31 confiderable accuracy, ftill remains to be tried. It will be mentioned in due time. Such then are the general laws observed in the mutual action of magnets. We think it fcarcely neceffary to enter into a farther detail of their confequences, correfponding to the innumerable varieties of pofitions in which they may be placed with refpect to each other. We are confident, that the fenfible actions will always be found agreeable to the legitimate confequences of the general propofitions which we have established in the preceding paragraphs. We proceed therefore to confider fome phyfical facts not yet taken notice of, which have great influence on the phenomena, and greatly affift us in our endeavours to underftand fomething of their remote cause. Magnetifm Magnetifm, in all its modifications of attraction, re is tempora- pulfion, and direction, is, in general, of a temporary or ry and pe- perifning nature. The beft loadftones and magnets, rifhing. unlefs kept with care, and with attention to certain circumftances, are obferved to diminish in their power. Natural loadstones, and magnets made of fteel, tempered as hard as poffible, retain their virtue with greatest obftinacy, and seldom lose it altogether, unless in fituations which our knowledge of magnetifm teaches us to be unfavourable to its durability. Magnets of temper ed fteel, fuch as is used for watch-fprings, are much fooner weakened, part with a greater proportion of their force by fimple keeping, and finally retain little or none. Soft fteel and iron lofe their magnetifm almoft as soon as its producing caufe is removed, and cannot be made to retain any fenfible portion of it, unlefs their metallic ftate fuffer fome change. 3. Hurt by 1. Nothing tends fo much to impair the power of a improper magnet as the keeping it in an improper pofition. If pofition. its axis be placed in the magnetic direction, but in a contrary pofition, that is, with the north pole of it where the fouth pole tends to fettle, it will grow weaker from day to day; and unless it be a natural load ftone, or be of hard tempered fteel, it will, after no very long time, lofe its power altogether. 2. By heat; effects of thunder city. 2. This diffipation of a strong magnetic power is greatly promoted by heat. Even the heat of boiling water affects it fenfibly; and if it be made red hot, it and electri-is entirely deftroyed. This laft fact has long been known. Dr Gilbert tried it with many degrees of violent heat, and found the confequences as now ftated; but having no thermometers in that dawn of science, he could not fay any thing precife. He only obferves, that it is deftroyed by a heat not fufficient to make it vifible in a dark room. Mr Canton found even boiling water to weaken it; but on cooling again the greatest part was recovered. 3. By violent treat ment. 3. What is more remarkable, magnetifm is impaired by any rough ufage. Dr Gilbert found, that a magnet which he had impregnated very firongly, was very much impaired by a fingle fall on the floor; and it has been obferved fince his time, that falling on ftones, or receiving any concuffion which caufes the magnet to ring or found, hurts it much more than beating it with any thing foft and yielding. Grinding a natural loadftone with coarse powders, to bring it into fhape, weakens it much; and loadftones fhould therefore be reduced into a shape as little different from their natural form as poffible; and this fhould be done briskly, cutting them with the thin difks of the lapidary's wheel, cutting off only what is neceffary for leaving their moft active parts or poles as near their extremities as we can. All thefe caufes of the diminution of magnetifm are more operative if the magnet be all the while in an im. proper pofition. 4. Lastly, magnetifm is impaired and deftroyed by 4. By other placing the magnet near another magnet, with their fi- magnets. milar poles fronting each other. We have had occafion to remark this already, when mentioning the experiments made with magnets in this pofition, for afcertaining the general laws or variations of their repulfion. We there obferved, that magnets fo fituated always weakened each other, and that a powerful magnet often changed the species of the nearest pole of one lefs powerful. This change is recovered, in part at least, when it has taken place in a loadstone or a magnet of hard fteel; but in fpring tempered fteel the change is generally permanent, and almost to the full extent of its condition while the magnets are together. It is to be remarked, that this change is gradual; and is expedited by any of the other caufes, particularly by heat or by knocking. 