KLM, is kept steady, the compass may be turned round, in order to place the sights, F, G, in the direction of the sun.

There are, on the inside of the box, two lines drawn perpendicularly along the sides of the box, just from the points where the thread III touches the edge of the box. These lines serve to show how many degrees the north or south pole of the needle is distant from the azimuth of the sun; for which purpose the middle of the apertures of the sights F, G, the thread HI, and the said lines, must be exactly in the same vertical plane. The use of the thread III, which is often omitted in instruments of this sort, is likewise to show the degrees between the magnetic meridian and the azimuth, when the eye of the observer stands perpendicularly over it. On the side of the box of this sort of compasses there is generally a nut or stop, which, when pushed in, bears against the card and stops it, in order that the divisions of the card, which coincide with the lines in the box, may be more commodiously

read off.

If the magnetic poles always agreed with the astronomical poles of the globe, the compassneedle must of necessity point due north and south in every part of the earth. This, however, is not the fact, as the needle is found to vary considerably, not only after the lapse of ages, but also at stated periods, within a few hours of its maximun in either direction. In other words, the magnetic meridian, and the real meridian, seldom coincide. The angle which they make is called the angle of declination, and this is said to be east or west, according as the north pole of the needle is eastward or westward of the true meridian of the place. From the minuteness of the daily variation, and the extreme difficulty of measuring it, excepting with the nicest instruments, its laws, and consequently its cause, are still undiscovered.

The following are a series of axioms respecting the variation of the magnetic needle collected by L. Cotte; to which he adds others respecting the northern lights; as being concernel in the movements of the needle.

1. The greatest variation of the needle from the north towards the west takes place about two in the afternoon, and the greatest approximation of it towards the north about eight in the morning; so that, from the last mentioned hour till about two in the afternoon, it endeavours to remove from the north, and, between two in the afternoon and the next morning, to approach it. 2. The annual progress of the magnetic needle is as follows:-between January and March it removes from the north; between March and May it approaches it; in June it is stationary; in July it removes from it; in August, September, and October, it approaches it; its declination in October is the same as in May; in November and December it removes from the north; its greatest western declination is at the vernal equinox, and its greatest approximation to the north at the autumnal equinox.

3. The declination of the magnetic needle is different, according to the latitude: among us it has always increased since 1657; before that period it was easterly.

4. Before volcanic eruptions and earthquakes, the magnetic needle is often subject to very extraordinary movements.

5. The magnetic needle is agitated before and after the appearance of the northern lights: its declination on those occasions is about noor. greater than usual.

6. The greater or less appearance of these northern lights is variable; some years this phenomenon is very frequent, in others uncommon. 7. The northern lights are more frequen about the time of the equinoxes than at other periods of the year.

8. This phenomenon is almost constant during the long winter in the polar regions, and is the more uncommon the nearer the equator.

9. Southern lights have been observed also in the regions near the south pole.

10. The northern lights are often accompanied with lightning, and a noise like that of electricity; while the lightning proceeds partly from the middle of the northern lights, and partly from the neighbouring clouds.

It happens that the declination is sometimes subject to interruptions, so that the needle manifestly remains stationary for a while. For example, at Paris the needle experienced no alteration from the year 1720 to 1724. during which it was constantly at 13° from the meridian.

Observation proves, also, that the variations of the declination compared with one another, in different points of the globe, follow different relations. But there is a fact extremely worthy of attention, that has been remarked by the celebratel Halle on the mere inspection of the table of declination published by Van Swinden, whose notice it had escaped. In the table, three places are pointed out where the needle has experiencel the greatest declination: and these are, first, in the middle of the Indian Ocean, from 10° to 15° of south latitude, and from 82° 87° of east longitude (reckoning from the island of Ferroe), where the variation, from the year 1700 to that of 1756, was from 11° to 11° 15′. Secondly, in the Ethiopian Ocean, from 5° of north to 20 or 25° of south Latitude, and in the interval of 10°, 15°, and 20°, of east longitude; the variation relative to this space, during the same period of time, was from 19° to 10° 45', principally under the line, and to 5° southward. Thirdly, at 50° north latitude, and between 17° of east and 10° of west longitude; where, again, in the same period, there was a variation of from 11° to 11° 45′.

Looking at Van Swinden's table, Hallé perceived that these three places formed, as it were, three centres, round which the numbers indicating the quantities of variation insensibly decreased in proportion as we departed from each centre; so that we have here a new order of observations, answering to the places where the variation was least in the same course of years.

