edges of the plate move, that edge of the plate nearest to either edge of the equator producing the greatest effect on it."

From another set of experiments, Mr Christie also found that the effect produced on the iron by its rotation is permanent so long as the plate remains stationary; that it is independent of friction; that it is so far independent of velocity, that the iron can scarcely be moved so slowly that the whole effect shall not be produced; and that the whole effect is produced by making it perform one fourth of a revolution. After Mr Christie had discovered these peculiar effects, he exhibited some of the phenomena to Mr Barlow, who conceived that the effect would be increased by rapid rotation, and who was thus led to make the experiments of which we have already given an account; but the phenomena differ essentially from those observed by Mr Christie, the former being temporary and dependent on velocity, while the latter are permanent, and independent of the rapidity of rotation.

In comparing the magnetic forces produced by rapid and slow rotation, Mr Christie found that the forces exerted on the needle during the rapid rotation of the plate are always in the same direction as the forces which are derived from the slowest rotation, and which continue to act after the rotation has ceased; but that the former forces are greater than the latter. From a mean of all the observations, the forces seem to be in the ratio of seventeen to thirteen, or very nearly of three to two. Hence Mr Christie conceives that the polarising of the iron in the same direction will account for the phenomena in both cases; but that the intensity of the polarity during the rapid rotation is greater than that which appears to be permanent after the rotation, whether slow or rapid, has ceased; and that the phenomena observed during rapid rotation are such as should be expected from what have been described as arising from rotation, without regard to its velocity.

We have already seen that Messrs Babbage and Herschel interposed plates of various metals between the revolving magnet and the copper disc, and found no perceptible effect to be produced. Mr Snow Harris, however, Influence of Heat on Magnetism.

has recently shown that several substances not supposed to contain iron have the power of intercepting the influence of a revolving magnet. A circular magnetic disc being delicately balanced on a fine central point by means of a rim of lead, was put into a state of rotation on a small agate cup, at the rate of 600 revolutions in a minute; and a light ring of tinned iron, also finely balanced on a central pivot, was placed immediately over it, at about four inches distance, by means of a thin plate of glass, on which its pivot rested. When the ring of tinned iron began to move slowly on its pivot by the influence of the magnet revolving below, a large mass of copper, about three inches thick, and consisting of plates a foot square, was carefully interposed between the magnet and the iron ring. The interposition of the copper soon sensibly diminished the motion of the iron disc, and at length arrested it altogether. On again withdrawing the copper, the motion of the disc was restored; and the same effects were repeatedly obtained. In this experiment both the magnet and the disc were enclosed by glass shades, and supported on a firm base.

The same effects were produced by a mass of silver and zinc; but when their thickness was considerably diminished by removing the central plates, the motion of the disc was not impeded. A very great thickness of lead was necessary to stop the disc, in consequence, as Mr Harris supposes, of its magnetic energy being so much less than that of copper.

CHAP. V.—ON THE INFLUENCE OF HEAT ON MAGNETISM.

This interesting department of magnetism divides itself into three parts: 1st, On the effect of heat on the development of free magnetism; 2ndly, on the anomalous attraction observed during the bright red and red heats; and, 3rdly, on the effect of heat on the distribution of magnetism in magnets.

Sect. I.—On the Effect of Heat on the development of Magnetism in Cast and Malleable Iron.

In the course of his experiments on the relative magnetic powers of different kinds of iron and steel, already given in the history of magnetism, Mr Barlow was led to the conclusion, that the harder the metal was, the less it exhibited a magnetic quality; a result which was highly favourable to the hypothesis, that the cohesive power of hardened steel not only prevented the entire development of its magnetism, but also the re-combination of the two kinds of magnetism when they were displaced by the action of a powerful magnet. With the view of establishing this hypothesis, Mr Barlow found it necessary to ascertain whether these different kinds of iron and steel would exhibit the same magnetic powers when reduced to the same degree of softness, which could only be done by heating them in a furnace, and trying their magnetic qualities in that state.

Having procured a bar of soft iron twenty-five inches long and an inch and a quarter square, and a cast-iron one of nearly the same dimensions, he inclined the bars in the direction of the dip; and having placed a magnetic needle nearly on a level with the upper extremity, and at the distance of six inches from it, he observed the deviations produced by the bars in different states of heat. Thus,

| Material | State | Deviation | |----------------|----------------|-----------| | Cast iron | Cold | Mean deviation 21° 30' | | Ditto | White heat | Ditto | 0 0 | | Ditto | Blood-red heat | Ditto | 52 0 | | Malleable iron | Cold | Ditto | 40 0 | | Ditto | White heat | Ditto | 0 0 | | Ditto | Blood-red heat | Ditto | 55 0 |

These experiments were often repeated with the same results. It deserves to be remarked as a singular result, that cast iron is decidedly inferior in its action when cold, and when hot possesses a superior power to malleable iron.