32 On the other hand, magnetism is acquired by the Maguetifm fame means, when fome other circumftances are at- may be ac. quired, tended to. 1. A bar of iron, which has long ftood in the mag-1. By magnetic direction, or nearly fo, will gradually acquire netical po magnetifm, and the ends will acquire the polarity cor-fition; refponding to their fituation. In this country, and the north of Europe, the old fpindles of turret vanes, old bars of windows, &c. acquire a fenfible magnetism ; their lower extremity becoming a north pole, and the other end a fouth pole. Gilbert fays, that this was firft obferved in Mantua, in the vane spindle of the Auguftine church-" Vento flexa (fays he) de prompta, et apothecario cuidar conceffa, attrahebat ferrea ramenta, vi perquam infigni." The upper bar of a hand rail to a fair on the north fide of the highest part of the steeple of St Giles's church in Edinburgh is very magnetical; and the upper end of it, where it is lodged in the ftone, is a vigorous fouth pole. It is worth notice, that the parts of fuch old bars acquire the ftrongest magnetifm when their metallic ftate is changed by exposure to the air, becoming foliated and friable. It would be worth while to try, whether the æthiops martialis, produced by fteam in the experiments for decompofing water, will acquire magnetifm during its production. The pipe and the wires, which are converted into the fhining æthiops, fhould be placed in the magnetic direction. 2. If a bar of fleel be long hammered while lying in 2. By ham the magnetic direction, it acquires a fenfible magnetifm mering; (See Dr Gibert's plate, reprefenting a blacksmith hammering a bar of iron in the magnetic direction). The points of drills, especially the great ones, which are urged by very great preffure; and broaches, worked by a long lever, fo as to cut the iron very fast, acquire a ftrong magnetifm, and the lower end always becomes the north pole (Phil. Tranf. xx. 417.) Even driving a hard fteel punch into a piece of iron, gives it magnetifm by a single blow. In fhort, any very violent squeeze given to a piece of tempered fteel renders it magnetic, and its polarity correfponds with its pofition during the experiment. We can scarcely take up a cutting or boring tool in a fmith's fhop that is not magnetical. Even foft ing; foft fteel and iron acquire permanent magnetism in this way. Iron alfo acquires it by twifting and breaking. It is therefore difficult to procure pieces of iron or steel totally void of determinate and permanent magnetifm; and this frequently mars the experiments mentioned in the first paragraphs of this article. The way therefore to enfure fuccefs in thefe experiments is to deprive the rods of their accidental magnetism, by fome of the methods mentioned a little ago. Let them be heated red hot, and allowed to cool while lying in a direction perpendicular to the magnetic direction (nearly E. N. E. and W. S. W. in this country). 3. By heat. 3. As heat is obferved to deftroy magnetism, fo it may also be employed to induce it on fubftances that are fufceptible of magnetifm. Dr Gilbert makes this obfervation in many parts of his work. He fays, that the ores of iron which are in that particular metallic ftate which he confiders as moft fufceptible of magne tifm, will acquire it by long continuance in a red heat, if laid in the magnetic direction, and that their polarity is conformable to their pofition, that end of the mass which is next the north becoming the north pole. He alfo made many experiments on iron and steel bars expofed to ftrong heats in the magnetical direction. Such experiments have been made fince Gilbert's time in great number. Dr Hooke, in 1684, made experiments on rods of iron and fteel one-fifth of an inch in diameter, and feven inches long. He found them to acquire permanent magnetifmn by expofure to Arong heat in the magnetic direction, and if allowed to cool in that direction. But the magnetiẩm thus acquired by fteel rods was much stronger, and more permanent, if they were fuddenly quenched with cold water, fo as to temper them very hard. He found, that the end which was next to the north, or the lower end of a vertical bar, was always its permanent north pole. Even quenching the upper end, while the reft was fuffered to cool gradually, became a very fenfible fouth pole. No magnetilm was acquired if this operation was performed on a rod lying at right angles to the magnetical direction. In these trials the polarity was always eftimated by the action on a mariner's needle, and the intenfity of the magnetifm was estimated by the deviation caufed in this needle from its natural pofition. Dr Gilbert made a very remarkable obfervation, which has fince been repeated by Mr Cavallo, and