These places are, first, the whole American Ocean, without including the gulf of Mexico, that is to say, from the eastern point of Africa to the farthest of the Bermudas Islands. And here also we must remark, that, in the ocean between Africa and North America, the variation is much less towards the American than towards the

African coasts. Secondly, the environs of the isle of Madagascar, and part of the coast of Zanguebar. Thirdly, that part of the ocean which is to the south and south-east of the Sunda Islands between those islands and New Holland. And, lastly, in the same sea, about the fourth degree of south latitude, and the ninety-seventh of east longitude, that is, in the middle of the space comprised between the western angle of New Holland and the southern point of Africa. In all these different places the declination of the needle has not varied, during the whole fifty-six years, as much as one degree.

If similar observations had been made in the Pacific Ocean, in the northern and southern oceans, and even the principal divisions of large seas, such as the Baltic, the Mediterranean, the Gulf of Mexico, &c., these places would probably have furnished similar points: and how greatly would the study of natural magnetism be facilitated, by referring to a series of such facts in subordination to a number of centres round which they would range themselves according to the order of their respective relations.

A memoir was presented to the French National Institute, by Humboldt and Biot,' On the variations of the terrestrial magnetism in different latitudes. These philosophers, having first determined the position of the magnetic equator by direct observations, proved that the magnetic force must increase in proceeding from that equator to the poles; and the same distinguished foreigners have given a mathematical hypothesis, which, when reduced to a formula, accords with all the inclinations of the needle hitherto observed.

As to the position of the magnetic equator, supposing it to be a great circle of the terrestrial sphere, an hypothesis which is conformable to observations: the inclination of this plane to the astronomical equator is equal to 12-2025° of the decimal division (10° 58′56′′ of the common division), and its occidental node on that equator is at 133-3719° (120° 2′ 5′′) longitude west from Paris, that is, a little beyond the continent of America, near the Gallipagos, in the South Sea; the other node is at 66-6281° (59° 57′ 55′′) eastward of Paris, that is to say, in the Indian Seas. The points where the axis of the magnetic equator pierces the earth's surface, are, the northern point at 87-7975° (79° 1′ 4′′) of north latitude, and at 33-3719° (30° 2′ 5′′) of longitude west from Paris; the southern point is situated in the same latitude south, and 166-6281 (149° 67′ 55′′) of longitude east from Paris. It is remarkable that this determination of the magnetic equator agrees almost perfectly with that given half a century back, by Wilke and Lemonnier.

With respect to the intensity of the magnetic force, in different parts of the earth, these philosophers have ascertained that it varies in different latitudes; its increase proceeding from the equator towards the poles. The needles of Humboldt's compass, which, at his departure, gave at Paris 245 oscillations in ten minutes, gave no more in Peru than 211, and it constantly varied in the same direction; that is to say, the number of the escillations always decreased by approach

ing the magnetic equator, and always increased by advancing towards the north. The differences can neither be ascribed to a diminution of magnetic force in the compass, nor to the effects of heat or of time; for, after three years' residence in the warmest countries of the earth, the same compass gave again in Mexico oscillations as rapid as at Paris.

There are some anomalies, however, occasioned by local causes. Thus Biot, having in the summer of 1804 carried to the Alps the magnetic needle employed in one of his previous aerial excursions, found that its tendency to return to the magnetic meridian was constantly stronger in these mountains than it was at Paris before his departure, and than it had been since his return. This needle, which made at Paris 83.9 oscillations in ten minutes of time, gave oscillations as below, at the places mentioned, in the same interval of ten minutes: viz. Paris, before departure, 83-9; Turin 87-2; on Mount Genevre 88.2; Grenoble 87·4; Lyons 873; Geneva 86.5; Dijon 84.5; Paris, after his return, 83.9. It appears to result from these observations, that the action of the Alps has a perceptible influence on the intensity of the magnetic force. Humboldt observed analogous effects at the bottom of the Pyrenees, for instance, at Perpignan. It is not improbable that they arose from the mass of these mountains, or the ferruginous matters contained in them; but, whatever may be the cause, it is hence manifest that the general action of terrestrial magnetism is sensibly modified by local circumstances, the differences of which may be perceived in places very little distant from each other.

