bars already made magnetic, he may communicate a | bars with their friendly poles so situated as that the stronger power to larger and harder steel bars.

the south pole at one end by a large pin, and place | greater distance than its attractive power; for inthe bars perpendicularly, with that end downwards, stance, if a magnet be freely suspended, another on the middle of one of the parallel bars, the two magnet properly situated within a certain distance of north poles towards the south, and the two south the former will turn it out of its ordinary direction; poles towards its north end; slide them backward and yet the degree of attraction exerted by these magnets forward three or four times the whole length of the against each other is not sensible at that distance, bar, and, removing them from the middle of this, which may be easily tried by fixing one of the magplace them on the middle of the other bar, as before nets to the scale of a balance. The reason of this directed, and go over that in the same manner; then property is, that the directive power depends both turn both the bars the other side upwards, and repeat upon the attraction of the poles of different names the former operation. This being done, the two bars and on the repulsion of those of the same name; that have been thus treated are to change places with whereas the attraction takes place only between poles two of the touching bars, which are to be subjected of different names. In order to render this view of to the same process, and so with the two other touch- the matter more intelligible, we may imagine a maging bars.] These were then changed for two others, netic needle freely suspended, and placed within the and these again for the last two. After treating each influence or sphere of action of a magnet. In this pair of bars in this way a number of times, and disposition, suppose that the north pole of a magnet changing them whenever the manipulations had been attracts the south pole of a magnetic needle with a continued for about a minute, the whole of the bars force equal to ten grains, and, as the attraction bewere at length found to be magnetized to saturation, tween poles of different names is nearly equal to the each pair readily lifting above eight ounces. In ac- repulsion between poles of the same name, it follows complishing this object, I took particular care that that the same north pole of the magnet repels the no magnetic substance was used in the process. All north pole of the magnetic needle with a force equal the bars were freed of magnetism before the experi- to ten grains: but these two forces both concur in ment, so that none of them, not even the largest, altering the direction of the needle; therefore, the produced a deviation of five degrees on the compass endeavour of the magnet to turn the needle's direction at three inches distance. Any bars which had been is equal to twenty grains; whereas the attraction, or strongly magnetized, and had had their magnetism the force by which the needle is drawn towards the destroyed or neutralized (either by hammering, heat-magnet, is only equal to the difference between the ing, or by the simultaneous contact of the two poles two above-mentioned opposite forces, which difference of another magnet placed transversely), I always arises from the pole of the magnet being nearer to found had a much greater facility for receiving po- one than to the other of the poles of the needle. The larity in the same direction as before than the con- same reasoning may be applied to the action between trary. Hence it generally happened that one blow the south pole of the magnet and the suspended with the original north end downwards produced as needle.* much effect as two or three blows did with the original south end downward." The correspondence between magnetism and electricity, in many of their phenomena, has, as we have already stated, led philosophers to refer both to a common principle.

The power of a magnet, and of iron or steel impregnated with the magnetic virtue, may be impaired by long lying in a wrong position, with regard to the earth or with respect to each other. Thus, if two magnets be placed so that their contrary poles may be contiguous, they will preserve one another's power; but if the north pole of one be placed near the north pole of the other, and the south near the south, then they will entirely destroy, or diminish each other's magnetism; and, if their original powers were very unequal, the polarity of the weaker magnet will be changed by the action of the stronger one.

The most remarkable phenomenon of the magnet, in relation to the earth, is the variation of the magnetic meridian in most parts of the globe, upon which depends the declination of the needle. Accurate observation of this phenomenon has ascertained the following facts:-There are certain points on the earth where no declination exists. The lines formed by their series, however, do not coincide with the geographical meridians; but, on the contrary, deviate from them very irregularly. According to the most recent observations, there exists a line without declination in the Atlantic Ocean, between the old and the new world. It intersects the meridian of Paris at a southern latitude of about 65°; thence it mounts to the north-west, to about 35° west longitude from this meridian, or 32° 39′ 37′′ from Greenwich, as high as the latitude of the coast of Paraguay; after which, In general, the same means which facilitate the becoming again almost north and south, it skirts the communication of magnetism, when pieces of iron, coast of Brazil, and proceeds to the latitude of &c., are properly situated with respect to the poles of Cayenne. Then, turning suddenly to the north-west, the earth, or of other magnets, will likewise facilitate it takes the direction of the United States of America, the loss of magnetism when the magnets are im- and thence proceeds to the northern parts of that properly situated; thus a red heat destroys in a great continent, which it traverses in the same direction. measure, or entirely, the power of a magnet. A steel The position of this line on the globe is not immutabar, strongly magnetic, will have its power much ble; at least for a century and a half it has been diminished by being repeatedly struck between two tending considerably from the east to the west. It stones, especially if it be struck standing in a direc-passed London in 1657, and Paris in 1664. Thus, in tion perpendicular to the magnetic meridian. A bar its present direction, it has traversed in the latitude of hard iron, which has acquired some degree of per- of these places nearly 80° of longitude in 150 years. manent magnetism, by being made red-hot and then But there is no doubt that this change is not uniform. cooled in the direction of the magnetical line, will It is even very unequal in different parallels. In the have that power destroyed, or much diminished, by

a few blows on its middle.

See "Manual of Natural Philosophy," by the Editor of

The directive power of a magnet is extended to a this work.

West Indies, for example, the declination of the needle | magnetic meridian, and is displaced by foreign power, has hardly varied for 140 years. In general, the slow-it returns, when the power ceases to act, to its former ness of this movement leaves it uncertain whether it situation by a series of oscillations. The time of an is constantly progressive or whether it must con- oscillation, in the case of the same needle, has a certinue in any particular direction. The very accurate tain relation to the magnetic power of the earth, and observations habitually made in several observatories serves as a measure of it, in a similar way as the osof England and France have appeared to indicate, cillations of the pendulum serve for the measurement for some years, a commencing retrogradation towards of the degrees of gravity. Humboldt found that a the east; but, even in the years 1790 and 1791, a needle which, in Paris, made 245 oscillations in ten similar retrogradation had been observed, which did minutes, made, in Peru, but 211 in the same time, not, however, continue. which would give the proportion of the magnetic power of the earth at Paris to that in Peru nearly as 135: 100. On the other hand, according to GayLussac, an elevation of 3532 toises (about 22,600 feet) over the level of the sea (in a balloon) showed no influence upon the magnetic power. The number of the oscillations, and of course the intensity of the magnetic power of the earth, always diminish in approaching the magnetic equator and increase in approaching the magnetic pole.

Another remarkable and evident manifestation of the influence of the magnetism of the earth upon the needle is the inclination or dip of the latter; i. e. a deviation from the horizontal plane, in northern regions of the north pole of the magnet, in the southern regions of the south pole of the magnet, and which, in the region of the magnetic equator, is 0, but increases towards the poles. This phenomenon, also, of the earth cuts the terrestrial equator, and winds through it in a serpentine line, in which it reaches twice on each side its maximum of distance from the earth's equator, which is nowhere more than 40° 10′. The inclination, in the northern hemisphere of the earth, is the strongest between 70° and 80° latitude. Under 74° 47', where Parry remained during the winter, the inclination amounted to 88° 43′ 45′′. The cause of all these phenomena is as yet unexplained. That there are great magnets in the earth, which move periodically, or (according to Professor Steinhauser) that an interior planet (Minerva) revolves round the centre of the earth once in 440 years, and thus produces the magnetic phenomena on the surface, or that (as Sander supposes) these are to be ascribed to a magnetic planet on the other side of Herschel, completing a revolution only once in 1720 years, may be matter of interesting speculation, but can hardly be looked on as any thing more.

The very exact measures of the inclinations or dip of the needle made at different periods, by Gilpin and Cavendish, in London, have proved that this element is also variable, though much less so than the declination. The inclination was, at London, in 1775, 72° 30′; in 1805, 70° 21'. This result has been confirmed in France, by the observations of Humboldt. It has been also proved, in a still more striking manner, by the successive measures of the inclination made by different navigators, between 1751 and 1792, at the Cape of Good Hope, which indicate, during this time, a progressive increase of inclination amounting to 5o. There is another line without declination, almost opposite to the preceding, which beginning in the great Southern Ocean, and running constantly in a north-western direction, cuts the western point of New Holland, traverses the Indian Ocean, enters the continent of Asia at Cape Comorin, and thence, pass-is subject to differences, because the magnetic equator ing through Persia and Western Siberia, ascends to Lapland. This line, however, divides near the great archipelago of Asia, and gives rise to another branch, which, running almost directly north and south, passes this archipelago, crosses China, and runs into the eastern part of Siberia. The two branches which intersect this line either experience no change of place or move with much slowness. The declination of the needle does not appear to have varied sensibly for 140 years at New Holland. Indications of a fourth line without declination were observed by Cook in the South Sea, towards the point of greatest inflexion of the magnetic equator. On the other hand, the points where the greatest declination of the needle has been observed are in high latitudes north and south. The greatest observed by Cook in the southern hemisphere was at 60° 40′ of latitude, and 91° 24′ 37′′ west from Greenwich. In the northern hemisphere, where the magnetic pole has been much more nearly approached, much greater declinations have been observed, amounting, in fact, to nearly 90° west. If the magnetic pole had been crossed, the north pole of the needle would have been turned to the south, and, directly over the pole, its direction would have been vertical, and of course it would have had no horizontal direction. It appears, therefore, that the horizontal direction will be very weak when the dip or inclination is great; so that a very slight extraneous influence, such as the iron on ship-board, may render the compass useless.

Besides these variations, others occur daily, and others according to the seasons. From eight o'clock A. M., the declination increases until about three o'clock; then it decreases until eight o'clock P. M., and remains unaltered until eight A. M. The amount of these daily deviations is the greatest from April to July, when it is from 13' to 16'; in the other months, it is from 8' to 10'. The direction of the needle is said to be affected by approaching earthquakes, or eruptions of volcanoes. If a needle stands in the

It had long been doubted whether any real analogy could be traced between light and magnetism; that problem is now, however, solved by Dr. Faraday's experiments. The only apparatus which serves to show on a large scale the identity which exists between light and magnetism is now placed in the National Gallery of Practical Science. It is delineated in figs. 3 and 4, Plate II., MAGNETISM, and the same letters refer to both figures. This instrument is of the horse-shoe form, and is composed of twelve sheer-steel plates, aa, each twenty-eight inches in length from the poles to the centre edge. At the greatest width of the curve the horse-shoe is seven inches across, and the extremities of the poles, b, c, are an inch and a half asunder. The keeper, or lifter, d, which is made of the purest soft iron, is five inches in length, two inches and a half wide, and one inch thick. Around the middle of the keeper, and occupying with its lower section the space between the poles, is a wooden winder, having about 100 yards of common threaded bonnet-wire, from which the