published in the Philofophical Tranfactions as a remarkable discovery. Dr Gilbert fays, p. 69. "Bacillum ferreum, valide ignitum ap. pone verforio excito; flat verforium, nec ad tule ferrum convertitur: fed flatim ut primum de candore aliquantulum remiferit, confluit illico." In feveral other parts of his treatise he repeats the fame thing with different circumftances. It appears, therefore, that while iron is red hot, it is not fufceptible of magnetifm, and that it is during the cooling in the magnetic direction that it acquires it. Gilbert endeavoured to mark the degree of heat moft favourable for this purpofe; but being unprovided with thermometers, he could not determine any thing with precifion. He fays, that the verforium, or mariner's needle, was moft deranged from its natural pofition a little while after the bar of iron ceased to thine in day-light, but was ftill pretty bright in a dark room. But there are other experiments which we have made, and which will be mentioned by and bye; by which it appears, that although a bright red or a white 1 heat makes iron unfufceptible of magnetism while in that ftate, it predisposes it for becoming magnetical. When a bar of fteel was made to acquire magnetifm by tempering it in the magnetical direction, we found that the acquired magnetifm was much stronger when the bar was made firit of all very hot, even although allowed to come to its molt magnetical ftate before quenching, than if it had been heated only to that degree; nay, we always found it ftronger when it was quenched when red hot. We offer no explanation at prefent; our fole bufinefs juft now being to ftate facts, and to generalize them, in the hopes of finding fome fact which fhall contain all the others. 4. The moft diftin&t acquifitions and changes of mag-4. By juxnetifm are by juxtapofition to other magnets and to iron, tapofition. As the magnetifm of a load ftone or magnet is weaken. ed by bringing its pole near the fimilar pole of another magnet, it is improved by bringing it near the other pole; and it is always improved by bringing it near any piece of iron or foft fteel. But this action, and the mutual relation of magnets and common iron, being the moft general, and the moft curious and inftructive of all the phenomena of magnetifm, they merit a very particular confideration. Of the communication of Magnetifm. 33 THE whole may be comprehended in one propofition, Communi which may be faid to contain a complete theory of mag-cation of netifm. magnetifm. Fundamental propofition. Any piece of iron, when in the neighbourhood of a magnet, is a magnet, and its polarity is fo difpofed that the magnet and it mutually attract each other. The phenomena which refult from this fundamental principle are infinitely various, and we must content ourselves with defcribing a fimple cafe or two, which will fufficiently enable the reader to explain every other. 34 Attractive Take a large and frong magnet NAS (fig. 7.), of which N is the north, and S the fouth pole. Let it be power com properly supported in a horizontal pofition, with its municated. poles free, and at a distance from iron or other bodies. Take any fmall piece of common iron, not exceeding two or three inches in length, fuch as a small key. Take allo another piece of iron, fuch as another fmaller key, or a bit of wire about the thickuefs of an ordinary quill. 1. Hold the key horizontally, near one of the poles, (as fhewn at no 1.), taking care not to touch the pole with it; and then bring the other piece of iron to the other end of the key (it is indifferent which pole is thus approached with the key, and which end of the key is held near the pole). The wire will hang by the key, and will continue to hang by it, when we gradually withdraw the key horizontally from the magnet, till, at a certain diftance, the wire will drop from the key, because the magnetism imparted from this diftance is too weak. That this is the fole reafon of its dropping, will appear by taking a fhorter, or rather a flenderer, bit of wire, and touch the remote end of the key with it: it will be fupported, even though we remove the key ftill farther from the magnet. 2. Hold the key below one of the poles, as at no 2. or 3. and touch its remote end with the wire. It will be fufpended in like manner, till we remove the key too far from the magnet. 3. Hold 1 35 Alfo a di. rective power. 