The important discovery of the inclination or dip of the magnetic needle was made about the year 1576, by Robert Norman, a compass maker, residing in Wapping, who in 1580 published a pamphlet entitled, The Newe Attractive, showing the Nature, Propertie, and manifold Vertues of the Loadstone; with the Declination of the Needle, touched therewith, under the Plaine of the Horizon. Found out and discovered by Robert Norman.' The sole credit of the discovery was awarded to him by Dr. Gilbert and Mr. William Burrows, contemporary writers on the subject of magnetism. The Newe Attractive was dedicated to Mr. Burrows, and Dr. Gilbert speaks of the author in the following terms: This is that Robert Norman, that skilful seaman, and ingenious artificer, who first found out the inclination of the magnetic needle.' Mr. Bond also assures us, about a century after the invention, That he himself saw an inclinatory needle of one Dr. Merrets, which was made by this Robert Norman, A. D. 1578, and which he fitted up for the Dr's use.' This curious pamphlet was reprinted in 1720 by Mr. William Whiston, and subjoined to a work of his own on the discovery of the longitude and latitude by the inclinatory or dipping needle.

The attraction or repulsion of two magnets decreases as the distance increases, but not according to any ratio of the distance. On this account a magnetical bar, which is at liberty to assume any horizontal position, as, for example, a needle floated on water by means of cork, or

the needle of a mariner's compass, being brought into the vicinity of another magnet, will assume such a situation as shall conform to the attractive and repulsive powers as much as possible. Thus, if a suspended magnetical needle be brought near another magnet, it will place itself in a position art as the magnet, if the poles of contrary names in each be mutually equi-distant; but, if the north pole of the suspended needle be nearer the south pole of the magnet than the two other poles are to each other, its north end will be most attracted, and consequently must incline, so that the axis of the two magnets will form an angle greater or less, according to circumstances. Suppose now a small magnetical bar, suspended so as to be capable of assuming any position whatever, be placed upon, or near, the surface of a very large globular magnet. It is evident, in this case, that the two ends of the small bar, being respectively attracted by the contrary poles of the globe, will always be found in a plane passing through these poles; or in other words, if circles or meridians be supposed to be described on the globe, intersecting each other in those poles, the magnetical bar must always be in the plane of one of them. But its situation, with regard to the spherical surface, will be governed by the excess of attraction in the nearest pole. If the bar be suspended immediately over the north pole of the magnet it must stand perpendicularly, with its south end downwards; but, if it be gradually removed along the surface towards the south pole, the increasing action of this last pole will cause it gradually to incline that way. At the equator it will rest parallel to the surface; and, in approaching still nearer the last mentioned pole, its north end will incline towards the surface, till at length it will stand perpendicularly over the south pole of the great magnet, with its north end downwards. For the sake of conciseness, the poles of the great magnet are supposed to be equally strong; which, however, is seldom the

case.

mities of the axis, like those of the beam of a balance, rest upon its supports, so that the nee dle may move itself vertically round, and, when situated in the magnetic meridian, may place itself in the magnetic line. When it is used a sea it is suspended by a ring. When it placed upon a stand a spirit-level is attache to it.

The greatest imperfection in this instrume is in the balancing of the needle, and the di culty of ascertaining whether the needle retai its equipoise. In observing the dip of the n dle, at any particular place, the best method avoid the error arising from the want of bala is, first to observe the dip of the needle, the reverse its magnetism, by the magnetic bars that the end of the needle which before was vated above the horizon may now be belo and, lastly, to observe its dip again; for a of the two observations will be pretty ne truth, though the needle may not be per balanced.

From a series of numerous and accura periments made by captain Sabine, he ob the following measures of the dip of the netic needle at London :By ten experiments with Tobias Mayer's

needle

By the times of oscillation in the magnetic

meridian, and in the plane perpendicular to it. Mean by three needles

By the times of vertical and horizontal

oscillation

Mean

Hence captain Sabine concludes, th was the mean dip of the needle at Lo August and September 1821, within a fe of noon.

As the observations of Nairne and C give 70° 25' for the dip in 1774, we obta as the mean annual rate of diminution 1774 and 1821.

Taking Mr. Whiston's determination dip in 1720 at 75° 10', which Mr. C considered as accurate, the annual dim 3' 05".

Having thus illustrated the nature of netic dip, it may now be advisable Biot's hypothesis, in which he attemp termine the law which regulates the i of the needle in different parts of M. Biot supposes two centres of attr northern and southern poles, situated distances from the axis of the m culated the action of earth's the

entre of the earth, equator. He th h ought to resul upon any po ng the attractiv f the squares was, that the ne rought to each lculation agre

