(210), the end A becoming the north, and в the south pole,

if the end s of the magnet were a south, and N a north pole.

A

B

T

If the magnet SN be brought in contact

with the middle of a bar of iron AB, the centre c will become a north pole, and the ends AB both south poles. And if the pole of a magnet be placed in the centre of a circular piece of sheet iron, the whole circumference will assume magnetic properties of the same kind as that of the pole of the magnet, whilst the centre with which it is in contact will assume an opposite polarity.

N

N

sn

S

211*. If a magnetic bar Ns, be broken in half in the centre, the half s will not be found to possess all southern, and N all northern polarity, as might perhaps be expected, but each portion will become a perfect magnet, each of the fractured ends exhibiting a polar state, as perfect as the entire magnet; the fractured lend s,' becoming a south and n a north pole; although at this middle point where s' and n' join, no magnetism could, before breaking it, be detected.

212. From these and similar experiments, a tolerably satisfactory theory of magnetism has been framed, which, if not correct, is certainly very convenient, as affording a key to all the ordinary magnetic phenomena, and may be admitted as at least a conventional hypothesis. According to this, two distinct magnetic fluids exist, one consisting of austral, the other of boreal magnetism, and under the influence of either, in a free state, the bar of iron or other metal, will point to the north or south poles of the earth, according to circumstances. In ordinary iron, these fluids exist in a

CONVENTIONAL HYPOTHESIS.

147

combined state, and therefore are perfectly latent, the metal appearing to be destitute of magnetism. These fluids exist in a certain proportion united to each molecule, or atom of the metal, and from which they can never be disunited; the only change which they are capable of undergoing being their decomposition into the separate fluids, one of which in a permanent magnet, is always collected on one, and the other on the opposite side of each particle or molecule of metal. 213. This theory explains the curious circumstances of a magnet possessing no attractive influence in its centre, and of its magnetism being apparently concentrated in the poles; for

if

nearest A.

AB represent a bar magnet, consisting of two rows of spherical molecules; the austral fluid will all be collected on the sides of the atoms nearest B, and the boreal fluid on those Then the effects of the austral fluid collected on one side of the molecule c, will be completely counteracted by the boreal fluid on the opposite side of D, the austral of this by the boreal of e, and so on, until we come to the last molecule h, whose austral side, having no other atom to oppose its action, will exert the ordinary attractive and repulsive effects of free magnetism. In the same manner the boreal side of c, will exhibit the phenomena of free magnetism; the particles in the second row will also be similarly arranged and exhibit similar phenomena. Thus

we see that the central portions of a bar magnet cannot exhibit evidence of free magnetism, because the magnetic fluid in one particle is held virtually neutralised, or disguised, by that next to it and so on.

An extension of this mode of reasoning will show why a steel ring may be converted into a magnet, by passing it

over the pole of a permanent magnet, without its exerting any attractive influence on iron, or exhibiting any other phenomena characteristic of free magnetism; for here every portion of the ring being continuous, the separated fluid on the side of every atom is held disguised by the free fluid of the opposite kind, on the opposed side of the next atom in the series. On breaking such a ring in half, the terminations of the fractured portions will be found to be energetic magnetic poles, from the portions which disguised their polar state being removed. And thus every fragment of a fractured bar is a perfect magnet, a fact so interesting and extraordinary that the Abbe Hauy has wittily termed magnets les polypes du regne mineral.* A German philosopher Eschenmaier,† has proposed the following formula as exhibiting an hypothetical view of the arrangement of magnetism in a magnetic bar; it certainly points out the absence of polar properties in the centre, end their gradual increase as we approach the extremities of the bar:

Mn

M3 M2 M1 M° M-1 M-2 M-3

M-n

the letter м, with the positive exponents 1, 2, 3, &c. representing one (as the austral) fluid, and with the negative exponents-1-,2,-3, the other or boreal fluid.

The phenomenon of induction admits of a similar explanation; for if A in the above figure be the austral pole, (or that which, if freely suspended, would point to the north,) of a large magnet, placed nearly in contact with a bar of soft iron ch the combined magnetism will be decomposed, its boreal fluid will be attracted to the sides of the atoms of iron nearest A, its austral fluid repelled to the opposite sides, and the bar of iron will become a magnet. If the magnet A be then removed, the separated magnetic fluids recombine, and the bar of iron is left free from magnetic properties; but if the bar be of hard iron or steel, the inductive action (210) of

* Traité de Physique. ii. p. 89.

↑ Gesetze magnetisch. Erscheinung. Tubingen, 1798.

the magnet A, although far less powerful, is considerably more permanent, for the magnetic fluids remain separated. after the removal of the magnet which induced their separation, or decomposition. Indeed, it would appear that the closer texture, and greater density of hard iron, or steel oppose themselves mechanically to the free and ready movement of the imponderable fluids imprisoned in the interspaces existing between their molecules.

214. A magnetic bar properly balanced upon a pivot is generally termed a needle, and constitutes the active agent in the well-known mariner's compass; guiding the sailor when all other indications of his course fail him. This valuable instrument was used in Europe in 1180, according to some poems of Guy of Provence; it is tolerably certain that it was known to the Chinese nearly 1000 years before the Christian era. Marco Paolo was the first European navigator who applied the compass needle to the practical, and important purposes for which it is now constantly used, in his return to Europe from the East Indies in 1295. This important property of a magnetic needle pointing towards the north and south poles of the earth, has been variously accounted for; thus Cardan has supposed that a star lodged in the constellation of Ursa Major attracts the needle, whilst others with more probability have supposed the earth to be, or to contain an enormous magnet, whose poles nearly correspond to the geographical poles of the globe. If this be admitted, we must suppose that a large collection of free boreal fluid is laid up in the northern, and of austral in the southern hemisphere. And in this

case, that the pole of the magnetic needle which points to the north, contains free southern, or austral magnetism; because poles of the same name repel each other (209), and accordingly that pole of the needle which points towards the north has been termed austral, and that towards the southern boreal.

Some philosophers, as Berzelius, have preferred the terms negative and positive fluids, to austral and boreal. It sig

nifies but little which are adopted, provided their conventional meanings are well understood, and as the terms austral and boreal are almost universally used, I have preferred them. It is only necessary to recollect, in reference to a magnetic bar, that boreal, southern, and positive, all refer to that pole which would point towards the south; and austral, northern, and negative, all refer to that which would, if freely suspended, point towards the north.

A

N

215. The magnetic needle does not point exactly north and south; and consequently the magnetic meridian, or place bisecting the earth in the direction of the needle, does not coincide with the geographic meridian. The magnetic meridian is not constant, sometimes being on the east, and sometimes on the west of the geographic meridian; this difference is termed the mag

netic declination, or more commonly magnetic variation. Thus, if AB represent the geographic meridian, Ns will represent the direction assumed by a compass-needle, or magnetic meridian, and the angle NCA is termed the angle of declination, or variation. In certain portions of the earth the magnetic and geographic meridians appear to coincide, as in some parts of North America, the north-eastern point of South America, western part of Australia, &c. These places are connected by an imaginary irregular curved line, termed the line of no variation. This line appears to move progressively over the surface of the globe, it passed over London in 1660, in which year the needle there pointed exactly to the north, and in 1663 it passed over Paris. At London the needle at present points about 24° west of the true north pole, the maximum variation having been attained in 1818, when it amounted to 24° 30'. The following table presents a view of the variations of the magnetic needle in London and Paris during several years: