SECTION XXX...

Terrestrial Magnetism-Magnetic Poles-Lines of equal and no Variation -The Dip-The Magnetic Equator-Magnetic Intensity-Secular, periodic, and transitory Variations in the Magnetic Phenomena-Origin of the Mariner's Compass-Natural Magnets-Artificial Magnets-Polarity -Induction-Intensity-Hypothesis of two Magnetic Fluids-Distribution of the Magnetic Fluid-Analogy between Magnetism and Electricity. In order to explain the other methods of exciting electricity, and the recent discoveries in that science, it is necessary to be acquainted with the general theory of magnetism, and also with the magnetism of the earth, the director of the mariner's compass-his guide through

the ocean.

The distribution of terrestrial magnetism is very complicated, and the observations simultaneously made at the various magnetic establishments recently formed in both hemispheres have changed many of the opinions formerly received with regard to that science.

Its influence, arising from unknown causes in the interior of the earth, extends over every part of its surface, but seems to be independent of the form and of the peculiarities of the exterior of our planet (a). Its action on the magnetic needle determine's the magnetic poles of the earth, which do not coincide with the poles of rotation.

Mr. Hansteen of Copenhagen computed, from observations in various parts of the world, that there are two magnetic poles in each hemisphere, while M. Gauss has concluded there is only one in each (A). The position of one of these poles was determined by our gallant countrymen when endeavoring to accomplish the north-west passage round America. It is situate in 70° 5′ 17′′ north latitude, and 96° 46′ 45′′ west longitude. Another northern magnetic pole is known by observation to be in Siberia, somewhat to the north of 60° north latitude and in 102° east longitude, so that the two poles are 198° 46′ 45′′ asunder. In his recent voyage to the Antarctic regions Sir James Ross ascertained that one of the southern magnetic poles is in 70° south latitude,

and about 162° east longitude. The position of the other south magnetic pole, if it exists, is unknown.

In consequence of the attraction and repulsion of these poles, a needle suspended so as to move freely in a horizontal direction, whether it be magnetic or not, only remains in equilibrio when in the magnetic meridian, "that is, when it is .in a place which passes through a north and a south magnetic pole. In some places the magnetic meridian coincides with the terrestrial meridian, and in these a magnetic needle freely suspended, as in the mariner's 'compass, points to the true north; but if it be carried successively to different places on the earth's surface its direction will deviate, sometimes to the east, and sometimes to the west of the true north. Imaginary lines drawn on the globe through all the places where the needle points due north and south are called lines of no variation. Imaginary lines drawn through all those places where the needle deviates from the geographical meridian by an equal quantity, are lines of equal variation.

A magnetic needle suspended so as to be movable only in a vertical plane dips, or becomes more and more inclined to the horizon the nearer it is brought to a magnetic pole, and there it becomes vertical. Lines of equal dip are such as may be imagined to pass through all those points on the globe where the dipping needle makes the same angle with the horizon. In some places the dipping needle becomes horizontal, and there the influences of the north and south poles are balanced, and an imaginary line passing through all such places is the magnetic equator. In going north from the magnetic equator one end of the dipping needle dips more and more till it becomes perpendicular at the north magnetic pole, while in proceeding south from the magnetic equator the other end of the dipping needle dips, and at last becomes perpendicular at the south magnetic pole. The magnetic equator does not coincide with the terrestrial equator: it appears to be an irregular curve passing round the earth, inclined to the earth's equator at an angle of about 12°, and crossing it in several points, the position of which seems still to be uncertain. According to some accounts, three

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points have been ascertained in which that curve cuts the equator; yet Captain Duperry, who crossed it repeatedly, affirms, from his own observations combined with those of M. Jules de Bosville and of Colonel Sabine, that it crosses the terrestrial equator in two points only, and those diametrically opposite one to the other, and not far from the meridian of Paris. One of these nodes he places in the Atlantic, the other in the Pacific ocean. He finds that the magnetic equator deviates but little from the terrestrial equator in that part of the Pacific where there are only a few scattered islands (b), that as the islands become more frequent the deviation increases, and arrives at a maximum both to the north and south in traversing the African and American continents; and that the symmetry of the northern and southern segments of this curve is much greater than was imagined.

