Magnetic meridian

“ The magnetic meridian runs indeed in general from pole to pole, but as it is not the earth's axis, which determines the magnetism, but the metallic masses, so164 the direction of the meridian deviates from the earth's axis in accordance with these masses. ”

“ The inclination is removed just the same as with the needle of the compass. One has merely to alter the centre of gravity, and this can be done in several ways. Thus, all that remains is the direction in the magnetic meridian. ”

“ It is probable that the meridians of all these and similar magnets occurring in the basaltic rocks, or in other rocks, are coincident with the magnetic meridian, and that they acquire this virtue as masses of iron are often known to do by long continued rest in favourable positions, combined probably with circumstances of which we are as yet doomed to remain in ignorance, little acquainted as we are with the history and causes of this obscure power. ”

“ Each observer sees the summit of the auroral arc at his magnetic meridian; it is, therefore, only those who are on the same magnetic meridian who see the same summit, and who are able by simultaneous observations to take its height.
If the summit of the arc pass beyond the zenith of the observer, the latter is surrounded by the matter of the aurora borealis. ”

“ As we travel north from the isoclinal of 0° along the meridian containing the magnetic pole the dipping needle’s north pole dips more and more, until when we reach the magnetic pole the needle is vertical. Going still farther north, we have the dip diminishing. The northerly inclination is considerably less in Europe than in the same latitudes of North America; and correspondingly the southerly inclination is less in South America than in the same latitudes of Africa. ”

“ The dipping needle, being landed on the South-West point of the islet, was adjusted as nearly as possible on the magnetic meridian by the sun’s bearings, and found to vibrate freely when the face of the instrument was directed to the east or west. The mean dip it gave was 80° 37′ 50′′. When the instrument was removed from the North-West to the South-East point about twenty yards distant and placed on the meridian the needle ceased to traverse but remained steady at an angle of 60°. ”

“ Since the earth's surface is diversified by elevations of land and depths of seas, great continental lands, oceans and seas differing in every way while the power that produces all magnetic movements comes from the constant magnetic earth-substance which is strongest in the most massive continent and not where the surface is water or fluid or unsettled, it follows that toward a massive body of land or continent rising to some height in any meridian, there is a measurable magnetic leaning from the true pole toward the east or the west. ”

“ It has been erroneously conceived that the height might be determined by observation of the corona, which is only an effect of perspective, owing to the convergence of parallel rays; each individual seeing his own aurora, as his own rainbow, from his particular point of view. As the centre of the arc is always in the magnetic meridian, simultaneous observation from two stations on the same meridian, with an interval sufficient to constitute a reliable base, might however effect the desired object. ”

“ From half-past nine to ten there was a large, luminous, horizontal arch to the southward, and several faint concentric arches northward. It was particularly noticed that all the arches seemed exactly bisected by the plain of the magnetic meridian. At half-past ten o’clock streamers appeared, very low in the south-east, running to and fro from west to east. They increased in number, and began to approach the zenith apparently with an accelerated velocity, when all on a sudden the whole hemisphere was covered with them, and exhibited such an appearance as surpasses all description. ”

“ The international magnetic committee have recommended that Y be taken positive to the east, but the fact that the declination is westerly over most of Europe has often led to the opposite procedure, and writers are not always as careful as they should be in stating their choice. Apart from mathematical calculations, the more usual course is to define the force by its horizontal and vertical components—usually termed H and V—and by the declination or angle which the horizontal component makes with the astronomical meridian. ”

“ After this geometrical representation of the relations of the horizontal magnetic force, we proceed to develope the mode of submitting them to calculation. On the surface of the earth becomes a simple function of two variable magnitudes, for which we will take the geographical longitude reckoned eastward from an arbitrary first meridian,—and the distance from the north pole of the earth ”

“ The variation of the magnetic needle, observed when we passed under the meridian of Isle de France, at the distance of more than 700,000 toises south, had been greater by twelve degrees than we found it near the coast of that land—a great difference when considered as arising merely from change of latitude. ”

“ But it very often happens at sea and on land that the magnetick iron does not point to the true pole, and that not only a versorium and magnetick pieces of iron, and the needle of a compass, or a mariners' compass, but also a terrella in its boat, as well as * iron ore, iron stones, and magnetick earths, properly prepared, are drawn aside and deviate towards some point of the Horizon very near to the meridian. For they with their poles frequently face termini away from the meridian. ”

“ Hence on the outermost surface of the earth magnetical bodies are slightly perverted from the true meridian. Moreover, since the surface of the globe is divided into high lands and deep seas, into great continental lands, into ocean and vastest seas, and since the force of all magnetical motions is derived from the constant and magnetick terrestrial nature which is more prevalent on the greater continent and not in the aquæous or fluid or unstable part ”

“ Hence the direction falls off and at the pole is itself nothing: and for that reason a feebler direction is easily vanquished by the stronger causes of variation, and near the pole the versorium deflects the more from the meridian. It is demonstrated by means of a terrella: if an iron wire of two digits length be placed on its æquator, it will be strongly and rapidly directed toward the poles along the meridian, but more weakly so in the mid-intervals; while near the poles one may discern a precipitate variation. ”

“ Besides, to observe the altitude on both sides of the meridian is just as expeditious as to observe on one side only and at the same time to find the elevation of the pole. And he who can take one altitude by the instrument can also take another; but if the one altitude be uncertain, then all the labour with the globe, numbers, sines and triangles is lost; nevertheless those exercises of ingenious mathematicians are to be commended. It is easy for anyone, if he stand on land, to learn the variation by accurate observations and suitable instruments, especially in a nearly upright sphere ”