It appears, from what Lucretius says in the passage above quoted, that the Greeks and Romans were aware, not only that the loadstone, or magnetic iron ore, attracted iron, but also that it endued iron in contact with it with its own peculiar property. Thus an iron ring will hang suspended by the attraction of a loadstone, and from that ring another, and so on, up to a certain number, depending on the power of the stone and the weight, &c., of the rings. They were also aware that the attraction was confined to iron, or at all events was not indiscriminate, and that it was not destroyed by the intervention of other bodies, such as brass, between the magnet and the iron. It appears, too, from the passage—

Fit quoque ut a lapide hoc ferri natura recedat

Interdum, fugere atque sequi consueta vicissim, &c.— that they had an idea that, under certain circumstances, the attraction might be replaced by a repulsion. If, however, we understand aright the latter part of Lucretius’s somewhat obscure description of what seems to have been an actual experiment of his own, this notion was in reality a hasty generalization, not justified by the observed facts. In any case there seems no warrant for assuming, as some have done, that the ancients had any definite conception of magnetic polarity.

What they wanted in definite experimental knowledge they supplied by an abundant use of the imagination.

We are told, for instance, that the magnet attracts wood and flesh, which was certainly beyond their powers of observation; that it is effective in the cure of disease; that it affects the brain, causing melancholy; that it acts as a love philtre; that it may be used in testing the chastity of a woman; that it loses its power when rubbed with garlic, but recovers it when treated with goat’s blood; that it will not attract iron in the presence of a diamond, and much else that was eagerly copied by the wonder-loving writers of the Middle Ages.

The science of magnetism made no real progress till the invention of the mariner’s compass. The early history of this instrument is very obscure. According to some authorities it was invented in China, and found its way into Europe probably through Arabian sources. The light thrown by recent researches on the literature of the Chinese has apparently thrown doubt upon their claim to this invention, although the knowledge of the loadstone and its attractive property may have been older among them than even among the Greeks. The first accounts of the compass in Europe go back to the 12th century, and, although the instrument described is very rough, it is not spoken of as a new invention. In its earliest form it seems to have consisted simply of an iron needle which was touched with the loadstone and placed upon a pivot, or floated on water, so that it could turn more or less freely. It was found that such a needle came to rest in a position pointing approximately north and south (some accounts say east and west, in which case there must have been a cross piece on the needle to indicate what was probably the important direction for the mariner). As these compasses were made of iron (steel was not used till much later), and were probably ill-pivoted, they must have been very inaccurate; and the difficulty of using them must have been much increased by the want of a card, which was a later addition made apparently by the Dutch.

It is unnecessary to enter into more detail here respecting the early history of the compass, as the matter has been very fully treated in the article Compass. We proceed therefore to show the bearing of the invention upon the science of magnetism. It will at once be seen that it involves two scientific discoveries of capital importance :— first, that the loadstone can transmit to iron with which it comes in contact a permanent property like its own; and, secondly, that a loadstone or magnet if suspended freely will turn so that a certain direction in it assumes a fixed position relative to the geographical meridian, a certain part of the magnet turning always towards the north, and the part opposite towards the south. These opposite parts of the magnet are called its “poles.”

To fix our ideas we shall describe a process by which we might definitely determine this direction in the magnet. Following· the example of Gilbert, let us consider a spherical magnet. Our reason for dealing with this form in the first instance is to make it perfectly clear that the phenomena depend essentially on something apart from the form of the body. We shall suppose that the magnet is homogeneous as to its mass, so that its centre of gravity coincides with its centre of figure. Suspend this spherical magnet by a fine thread of untwisted silk, attached to any point of its surface, say P. After the magnet has come to rest, mark the vertical plane through the centre which falls in the geographical meridian; this may be done by tracing a great circle on the surface of the magnet. Next find the point P' in which the vertical through P cuts the surface again, and suspend the magnet by P', again marking the plane which falls in the meridian. Now, find the plane which bisects the acute angle between the two former planes, mark it by a great circle, and call it the axial plane of P. If we thus find the axial planes of any number of points, we shall find that they all intersect in one common line passing through the centre of the sphere. We may call this line the “axis” of the magnet. Let us mark the points where it cuts the surface; we may call these the “poles” of the magnet. We shall then observe that, however we suspend it, the magnet will always come to rest so that the vertical plane through the axis makes a definite angle with the meridian. This angle (δ) is called the “declination” (also, by sailors, the “variation”.); it varies from place to place, and from time to time, but very slowly, so that throughout a limited area of the earth’s surface, and for a limited time, it may be regarded as constant.