3. Hold the key above the poles, as at n° 4. or 5. and touch its adjacent end with the wire (taking care that the wire do not alfo touch the magnet). The wire will still be supported by the key, till both are removed too far from the magnet. Thus it appears, that in all thefe fituations the key has fhewn the characteristic phenomenon of magnetism, namely, attraction for iron. In the experiment with the key held above the pole, the wire is in the fame fituation in refpect to magnetism as the key is when held below the pole; but the actions are mutual. As the key attracts the wire, fo the wire attracts the key. If the magnet be fupported in a vertical pofition, as in fig. 8. the phenomena will be the fame; and when the key is held directly above or directly below the pole, it will carry rather a heavier wire than in the horizontal pofition of the magnet and key. Instead of approaching the magnet with the key and wire, we may bring the magnet toward them, and the phenomena will be still more palpable. Thus, if the bit of wire be lying on the table, and we touch one end of it with the key, they will fhew no connection whatever. While we hold the key very near one end of the wire, bring down the pole of a magnet toward the key, and we shall then fee the end of the wire rife up and ftick to the key, which will now fupport it. In like manner, if we lay a quantity of iron filings on the table, and touch them with the key, in the abfence of the magnet, we find the key totally inactive. But, on bringing the magnet anyhow near the key, it immediately attracts the iron filings, and gathers up a heap of them. In the next place, this vicinity of a magnet to a piece of iron gives it a directive power. Let NAS (fig. 9.) be a magnet, and BC (n° 1.) a key held near the north pole, and in the direction of the axis. Bring a very fmall mariner's needle, fupported on a sharp point, near the end. C of the key which is fartheft from N. We fhall fee this needle immediately turn its fouth pole towards C, and its north pole away from C. This pofition of the needle is indicated at c, by marking its north pole with a dart, and its fouth with a crofs. Thus it appears that the key has got à directive power like a magnet, and that the end C is performing the office of a north pole, attracting the fouth pole of the needle, and repelling its north pole. It may indeed be faid, that the needle at c arranges itself in this manner by the directive power of the magnet; for it would take the fame pofition although the key were away. But if we place the needle at b, it will arrange itself as there reprefented, fhewing that it is influenced by the key, and not (wholly at least) by the magnet. In like manner, if we place the needle at a, we fhall fee it turn its north pole toward. B, notwithstanding the action of the magnet on it. This action evidently tends to turn its north pole quite another way; but it is influenced by B, and B is performing the office of a fouth pole. In like manner, if we place the key as at n° 2. we fall obferve the end B attract the fouth pole of the needle placed at a, and the end C attract the north pole of a needle placed in b. In this fituation of the key, we fee that B performs the office of a north pole, and C performs the office of a fouth pole. Thus it appears that the key in both fituations has become a magnet, poffeffed of both an attraÂive and a 36 is owing to tion of its Laitly, the magnetifm of the key is fo difpofed, that The attra the two magnets NAS and BC muft mutually attract tron of each other; for their diffimilar poles front each other. the dipoh Now, it is a matter of uniform and uncontradicted obfervation, that when a piece of iron is thus placed near own tem a magnet, and the difpofition of its magnetifm is thus porary examined by means of a mariner's needle, the difpofi- magnetin tion is fuch that two permanent magnets with their poles fo difpofed muft attract each other. The piece of iron, therefore, having the fame magnetic relation to the magnet that a fimilar and fimilarly difpofed magnet has, muft be affected in the fame manner. We cannot, by any knowledge yet contained in this article, give any precife intimation in what way the polarity of the piece of iron will be difpofed. This depends on its fhape as much as on its pofition. By defcribing two or three examples, a notion is obviously enough fuggefted, which, although extremely gratuitous, and perhaps erroneous, is of fervice, because it has a general analogy with the obferved appearances. If one end of a slender rod or wire be held near the north pole of the magnet, while the rod is held in the direction of the axis (like the key in fig. 7. n° 1.), the near end becomes a fouth, and the remote end a north pole. Keeping this fouth pole in its place, and turning the rod in any direction from thence, as from a centre, the remote end is always a north pole. And, in general, the end of any oblong piece of iron which is nearest to the pole of a magnet becomes a pole of the oppofite