MAGNETISM, ANIMAL, a fancied sympathy said by some persons to exist between the magnet and the human body: by means of which the former became capable of curing many diseases in an unaccountable manner. The fanciful system, to call it by no worse name, of animal magnetism, appears to have originated in 1774, from a German philosopher, named Father Hehl, who greatly recommended the use of the magnet in medicine. M. Mesmer, a physician of the same country, by adopting the principles of Hehl, became the direct founder of the system; but afterwards, deviating from the tenets of his instructor, he lost his patronage, as well as that of Dr. Ingenhousz, which he had formerly enjoy ed. Mesmer had already distinguished himself by A Dissertation on the Influence of the Stars upon the Human Body, which he publicly defended in a thesis before the university of Vienna; but he was so unable to stand before the opposition of Hehl and Ingenhousz, that his system fell almost instantly into disrepute. Mesmer appealed to the Academy of Sciences at Berlin; but they rejected his principles as destitute of foundation, and unworthy of the smallest attention. He then made a tour through Germany, publishing every where the great cures he performed by means of his animal magnetism, while his opponents every where pursued him with detections of the falsehood of his assertions. Mesmer, still undaunted by so many defeats, returned to Vienna; but meeting there with no better success than before, he retired to Paris in the beginning of 1778. Here he met with a very different reception. He was first patronised by the author of the Dictionnaire des Merveilles de la Nature; in which work a great number of his cures were published, Mesmer himself receiving likewise an ample testimony of his candor and solid reasoning. He soon collected patients, and, in April 1778, retired with them to Cretail, whence he in a short time returned with them perfectly cured. His success was now as great as his disappointment had been before. Patients increased so rapidly, that he was soon obliged to take in pupils to assist him in his operations. These pupils succeeded as well as Mesmer himself; and so well did they take care of their own emolument, that one of them, named M. Deslon, realized upwards of £100,000 sterling. In 1779 Mesmer published a Memoir on Animal Magnetism, promising afterwards a complete work upon the subject, which should make as great a revolution in philosophy as it had already done in medicine. The new system now gained ground daily; and soon became so fashionable, that the jealousy of the faculty was thoroughly awakened, and an application concerning it was made to government. In consequence of this, a committee was appointed to enquire into the matter, consisting partly of physicians, and partly of members of the Royal Academy of Sciences, with Dr. Benjamin Franklin at their head. This was a thunderstroke to the supporters of the new doctrine.-Mesmer himself refused to have any communication with the committee; but his most celebrated pupil Deslon was less scrupulous, and explained the principles of his art in the following manner: 1. Animal magnetism

is a universal fluid, constituting an absolute plenum in nature, and the medium of all mutual influence between the celestial bodies, and be twixt the earth and animal bodies. 2. It is the most subtile fluid in nature; capable of a flux and reflux, and of receiving, propagating, and continuing all kinds of motion. 3. The animal body is subjected to the influences of this fluid by means of the nerves, which are immediately affected by it. 4. The human body has poles and other properties analogous to the magnet. 5. The action and virtue of animal magnetism may be communicated from one body to another, whether animate or inanimate. 6. It operates at a great distance without the intervention of any body. 7. It is increased and reflected by mirrors; communicated, propagated, and increased by sound; and may be accumulated, concen trated, and transported. 8. Notwithstanding the universality of this fluid, all animal bodies are not equally affected by it; on the other hand, there are some, though but few in number, the presence of which destroys all the effects of animal magnetism. 9. By means of this fluid nervous disorders are cured immediately, and others mediately; and its virtues, in short, extend to the universal cure and preservation of mankind. From this extraordinary theory, Mesmer, or M. Deslon, had fabricated a paper, in which he stated that there was in nature but one disease and one cure, and that this cure was animal magnetism. To ascertain the truth of these assertions, the committee attended M. Deslon, in the room where his patients underwent his magnetical operations. The apparatus consisted of a circular platform made of oak, one foot and a half high; with a number of holes at the top, in which were iron rods with moveable joints, for the purpose of applying them to any part of the body. The patients were placed in a circle around it, each touching one of these rods, and joined to one another by a cord passing round their bodies. Each of them held an iron rod in his hand ten or twelve feet long, to concentrate the magnetism. M. Deslon also called in the aid of music from a piano-forte: on which some airs were played, accompanied with songs; alleging that music is a conductor of animal magnetism, which is transmitted to the patients by the sounds. The internal part of the platform was said to concentrate the magnetism, and was the reservoir whence the virtue was diffused among the patients. The committee satisfied themselves, by means of a needle and electrometer, that neither common magnetism nor electricity was concerned. Deslon also communicated the magnetism by his finger, and a rod which he held in his hand, and which he carried about the face, head, or such parts of the patient as were diseased. His principal application, however, was by pressure of his hands or fingers on the lower regions of the stomach. The effects of these operations upon his patients were very different. Some felt nothing; others spit, coughed, perspired, and felt, or pretended to feel, extraor dinary heats in different parts of the body. Many women, but very few men, had convulsions, which Deslon called their crisis, &c. The commissioners at last finding they could come to