One end of the axis always turns northwards, and the other always southwards; we shall call the former the “north” and the latter the “south pole,” although, for reasons to be afterwards explained, it would be more appropriate to invert the order of these names. Henceforth the vertical plane in which the axis of the magnet comes to rest will be called the magnetic meridian, and the two horizontal directions in this plane magnetic north and magnetic south respectively.

It must be carefully noticed that there is a certain amount of arbitrariness in our definition of the axis and poles of a magnet. In reality it is only the direction of the axis that is fixed in the body, and not its absolute position. This will be made plain if we repeat all our experiments with the spherical magnet after fastening to it a piece of wax or other non-magnetic body, so as to leave its magnetic properties unchanged, but to throw its centre of gravity out of the centre of figure. Everything will fall out as before, only the axial planes of the different points of suspension will now meet in a line, parallel, it is true, to the axis determined before, but passing through the new centre of gravity. In point of fact, therefore, we might choose any point in the body, draw a line through it iu the proper direction, and call this the axis. Hereafter we shall, unless the contrary is stated, draw the axis through the centre of gravity of the body, or through its centre of figure if it has one; and we define the poles, for the present, as the points in which the axis cuts the surface of the magnet, supposing, as will be generally the case, that the line cuts the surface iu two points and no more.

Having now obtained a definite idea of the axis of a magnet, and seen that it has, in the first instance at least, nothing to do with the external form of the body, let us proceed to make an artificial magnet of the particular kind usually called a “magnetic needle,” and briefly examine its properties. Take a tolerably thin flat piece of pretty hard-tempered steel, of the elongated symmetrical form NS shown in fig. 1. We suppose it, in the first place, in an unmagnetic condition. Let it be pierced by a well-turned axis ab, passing accurately through its centre of gravity, and perpendicular to its plane, so that, when the axis is placed on two horizontal knife edges, the needle will rest in any position indifferently. Further, let four very small hooks, c, d, e,f, be attached, two (c, d) to the ends of the axis, and other two (e,∕) to the edges of the needle in a line perpendicular to NS. Now rub the half of the needle towards N with the south pole of the spherical magnet whose properties we have just discussed, beginning the stroke at the middle and ending it at the point of the needle, and for symmetry’s sake let us do the same to the other side of the needle, and then repeat this process with the north pole of the sphere on the other half towards S. Let us examine the properties of the needle thus “magnetized.” If we suspend it first by the hook c and then by the hook d, we shall find that in both cases the line joining NS makes very nearly the same angle with the geographical meridian. Hence the magnetic axis must lie in a plane through NS perpendicular to the plane of the needle. A similar experiment with the two hooks e, f will show that the magnetic axis lies approximately in the plane of the strip, which we may suppose for the present to be infinitely thin. Hence the magnetic axis may be taken to be coincident with the line NS joining the points of the needle. This coincidence is, however, in general only approximate, and in delicate measurements corrections have to be made on that account, of which more hereafter. If we now mount our magnetized needle on a piece of cork or two straws, and float it in a basin of water, or replace its axle by a small cap and set it on a pivot, we have the mariner’s compass in its early form. We shall call it a magnetic needle, to distinguish it from the more elaborate compass of the present day. A favourite way of showing the directive property of a magnet, described by Gilbert, is to magnetize a sewingneedle, and lay it very gently, by means of a fork of wire, on the surface of water; it will float and turn until it takes up its position in the magnetic meridian.

A needle mounted in this way, so as to have great freedom to move in a horizontal plane, is of great use in magnetic experiments. Gilbert calls it a “versorium.” When very delicate applications are in view, the point of the pivot on which it is mounted must be very hard (say of hard tempered steel or iridium), and the cap should be fitted with an agate or other hard stone having a polished cavity of the form of a blunted cone to receive the pivot. A still better arrangement, also used by Gilbert, is to suspend a short and very light piece of steel wire—a fine sewing needle may be used— by means of a single fibre of silk. The most delicate arrangement of all is to use one of Sir W. Thomson’s light galvanometer mirrors with the magnets attached, and follow its movements by means of the lamp and scale as usual. See Galvanometer.

Such, with as much of modem accuracy imported into them as was necessary for clearness of exposition, were the facts of magnetism as known up to the beginning of the 16th century.

Another experiment with our magnetized needle will enable us to describe the next important magnetic dis covery. In its unmagnetized condition the needle rested indifferently in any position when its axis was placed on two horizontal knife edges. In the magnetized state this is no longer the case. The axis of the needle now takes up a fixed position, with its north end pointing downwards (fig. 2), and if disturbed will oscillate about that position, and finally settle into it again. The angle which the axis NS makes with the horizon is least when the plane of rotation of the needle is in the magnetic meridian: the angle (t) in this case is called the “dip,” or (by Continental writers) the “inclination.” It is greatest, viz., 90°, when the plane of rotation of the needle is vertical and perpendicular to the magnetic meridian. At Greenwich the dip is about 67° 30' at the present time. If we place the needle with its plane of rotation perpendicular to the line of dip, the equilibrium will be indifferent, as it was in all positions before magnetization; but there is no other position of the magnetized needle for which this is true.