name, while the remote end becomes a pole of the fame name with that of the magnet. If the iron rod be held perpendicularly to the axis, with its middle very near the north pole of the magnet, the two extremities of the iron become north poles, and the middle is a fouth pole. If the north pole of a magnet be held perpendicular to the centre of a round iron plate, and very near it, this plate will have a fouth pole in its centre, and every part of its circumference will have the virtue of a north pole. If the plate be shaped with points like a star, each of thefe points will be a very diftinct and vigorous north pole. Something like this will be obferved in a piece of iron of any irregular fhape. The part immediately adjoining to the north pole of the magnet will have the virtue of a fouth pole, and all the remote protuberances will be north poles. The notion naturally fuggefted by thefe appearances is, that the virtue of a north pole feems to refide in fomething that is moveable, and that is protruded by the north pole of the magnet toward the remote parts of the iron; and is thus conftipated in all the remote edges, points, and protuberances, much in the fame manner as electricity is obferved to be protruded to the remote parts and protuberances of a conducting body by the prefence of an overcharged body. This notion will greatly afft the imagination; and its confequences very much resemble what we obferve. As a farther mark of the complete communication of every magnetic power by mere vicinity to a magnet, we may here observe, that the wire D, of fig. 7. n° 2. and 3. will fupport another wire, and this another; and fo 37 on, to a number depending on the ftrength of the magnet. The key has therefore become a true magnet in every refpect; for it induces complete magnetifm on the appended wire. That this is not the fame operation of the great magnet (at least not wholly fo), appears by examining the magnetism of D with the needle, which will be feen to be more influenced by D than by A. This fact has been long known. The ancients fpeak of it: They obferve, that a loadstone caufes an iron ring to carry another ring, and that a third; and fo on, till the string of rings appears like a chain. Exception What has now been faid will explain a feeming exexplained. ception to the univerfality of the propofition. If the key be held in the fituation and position reprefented by fig. 10. the bit of wire will not be attracted by it; and we may imagine that it has acquired no magnetifm: But if we bring a mariner's needle, or a bit of wire, near to its remote end B, it will be ftrongly attracted, and fhew B to be a north pole. The needle held near to C will alfo fhew C to be a fouth pole. Alfo, if held near to D, it will fhew D to be a north pole. Now the ends C, both of the key and of the wire, being fouth poles, they cannot attract each other, but, on the contrary, they will repel; and therefore the wire will not adhere to the key. And if the key of fig. 17. n° 4. with the wire hanging to it, be gradually carried out. ward, beyond the north pole of the magnet, and then brought down till its lower end be level with the pole, the wire will drop off. There is, however, one exception to the propofition. If the key in fig. 7. with its appending wire D, be gradually carried from any of the fituations 2, 3, 4, or 5, toward the middle of the magnet, the wire will drop off whenever it arrives very near the middle. If we fuppofe a plane to pass through the magnetic centre A, perpendicular to the axis (which plane is very properly called the magnetic equatorial plane by Gilbert), a flender piece of iron, held anywhere in this plane, acquires no fenfible magnetifm. It gives no indication of any polarity, and it is not attracted by the magnet. It is well known, that the activity of a loaditone or magnet refides chiefly in two parts of it, which have been called its poles; and that thofe are the beft magnets or load ftones in which this activity is leaft diffufed; and that a certain circumference of every load tone or magnet is wholly inactive. When a loaditone or magnet of any fhape is laid among iron filings, it collects them on two parts only of its furface, and between thefe there is a fpace all round, to which no filings attach themselves. We presume that the reader already explains this appearance to himself. Many things thew a contrariety of action of the two poles of a magnet. We have already obferved, that the north pole of a strong magnet will produce a frong northern polarity in the remote end of a small steel bar; and, if it be then applied near to that end in the oppofite direction, it will deftroy this polarity, and produce a fouthern