Mr Barlow now compared malleable iron with soft and hard shear steel. The bars were twenty-four inches long and an inch and a quarter square, and the following were the results:

| Material | State | Deviation | |----------------|----------------|-----------| | Malleable iron | Cold | Mean deviation 15° 10' | | Ditto | White heat | Ditto | 0 0 | | Ditto | Blood-red | Ditto | 11 11 | | Soft shear steel| Cold | Ditto | 11 0 | | Ditto | White heat | Ditto | 0 0 | | Ditto | Blood-red | Ditto | 48 0 | | Hard shear steel| Cold | Ditto | 8 0 | | Ditto | White heat | Ditto | 0 0 | | Ditto | Blood-red | Ditto | 47 30 |

These experiments establish the curious fact of the total destruction of the magnetic virtue by a white heat; and also the no less important one, that every kind of iron or steel has a greater capacity for developing its magnetism when softened by fire than when cold.

Sect. II.—On the Anomalous Attraction observed in Cast and Malleable Iron during the Bright Red and Red Heats.

In pursuing the preceding researches, Mr Barlow was Anoma-led to observe a remarkable anomaly in the action of the iron at the red heat. When iron brought to a white heat has wholly lost its power, it again acquires, as it passes into the bright red and red, a magnetic power; but, what is truly strange, its power is attractive for the south end of the needle; that is, if the north pole of the needle was attracted when the iron was cold, the south end will be attracted when the iron is at a bright red heat.

In order to investigate this subject thoroughly, Mr Barlow made a very extensive series of experiments with four different bars, each twenty-five inches long and an inch and a quarter square, two of them being of cast and two of malleable iron. He used also other two bars, one of cast and one of malleable iron, of the same dimensions, which were kept as standards to determine the quantity of cold attraction. The time employed in each experiment was a quarter of an hour: the white heat generally continued about three minutes when the negative attraction commenced. This attraction lasted about two minutes more, when the usual attraction began. This sometimes reached its maximum with great rapidity, but at other times it increased very gradually. The following table contains the results of Mr Barlow's experiments. The letters CB denote the cast-iron bar, and MB the malleable-iron bar; and the sign + indicates when the ordinary attraction of the iron takes place; and — the anomalous or negative attraction. ### Table of the Results of Mr. Barlow's Experiments on the Effect of Iron on the Compass Needle at different Degrees of Heat.

N.B.—We have omitted the column for white heat, as no effect is ever produced at that temperature.

| No. | Description of Bar. | Height and Depth of Centre of Bar from Needles. | Distance from Needles. | Position of Needles. | Effect when Red Heat. | Effect at Blood Heat. | Remarks. | |-----|---------------------|-------------------------------------------------|------------------------|----------------------|-----------------------|-----------------------|----------| | 1 | C. B. No. 1. | 0° below. | 6°0 | S. 80° W. | + 0° | + 17° | South end drawn to the bar at red heat. | | | M. B. No. 2. | Ditto. | 6°0 | Ditto. | + 30° | 0° | This bar being left standing, it attracted the same three days after. | | | C. B. No. 2. | Ditto. | 6°0 | Ditto. | + 15° | 0° | Observed at the same time with two compasses. | | | M. B. No. 1. | Ditto. | 6°0 | Ditto. | + 29° | 0° | The needle suspected to touch the box. | | 2 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 3 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 4 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 5 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 6 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 7 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 8 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 9 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 10 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 11 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 12 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 13 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 14 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 15 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 16 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 17 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 18 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 19 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 20 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 21 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 22 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 23 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 24 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 25 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 26 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 27 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 28 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 29 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 30 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 31 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 32 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 33 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 34 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 35 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 36 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 37 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. | | 38 | Ditto. | 1° above. | 6°0 | Ditto. | + 29° | 0° | Ditto. |

*Note:* Negative attraction rather sudden. Motion of needle very slow. 100° very sudden, returned immediately. Both attractions gradual. The same as No. 32; both anomalous. Attractions very gradual. Attraction regular, but quick. No motion in the needle.

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**Influence of Heat on Magnetism:** One of the most remarkable results of these experiments is, that the anomalous action of the bar between a bright-red and blood-red heat increases as we raise the bar above the needle, and becomes a maximum at the centre of the bar; whereas at low temperatures the action of a bar of iron under the very same circumstances goes on diminishing as the bar is raised, and becomes a minimum at the centre. When the needle is placed at the height of the centre of the bar, when heated to produce the anomalous effect, the smallest displacement is sufficient to change the sign and the quantity of the deviation.

Mr Barlow made some experiments with a twenty-four pound ball of iron, but the heat was too intense to allow any very accurate observations to be made. The results, however, were as follow:

Cold attraction........... + 18° 30' deviation. Red heat.................. - 3 39