The remarks which we made as to variation in space and time of the declination apply also to the dip. The variation from place to place differs, however, in nature from that of the declination. Along a line running in the neighbourhood of the geographical equator, partly north and partly south of it, the dip is zero. North of this line, which is called the magnetic equator, the north end of the needle dips below the horizon; and the angle of dip increases as we go northwards, until, at a point in the Hudson’s Bay Territory, the needle dips with its north pole vertically downwards. South of the magnetic equator the south end dips below the horizon; and there is again a point in the southern hemisphere where the south end dips vertically downwards. These points are called the “magnetic poles” of the earth. For further details on this subject we refer the reader to the discussion of terrestrial magnetism in the article Meteorology.

It was in the accurate observation of the declination and dip of the magnetic needle that the science of magnetism arose. The dip appears to have been first observed by Georg Hartmann, vicar of the church of St Sebaldus at Nuremberg (1489-1564), who seems to have been in advance of his age in magnetical matters. In a letter to Duke Albrecht of Prussia, dated 4th March 1544, he writes :—

“Besides, I find this also in the magnet, that it not only turns from the north and deflects to the east about 9° more or less, as I have reported, but it points downwards. This may be proved as follows. I make a needle, a finger long, which stands horizontally on a pointed pivot, so that it nowhere inclines towards the earth, but stands horizontal on both sides. But as soon as I stroke one of the ends (with the loadstone), it matters not which end it be, then the needle no longer stands horizontal, but points downwards (fällt unter sich) sonie 9° more or less. The reason why this happens was I not able to indicate to his Royal Majesty.”

From this it will be seen that Hartmann had unquestionably observed the tendency of the magnetized needle to dip. His method of observing is of course unsuited for measurement, and it is not surprising that he got a result of 9° instead of somewhere about 70°.

In 1576 the dip was independently discovered by Robert Norman, a skilful seaman and an ingenious artificer, according to Gilbert. He was in the habit of making compass needles, and carefully balancing them so as to play Horizontally on their pivots before magnetization. He found that, after they were magnetized, they constantly dipped with the north end downwards, so that a counterpoise had to be added to bring them back to the horizon. This led him to construct a special instrument, the prototype of the modern dipping needle, to show this new phenomenon. With this instrument he made the first accurate measurement of the dip, and found it to be 71° 50' at London.

The early English magnetic observers, of whom Norman and Burroughs (who wrote an able supplement to Norman’s work) were admirable examples, must have done much for the introduction of precise ideas into magnetism. But their fame was speedily eclipsed by William Gilbert of Colchester (1540-1603), whom Poggendorff has justly called the Galileo of magnetism, and whom Galileo himself thought enviably great. In his great work entitled De Magnete Magneticisque Corporibus et de Magno Magnete Tellure Physiologia Nova, first published in 1600, we find a complete account of what was known of magnetic phenomena up to his time, with a large number of new ideas and new experimental facts added by himself. We find in Gilbert’s work, in a more or less accurate form, nearly all that we shall lay before the reader in the first section of this article, described very much in the language that we shall use. “How far he was ahead of his time is best proved by the works of those who wrote on magnetism during the first few decades after his death. They contributed in reality nothing to the extension of this branch of physical science.” Mutual Action of Like and Unlike Poles.— If we take a magnet whose poles N', S' have been determined and marked as above explained, and bring its north pole N' near the north pole N of a magnetic needle, N will move in a direction indicating repulsion between N and N'. The same result will follow if the south pole S' of the magnet be brought near the south pole S of the needle. But if S' be brought near N, or N' near S, attraction will be indicated. Hence the following fundamental law of the action between two magnets :— Like poles repel each other; unlike poles attract each other. It would appear, therefore, that the whole action of one magnet upon another is of a somewhat complicated character, even if we take the simplest view of it that the experimental facts will allow, viz., that the action may be represented by forces acting between the two pairs of points in each magnet which we have defined as north and south poles. On this assumption, the action of N'S' upon NS would consist of the four forces represented in fig. 3, for all these must exist in accordance with the law just established. Whether this is a sufficient representation of the most general case, and what the exact law of the forces ought to be, we are not yet in a position to decide. One thing, however, is clear, that the action between two poles must diminish when the distance between them increases; otherwise we should not have been able to make the action of N or S upon N' prevail, by bringing the...