The intensity of the magnetic force is different in different parts of the earth. If a magnetic needle, freely suspended so as to move horizontally, and at rest in a magnetic meridian, be drawn any number of degrees from that position, it will make a certain number of oscillations before it resumes its state of rest. The intensity of the magnetic force is determined from these oscillations, in the same manner that the intensity of the gravitating and electrical forces is known from the vibrations of the pendulum and the balance of torsion (c): and in all these cases it is proportional to the squares of the number of oscillations performed in a given time, consequently a comparison of the number of vibrations accomplished by the same needle during the same time in different parts of the earth's surface will determine the variations in the magnetic action. By this method it was discovered that the intensity of the magnetic force increases from the equator toward the poles; but the foci of the greatest total intensity of the magnetic force seem neither to coincide with the magnetic nor rotatory poles of the earth (d). One of these foci, according to Colonel Sabine's magnetic chart, is situate about the 47° south latitude and 140° east longitude, while another of less energy is in 60° south latitude and 235° east longitude. The point of least total magnetic intensity on the

whole globe is by the same chart about the 25° south latitude and 12° west longitude. In the northern hemisphere the foci of maximum intensity are in lat. 54° 32′ N., long. 261° 27′ E., and lat. 71° 20′ Ñ., long. 119° 57′ E., according to M. Gauss's calculations. The magnetic intensity appears to be doubled in the ascent from the equator to Baffin's bay.

Such are the principal phenomena of terrestrial magnetism, but it is subject to secular, periodical, and transient disturbances still imperfectly known. In the northern hemisphere, the poles, the lines of equal and no variation, the equator, and in short the whole system is gradually moving toward the east, so that the relations observed in Europe two centuries ago have now reached the limits between Europe and Asia, while other parts of the system have moved gradually over to us from the west. In the southern hemisphere the secular motion of the poles and of the whole system is in a contrary direction. The cause of these secular disturbances is altogether unknown.

The horizontal needle or compass at any one place is also subject to periodic and transient perturbations. Great disturbances occur on the same day, or nearly on the same day, in different years, from causes unknown.

There are also disturbances which, according to the observations of M. Kreil, in Milan, depend on the declination of the moon and her distance from the earth; others of shorter duration seem to be intimately connected with the motion of the sun in regard to the magnetic meridian of the place of observation. In consequence of the latter, the needle in the same place is subject to diurnal variations: in our latitudes the end that points to the north moves slowly westward during the forenoon, and returns to its mean position about ten in the evening; it then deviates to the eastward and again returns to its mean position about ten in the morning.

M. Kupffer of Casan ascertained that there is a nocturnal as well as a diurnal variation, depending in his opinion upon a variation in the magnetic equator. Magnetic storms, or sudden and great but transient disturbances, take place occasionally in the compass, which are per

ceived simultaneously over widely extended regions; while others of less magnitude and duration occur more frequently, and are, equally with the greater, not amenable to any known laws.

The dip is subject to a secular variation, and according to Colonel Sabine has been decreasing in northern latitudes for the last fifty years at the rate of three minutes annually, and is probably owing to the secular motion of the magnetic equator. There are disturbances also in the dip of a periodic nature, and others very transient, which M. Kreil attributes to weak shocks of earthquakes, having observed that the greatest vertical disturbances have almost always coincided with considerable earthquakes even when they occurred in remote regions.

The magnetic intensity is subject to various changes. M. Hansteen has found that it has been decreasing annually at Christiana, London, and Paris at the rate of its 235th, 725th, and 1020th parts respectively, which he attributes to the motion of the Siberian magnetic pole. The moon increases the magnetic intensity in our hemisphere: but her influence differs with her difference of position in the heavens. The times of vibration of the needle are less when the moon has south declination than when she has north, and they are less when she is in perigee than in apogee. It is still doubtful whether magnetic intensity varies with the height above the earth or not.

The diurnal variation in the horizontal intensity observed by M. Hansteen at Christiana is probably owing to the sun's influence: indeed the whole of the magnetic disturbances have been ascribed to that cause; and he has even found a general resemblance between the isothermal lines and the lines of equal dip on the surface of the earth: yet in the present state of our knowledge the magnetic phenomena can only be regarded as the effects of a combination of causes whose separate action is still unknown.

The inventor of the mariner's compass, like most of the early benefactors of mankind, is unknown. It is even doubted which nation first made use of magnetic polarity to determine positions on the surface of the globe.