polarity. In what ever these actions may confit, there is fomething not only different but oppofite. They do not blend their effects, as the yellow and blue making rays do in producing green. They oppofe each other, like mechanical preffures or impulfions. We have every mark of mechanical action; we have local motion, though unfeen, except in the gradual progreffion of the magneti. cal faculties along the bar; but we have it diftinctly in the ultimate effect, the approach or recefs of the mag- Adopting this principle, therefore, that the actions tion. 38 There is a very remarkable circumstance which ac- Magnetifm companies the whole of this communication of magne- not impairtifm to a piece of iron. It does not impair the powered by com. of the magnet; but, on the contrary, improves it. This municafact was obferved, and particularly attended to, by Dr" Gilbert. He remarks, that a magnet, in the hands of a judicious philofopher, may be made to impart more magnetifm than it poffeffes to each of ten thousand bars of fteel, and that it will be more vigorous than when the operations began. A magnet (fays he) may be spoiled: by injudicious treatment with other magnets, but never can touch a piece of common iron without being im proved by it. He gives a more direct proof. Let a magnet carry as heavy a lump of iron as poffible by its lower pole. Bring a great lump of iron clofe to its upper pole, and it will now carry more. Let it be loaded with as much as it can carry while the lump of iron touches its upper pole. Remove this lump, and the load will inftantly drop off. But the following experi ment fhews this truth in the moft convincing manner : Let NAS (fig. 11.) be a magnet, not very large, nor of extreme hardness. Let CD be a ftrong iron wire, hanging perpendicularly from a hook by a fhort thread or loop. The magnet, by its action on CD, renders D a north pole and Ca fouth pole, and the polarity of D's magnetifm fits it for being attracted. Let it affume the pofitios Ce, and let this be very carefully marked. Now bring a great bar of iron s Ba near to the other end of the magnet. We fhall inftantly perceive the wire Ce approach to the fouth pole of the magnet, taking a pofition Cf. Withdraw the bar of iron, and Cf will fall back into the pofition Ce. As we bring the iron bar gradually nearer to the magnet, the 39 Therefore the wire will deviate farther from the perpendicular, and when the bar B touches the magnet CD, will start a great way forward. It is alfo farther to be observed, that the larger the bar of iron is, the more will CD deviate from the perpendicular. Now this must be afcribed to the action of the bar on the magnet. For if the magnet be removed, the bar alone will make no fenfible change on the pofition of the wire. We know that the bar of iron becomes magnetical by the vicinity of the magnet. If we doubt this, we need only examine it by means of a piece of iron or a mariner's needle. This will fhew us that s has become a fouth, and ʼn a north pole. Here then are two magnets with their diffimilar poles fronting each other. In conformity with the whole train of magne. tical phenomena, we muft conclude that they attract each other, and muft improve each other's magnetism. This is a most important circumftance in the theory nothing is of magnetism. For it fhews us, that, in rendering a transferred. piece of iron magnetic, there is no material communica tion. There is no indication of the transference of any fubftance refiding in the magnet into the piece of iron; nor is there even any transference of a power or qua. lity. Were this the case, or if the subftance or quality which was in A be now transferred to B, it can no longer be in A; and therefore the phenomena refulting from its prefence and agency muft be diminished. We muft fay that the magnet has excited powers inherent, but dormant, in the iron; or is, at least, the occation of this excitement, by disturbing, in fome adequate manner, the primitive condition of the iron. We muft alfo fay, that the competency of the magnet and of the iron to produce the phenomena, is owing to the fame circumstances in both; becaufe we fee nothing in the phe. nomena which authorifes us to make any diftinction between them. Whatever therefore caufes one magnet to attract another, is alfo the reafon why a piece of iron in the neighbourhood of a magnet attracts another piece of iron; and we rauft fay that the caufe of polarity, or the origin of the directive power, is the fame in both. Now we understand perfectly the directive power of a magnet, as exerted on another magnet. We fee that it arifes from a combination and mechanical compofition of attractions and repulfions. It must be