MAGNETISM 221 two horizontal knife edges. In the magnetized state this is no longer the case. The axis of the needle now takes up a fixed position, with its north end pointing downwards (fig. 2), and if disturbed will oscillate about that position, and finally settle into it again. The angle which the axis NS makes with the horizon is least when the plane of rotation of the needle is in the mag netic meridian : the angle (t) in this case is called the " dip," or (by Con tinental writers) the "inclination." It is greatest, viz., 90, when the plane of rotation of the needle is vertical and perpendicular to the magnetic meridian. At Greenwich the dip is about 67 30 at the pre sent time. If we place the needle with its plane of rotation perpendi cular to the line of dip, the equili brium will be indifferent, as it was in all positions before magnetization ; but there is no other position of the magnetized needle for which this is true. The remarks which we made as to variation in space and time of the declination apply also to the dip. The variation from place to place differs, however, in nature from that of the declination. Along a line running in the neighbourhood of the geographical equator, partly north and partly south of it, the dip is zero. North of this line, which is called the magnetic equator, the north end of the needle dips below the horizon ; and the angle of dip increases as we go northwards, until, at a point in the Hudson s Bay Territory, the needle dips with its north pole vertically downwards. South of the magnetic equator the south end dips below the horizon ; and there is again a point in the southern hemisphere where the south end dips vertically downwards. These points are called the " magnetic poles " of the earth. For further details on this subject we refer the reader to the discussion of terrestrial magnetism in the article METEOROLOGY. It was in the accurate observation of the declination and dip of the magnetic needle that the science of magnetism arose. The dip appears to have been first observed by Georg Hartmaun, vicar of the church of St Sebaldus at Nuremberg (1469-1564), who seems to have been in advance of his age in magnetical matters. In a letter 1 to Duke Albrecht of Prussia, dated 4th March 1544, he writes : " Besides, 1 find this also in the magnet, that it not only turns from the north and deflects to the east about 9 more or less, as I have reported, but it points downwards. This may bo proved as follows. I make a needle, a finger long, which stands horizontally on a pointed pivot, so that it nowhere inclines towards the earth, but stands horizontal on both sides. But as soon as I stroke one of the ends (with the loadstone), it matters not which end it be, then the needle no longer stands horizontal, but points downwards (fallt unter sich) some 9 more or less. The reason why this happens was I not able to indicate to his Royal Majesty." From this it will be seen that Hartmann had unques tionably observed the tendency of the magnetized needle to dip. His method of observing is of course unsuited for Brought to light by Moser. See Dove s P.epertorium der Physik, ii., 1838. It does not appear that Hartmann s letter was ever before published. Moser is therefore scarcely justified in attacking Norman s priority in this matter, still less in attempting to deny him the credit of first observing the dip by a sound method. Had he read the Newe Attractive he could scarcely have fallen into such an error; for in respect of clearness and scientific precision Hartmann s letter, interesting as it is, cannot for a moment be compared with Norman s little work. measurement, and it is not surprising that he got a result of 9 instead of somewhere about 70. In 1576 the dip was independently discovered by Robert N< rman. Norman, a skilful seaman and an ingenious artificer, according to Gilbert. He was in the habit of making compass needles, and carefully balancing them so as to play Horizontally on their pivots before magnetization. He found that, after they were magnetized, they constantly dipped with the north end downwards, so that a counter poise had to be added to bring them back to the horizon. This led him to construct a special instrument, the proto type of the modern dipping needle, to show this new phenomenon. With this instrument he made the first accurate measurement of the dip, and found it to be 71 50 at London. 2 The early English magnetic observers, of whom Norman and Burroughs (who wrote an able supplement to Norman s work) were admirable examples, must have done much for the introduction of precise ideas into magnetism. But their fame was speedily eclipsed by William Gilbert of Colchester 3 Gilbert. (1540-1603), whom Poggendorff has justly called the Galileo of magnetism, and whom Galileo himself thought enviably great. In his great work entitled De Magnate Mayneticisque Corporibus et de Magno Magnete Tellure Physiologia Nova, first published in 1600, we find a com plete account of what was known of magnetic phenomena up to his time, with a large number of new ideas and new experimental facts added by himself. We find in Gilbert s work, in a more or less accurate form, nearly all that we shall lay before the reader in the first section of this article, described very much in the language that we shall use. " How far he was ahead of his time is best proved by the works of those who wrote on magnetism during the first few decades after his death. They contributed in reality nothing to the extension of this branch of physical science." 4 Mutual Action of Like and Unlike Poles. If we take a Like magnet whose poles N , S have been determined and magnetic marked as above explained, and bring its north pole N ^ ^. near the north pole N of a magnetic needle, N will move unlike in a direction indicating repulsion between N and N . attract. The same result will follow if the south pole S of the magnet be brought near the south pole S of the needle. But if S be brought near N, or N near S, attraction will be indicated. Hence the following fundamental law of the action between two magnets : Like poles repel each other; unlike poles attract each other. It would appear, therefore, that the whole action of one magnet upon another is of a some what complicated character, even if we take the simplest view of it that the experi mental facts will allow, viz., that the action may be represented by forces acting between the two pairs of points in each magnet which we have defined as north and south poles. On this assumption, the action of N S upon NS would consist of the four forces represented in fig. 3, for all these must exist in accordance with the law just established. Whether this is a sufficient 2 lie published a work, of which the following description is given in the Ronalds Catalogue : " The Newe Attractive, containing a short discourse of the Magnes or Lodestone, and amongst other his vertuus, of a new discovered secret and subtill propertie concemyng the Declyning of the Needle, touched therewith, under the plaine of the Horizon. Now first found out by Robert Norman, Hydrographer. 4to (black letter, scarce), London, 1581." 3 For details as to his life, see art. GILBERT.
4 Poggendorff, Geschichte der Physik, p. 28fi.