two ends, e, f, composed of four lengths of the wire | imparted to a professor might afterwards be comtwisted together, are carried out. One of the twisted municated to others without the agency of ferruginous ends passes beyond each end of the keeper, and rests bodies. So much of interest has lately been excited upon the respective poles of the magnet b, c. A short by the attempts to revive the long-exploded doctrines lever, g, placed in a frame, is so attached to the of animal magnetism that it may be advisable to winder and keeper as to admit of their being forced present our readers with a brief outline of its history, up by the hand applied to the longer end of the lever. and the phenomena said to be elicited. The principal Every time this is effected a brilliant coruscation of means used to produce the effects of animal magnetlight is produced at the end of the wire. The sparks ism are such as touching and stroking with the hands, thus elicited will ignite inflammable bodies, and the breathing on a person, fixing the eyes upon him, &c.; whole experiment proves to a demonstration the in- the magnetized person must always be of a weaker timate analogy which exists between light, magnet-constitution, and, if possible, of a different sex, from ism, and heat. the magnetizer; and it is indispensable that he should Under the article ELECTRO-MAGNETISM our readers be of a disposition to believe without doubting. The will find a series of experiments intended to illustrate phenomena themselves consist partly in bodily sensa the phenomena which result from combining voltaic tions (for instance, chilliness, heaviness, flying pains, electricity with the magnetic powers. We now re-oppressions, &c.), partly in a diminished activity of vert to the subject to point out a mode of preparing the external senses, partly in fainting, convulsions, a horse-shoe magnet of prodigious powers from a sleep, with lively dreams, in which the magnetized piece of common soft iron. The apparatus we are person is transported to higher spheres, observes the about to describe is the only one yet constructed on internal organization of his own body, prophesies, so large a scale, combining a perfect scientific ar- gives medical prescriptions, receives inspired views of rangement with enormous magnetic power. It is heaven, hell, purgatory, &c., reads sealed letters laid represented at figs. 1 and 2, Plate II., MAGNETISM, on his stomach, and, when awakened, is totally unand consists of a bar of soft iron bent into the form conscious of what he has experienced. At the same of a horse-shoe, and weighing about thirty-four time, the soul becomes so elevated and refined that pounds it is surrounded with systematic reduplica- the magnetized individual has an instinctive perceptions of copper wire. There are ten series of the tion of the presence of the impure, and falls into fits wire, each containing ninety feet. The commencing at the approach of disbelievers in animal magnetism, extremities are all soldered to a thick wire, b, while and of all who investigate it by the rules of ordinary those terminating are likewise affixed to another reason. Hence it is necessary to keep sceptics at a wire, c. This arrangement affords the means of trans-distance, when it is desired to witness the highest mitting an electric current through the whole series phenomena. of wires at the same time with great facility. The The magnetized person shows a remarkable convoltaic battery, d, employed to make the magnet, is nection with, and dependence on, the magnetizer, very small, and composed of a double concentric tasting what he eats, smelling what he holds before cylinder of copper, and a movable cylinder of zinc, e. his nose, and no one else can bring him back from By the aid of this apparatus the horse-shoc of the magnetic state. In the sequel we shall give a soft iron may in an instant be converted into a mag-brief exposition of the phenomena, as stated by Kluge, net capable of supporting a weight of 500 lbs. attached to the scale f, beneath. The whole is supported by a strong triangular stand, g.

who appears, in his Attempt at an Exhibition of Animal Magnetism, to have given the fullest account of them. A scientific investigation of the influence which we are considering is hardly consistent with the views entertained of it by its adherents; for they maintain that mere reason cannot approach nor conceive this great mystery; it can be rightly apprehended only by a believer. The whole of the effects of animal magnetism seem to be ascribable to a heated imagination, to an excitement, half spiritual, half sensual, and to a morbid sensitiveness. The history of animal magnet.

The compass furnishes a most important application of magnetism to practical science. A common compass, in its early form, is delineated at fig. 4, Plate I., MAGNETISM. It consisted of a cylindrical box, b, supported by two centres or pivots resting in the uprights h, h. In the middle of the box was placed the compass card, turning freely on a centre: motion was communicated to the card, shown also at fig. 3, by the bar magnet, klm. Now this ar-ism may be thus briefly detailed:-Anthony Mesmer, rangement admitted of motion but in one direction; but in the form delineated at fig. 2 a double motion is produced, and, whatever be the direction of the supporting box, the compass needle must of necessity hang flat.

in 1772, attempted cures with the mineral magnet, and excited some sensation in Vienna, but at length declared that not the magnet, but a mysterious power in his own person caused the effects ascribed to the magnet, and that this power was related not only to the magnetic power, but to the attraction dispersed throughout the universe. But, a fraud which he attempted (the pretended restoration of sight to a girl) having been discovered, he proceeded, in 1778, to Paris. The great supporters of animal magnetism have recently been Kieser in Jena, and Wolfart in Berlin; the former explains the phenomena by the striking difference between life by day and life by night, both in MAGNETISM, ANIMAL. This fanciful science ap- the case of animals and vegetables; the latter adopts pears to have originated with Mesmer and other the mystical jargon of Mesmer. In 1820, the PrusGerman illuminés, who believed that the power of asian government caused a prize to be offered for the common magnet might be made to act on the human | best treatise on this subject, but it was subsequently frame. They also believed that this power once withdrawn.

In the azimuth compass sights are added, shown at F, G, by which a star or any other distant body may be observed. The whole apparatus turns on the foot E, to which are attached the uprights C, D. B represents the ring which, by means of two screws, supports the central box. The compass card is shown at H. The azimuth compass will also answer for a steering compass.