the fame in this magnetifm now inherent in the iron. The piece of iron directs a mariner's needle, as a magnet would direct it; therefore, as there is something in a piece of iron which now attracts fomething in another piece of iron, fo there is fomething in the firft which repels fomething in the last. It may indeed be faid that it is not a piece of iron, Objections anfwered but a mariner's needle, or magnet, that is thus directed by a curious by our iron magnetised by vicinity to a magnet. This objection is completely removed by the most curious of all the facts which occur in this manner of producing magnetifm. Take a piece of common iron, fashion it, and fit it up precisely like a mariner's needle, and care. fully avoid every treatment that can make it magnetical. Set it on its pivot, and bring it near the north pole of a magnet, placing the end, made like the fouth pole of the needle, next to the north pole of the magnet. In fhort, place it by hand exactly as a real mariner's needle would arrange itself. It will retain that pofition. Now carry it round the magnet, along the circumference of a magnetic curve, or in any regular and continuous fact. 40 route. This piece of iron will, in every fituation, affume the very fame pofition or attitude which the real magnetical needle would affume if in the fame place, and it will ofcillate precifely in the fame way. Here then it is plain, that there is no distinction of power between the magnetifm of the iron and of the real needle. To complete the proof: Inftead of approaching the magnet with this iron needle, bring it into the vicinity of a piece of iron, which is itself magnetical only by vicinity to a magnet, it will arrange itfelf just as the real needle would do, with the fole difference, that it does not indicate the kind of polarity exifting in the extremities of the iron, because either end of it will be attracted by them. And this circumftance leads us to the confideration of the only diftinction between the magnetilm of a loadftone or magnet and that of common iron. tra: fitor and deter The magnetism of common iron is momentary, and Maguenfa therefore indifferent; whereas that of a magnet is per, of iron is manent and determinate. When iron becomes magne, and indes tic in the way now mentioned, it remains fo only rent; tat while the magnet remains in its place; and when that is that of removed, the iron exhibits no figns of magnetism, magnets Therefore when the north pole of a magnet has produ- is durable ced a fouth pole in the neareft end of an iron wire, and minate. a north pole at its remote end, if we turn the magnet, and prefent its fouth pole, the neareft end of the wire inftantly becomes a north pole, and the other a fouth pole; and this change may be made as often, and as rapidly, as we pleafe. This is the reafon which made us direct the experimenter on the iron needle to begin his operation, by placing the end marked for a fouth pole next to the north pole of the magnet. It becomes a real fouth pole in an inftant, and acts as fuch during its peregrination round the magnet. But in any one of its fituations, if we turn it half round with the finger, the end which formerly turned away from a pole of the magnet, will now turn as vigorously toward it. Therefore, in carrying the iron needle round the magnet, we directed the progrefs to be made in a continuous line, to avoid all chance of miftaking the polarities. INDUC TION. For all the reafons now adduced, we think ourselves MAGNE obliged to fay, that the magnetism produced on com-TISM OF mon iron by mere juxtapofition to a magnet, is gene. rated without any communication of fubitance or faculty. The power of producing magnetical phenomena is not shared between the magnet and the iron. We fhall call it INDUCED MAGNETISM; MAGNETISM BY INDUCTION. We have faid that induced magnetism of common iron is quite momentary. This must be understood with careful limitations. It is ftrictly true only in the cafe of the finest and pureft foft iron, free of all knots and hard veins, and therefore in its moft metallic ftate. Iron is rarely found in a state so very pure and metallic; and even this iron will acquire permanent and determi nate magnetifm by induction, if it has been twisted or hammered violently, although not in the magnetic di rection; alfo the changes produced (we imagine) on the pureft iron by the action of the atmosphere make it fufceptible of fixed magnetifm. But the magnetism thus inducible on good iron is fcarcely fenfible, and of no duration, unless it has lain in the neighbourhood of a magnet for a very long while. What has now been faid of common iron, is also true of it when in the state of foft fteel. But |