MABIXER'S COMPASS.] 



UNDULATORY FORCES. MAGNETISM. 



245 



would be more nearly reached by any body exposed to 

 their action. 



The attractive and repulsive powers of a magnet are 

 equal to each other ; and, indeed, they may be regarded 

 as the opposite conditions of one force. In this consists 

 the analogy between magnetism and the double electrical 

 theory adopted in reference to frictional electricity. 

 Just as we presume that the inside and outside of a 

 charged Leyden jar* are in opposite states of electrical 

 excitation, and yet that such are exactly equal in 

 amount ; and, again, as similarly electrical bodies repel, 

 and dissimilar attract, each otherf so we have iden- 

 tical conditions and affections of the magnet. We may 

 also illustrate the condition of a magnetised body in a 

 similar manner to that adopted in instances of electrical 

 excitation ; for as the alternate sides of the atoms of an 

 electrified body are supposed to be in opposite states of 

 electricity, so the particles of steel composing a magnet, 

 have respectively, on their opposite sides, a north and 

 a south pole the whole magnet being built up of such 

 until the two extremities form the two opposite poles. 



A very beautiful iDustration of the effects of mag- 

 netic attraction, is afforded by placing a sheet of writing- 

 paper on the poles of a horse-shoe magnet, and sifting 

 over it some fine steel or iron filings. The paper is then 

 to be gently tapped, when the filings will arrange them- 

 rig. 2. selves in various forms and at 



different distances, producing 

 what are termed magnetic 

 curves, the appearance of which 

 is illustrated in the annexed 

 engraving. The same effect 

 may be produced by a bar 

 magnet. Artificial magnets pro- 

 duced by the rubbing process, 

 never exhibit so great an at- 

 tractive power as those produced 

 by soft iron when surrounded 

 by a voltaic current, and hence 

 termed an electro-magnet. Wu 

 shall, however, defer speaking on this point until we 

 treat on the subject of Electro-Magnetism. 



Pieces of steel acquire the power of magnetic attrac- 

 tion, repulsion, and polarity, if they are kept in souio 

 positions for a considerable time. A poker will thus 

 often be found magnetic if it be kept in a position 

 nearly due north and south. Old iron window-frames 

 are similarly affected. In fact, masses of cast-iron and 

 steel are frequently found to have acquired magnetic 

 properties from such causes. This results from the 

 inductive action of the magnetism of the earth, which, 

 like electricity, can induce its effects in bodies at a 

 distance from its source. 



Nickel and cobalt have decided magnetic properties 

 under certain circumstances ; but such are extremely 

 feeble compared with those possessed by iron and its 

 compounds. The magnetic force exercises its influence 

 through bodies which are incapable of being acted on 

 by it. Thus, if a magnetised needle be placed on the 

 surface of water in a basin, and a horse-shoe or other 

 magnet be brought near to it whilst the material of the 

 vessel intervenes, it will be found that the attraction 

 and repulsion are as actively exhibited, as would occur 

 supposing that no substance was placed between the 

 two magnets. Most solids, liquids, and even gases, 

 may, however, be affected by the magnetic force. This 

 we shall more fully explain when we treat on Dia- 

 Magnetism. 



THE MARINER'S COMPASS AND THE DIPPING- 

 NEEDLE. 



WE have preferred to introduce our readers to a general 

 knowledge of magnetic attraction and polarity, before 

 directing their attention to the very interesting subjects 

 of the Mariner's Compass and the Dipping-Needle, be- 

 cause these depend on a special application of the laws we 

 have just explained. 



ftw tmtf, p. 179. f See ante, p. 17S. 



We have, however, hitherto confined our attention to 

 the artificial magnet, or the loadstone : we must now 

 extend our ideas beyond such limits, and transfer all the 

 powers and laws we have examined, to the earth, as the 

 great source of magnetic attraction. We may conceive, in 

 fact, the earth to be an immense magnet, having polarity, 

 attraction, <fcc. , as have the feeble instruments we employ 

 in our researches into the nature of this force. Indeed, 

 all development of magnetism which we observe, is due 

 to the inductive power of the earth on the bodies we 

 find magnetic. The magnetic poles of the earth vary, 

 from its natural polarity. The north magnetic pole is 

 situated in long. 90 45', and lat. 70 5'. The magnetic 

 equator is therefore, of course, not coincident with that 

 of the earth. 



The mariner's compass consists of a bar of steel which 

 has been well magnetised mounted on a pivot, so that it 

 can move as freely as possible in a horizontal direction. 

 Attached to, or placed beneath it, is a circular card, on 

 which are engraved the different points of the compass. 

 This card is divided into thirty-two parts. Each of 

 these parts, or points, are subdivisions of the four, car- 

 dinal points north, east, south, and west ; and the inter- 

 mediate points between any of them, acquire their 

 joint names; such as north-east, south-west, south- 

 south-east, <fec. 



Generally speaking, at least two compasses are carried 

 on board a vessel, so that in case of aay damage to either, 

 another may at once supply its place. The greatest care 

 is observed to place the compass in a position in wMch 

 any external object shall have as little effect as possible 

 upon it, as any derangement might lead to the most 

 serious results ; of which wo shall speak more fully as we 

 proceed. 



The needle, as we have already stated, does not point 

 due north ; that is, it does not lie in a plane parallel to 

 the meridian of a place which, of course, is due north 

 and south. This deviation is termed the variation of the 

 compass ; and it is undergoing constant, although gradual 

 change. This variation also differs for different places 

 on the earth's surface. About 300 years ago, the needle 

 pointed 11 east of north. In 1657, it pointed due north 

 and south; and at the present time its variation is 

 about 23 west of north ; each of the above referring to 

 observations made in London. Tables are constructed, 

 giving the variation of the needle for different places; 

 and we need scarcely remark that such are of great im- 

 portance to the mariner. 



But besides the horizontal motion of the magnetic 

 needle, a bar of steel, if properly balanced, so as to be 

 capable of a vertical motion, will, on being magnetised, 

 lose its balance, and bend at an angle towards the earth. 

 Such an arrangement is called a dipping -needle ; and the 

 variation from a horizontal line is called the dip. At 

 the present time, the dip for London is about 67"; and, 

 like the horizontal direction of a needle, is constantly 

 varying. If a needle were placed in lat. 70 5' north, 

 and long. 96 45' west, which is the position of the 

 magnetic pole of the earth, it would then assume a ver- 

 tical position. 



Apart from any accidental influence, the needle varies 

 daily in its indications ; the maximum being during the 

 day, and the minimum during the night, in any locality. 

 It has hence been assumed that the sun has a definite 

 action on it ; and some have gone the length to suggest 

 that that luminary is the great source of magnetic force. 

 When, however, we discuss the influence of electric 

 currents on a magnetised needle, we shall find abundant 

 reasons foi the effects wo have named. There are other 

 disturbing causes which act on a magnetised needle 

 occasionally. It has been noticed, for instance, that the 

 appearance of the aurora borealis has a decided influence 

 on the direction of a needle; causing it, at times, to 

 fluctuate considerably. Indeed, the needle indicates the 

 presence of that splendid phenomenon, when, from any 

 cause, it is invisible to the eye. 



The diurnal variations of the needle have been mado 

 the subject of anxious investigation by most of tho 

 leading astronomers in the observatories of Europe and 



