In an isoclinic chart by the same author and for the same epoch in the upper part of the chart, which is left white, the n. end of the needle dips; and in the lower part, which is tinted, the s. end of the needle dips The amount of dip is marked on lines. Thus, the line passing through the center of England is marked 70'. A dipping-needle, at any place et.t by the line, is inclined 70° to the horizon. The line 75' passes to the n. of the British isles. In Ireland and Scotland, therefore, the dipping needle has an inclination greater than 70', and less than 75'. The line marked 0° is the line of no dip; at station on it the dipping-needle is horizontal. This line is called the magnetic equator. It is not coincident with the geographical equator; it is not even a great circle of the earth, but is an irregular curve cutting the equator in two points, one near the w. coast of Africa, and the other in the middle of the Pacific ocean. The, points on the earth's surface where the dipping-needle stands vertical, and where, in consequence, as before mentioned, the compass-needle in any direction, are called the magnetic poles. The n. magnetic pole was found in Boothia Felix by capt. Ross at 70° 5' n. lat. and 14' e. long. According to Gauss's calculation, it should have been at the time (1831) some 3' n. of this point. From observations made at Hobart Town, the nearest station to it, the s. magnetic pole should lie 66° s. lat. and 146 e. long. These points are not diametrically opposite each other, as the geographical poles. If the lines of equal dip were drawn on a globe, they would form round the magnetic poles a system of irregular circles, somewhat resembling that of the parallels of latitude round the poles of the earth.
We do not add an isodynamic chart, as it would take up too much space. Col. Sabine's dynamical chart, along with the isogonic and isoclinic charts, will be found fully engraved and explained in Johnston's Physical Atlas (new edition). From this chart we learn that the magnetic intensity is least in the vicinity of the magnetic equator, and increases as we approach the magnetic poles. The lines of equal intensity, though running much in the same direction as the lines of equal dip, are neither coincident nor parallel with them. The line of least intensity, itself not an isodynamic line, runs nearly parallel to the magnetic equator, but lies, except in the western half of the Pacific, a few degrees to the s. of it. We thus learn that the changes in direction and intensity do not march together. We should fancy that at that point or points on the earth's surface where the dipping-needle stood erect we should be nearest to the center of free magnetic energy, and that there the force would be greatest; but this is not the case. The point in North America where the intensity is greatest is situated to the w. of Hudson's bay, some 18° s. of the n. magnetic pole. But this is not the only point of maximum force in the n. magnetic hemisphere. There is another, which was found by Hansteen in 1828, in Northern Siberia, about the longitude 120°. This maximum point is weaker than the American, in the proportion of 100 to 107 (Sabine). According to
Gauss, there can only be onemaximum point in the southern hemisphere which is stronger than either of the other two. It lies n.e. of the s. magnetic pole, and its intensity is 137 (Gauss) compared with 107, that of the principal northern center. At none of those points does the dipping-needle stand erect. This want of coincidence of the points of vertical dip and of maximum intensity has led to some confusion in the use of the term magnetic pole; some writers meaning by it a point of vertical dip, and others a point of maximum intensity. In adopting the former definition we are only adhering to the popular meaning of the word, and to the opinion of Gauss, perhaps the greatest author ity on the subject. Some of the best English authorities, however, attach to it the la:ter meaning.
Although the total intensity increases as we go northward or southward from the line of least intensity, the horizontal intensity diminishes. This arises from the fact that the greater the dip the less the horizontal intensity. Hence the compass-needle, which is affected alone by the horizontal intensity, oscillates more sluggishly as we leave the line of least intensity. A dipping-needle, for instance, oscillates faster at London than at Calcutta, because the total intensity which affects it is greater at London than at Calcutta; but with a compass-needle it is. the reverse, from the horizontal intensity being greater at the latter than at the former station.
Variations of the magnetic elements do not remain constant in the same place, but are subject to continual though small variations. These are regular and irregular. Under regular variations are included secular, annual, and diurnal variations. The secular variations take centuries for their completion. The following list of the declination and dip at London in different years will give an idea of the secular varia tions for these elements: At present the annual decrease of declination at Kew is 8'. At this rate it would take rather more than 84 years for the compass-needle to shift through a whole point. From the observations of the dip we find that it has been gradually decreasing for the last 150 years. The annual decrease of dip is at present about 2.6'. The time during which observations have been taken of the declination and dip is far from comprehend ing a cycle of change in either, and it is a mere matter of speculation how long that may take. The magnetic history of London does not apply to other places, each place, so far as has been ascertained, having a magnetic history of its own. Thus, in Paris, the time of no declination was 1669; and of maximum declination, 1814; the latter amounting to 22° 34' west. Every place, according to Barlow, appears to have its own magnetic pole and equator. Magnetic intensity has been observed for so short a time that little as yet is known of its secular variation. At present the horizontal intensity is increasing in Europe, but that may arise partly from decrease of dip.