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tion to a magnet freely suspended in it. The directive tendency of the com- pass, and other phenomena show that the 'earth is..surrounded by such a field. In fact, these phenomena are such as might be expected if we knew that a-bar magnet four or five thou- sands. miles -long,. extended. nearly north and south through the earth's center. This terrestrial magnetism is explained as being due to equatorial electric currents produced by the ac- tion of the sun, and modified by the motion of the earth. By observations of the north st. or in any other convenient way, mark a true north and south line of- your experimental table, or on the floor where it will remain stationary. Place a magnetic needle in this line, and de- termine the direction. and magnitude ofe the variation of the compass for your position. To verify the correct- ness of your work compare the. varia- tion determined by that given on a navigational chart. of your locality; that. given in the lighthouse boox, or that given on an isogonic chart of the world. | a Float a magnet on water as already explained. The float should be the lightest that will carry the load with safety, and the. body of water should be so large that surface tension will not urge the float toward the side of the vessel which should be a wooden basin, or other non-magnetic sub- stance. When the magnet is at rest near the middle of the liquid surface, determine the tendency of the magnet to drift toward the north or south. Repeat the experiment with a variety of magnets, and try to find one that always floats in one direction, that is; one in which the marked pole is stronger or weaker than the other. If you cannot find such a magnet, strong- ly magnetize, the blade of an old hack- saw, and test it on the float. If you have. not 'yet found that for which you. seek, break the blade in the mid- dle, and test each half. If necessary to 'the success of your search, break one of the halves in two, and repeat the tests. Make very careful notes of any magnet that you, find to have more magnetism of one kind than of the other. Such a magnet would not do for a compass needle, neither would it. do for the purpose of adjusting the compass. Tt will be more difficult to find unequal poles of the same mag- net among smaller magnets. than it will, larger ones. | Were it possible to get a magnet with only one pole, say a north pole, for instance, under the above condi- tions,. it would move on toward the north as far as the basin would permit THE Marine REVIEW Ft Ids Behe : nd it. The attractive effects being due to terrestrial magnetism. Some peo- ple have an idea that, under the above conditions, the magnet ought to float towards the north just. the same. This is because they entertain the idea that the north end of.the needle contains all the magnetism, and the south end none... The earth's mag- netism, which gives the needle direc- tive power, is, we might say, station- ary; because a compass needle (which is simply a bar magnet) always ar- ranges itself in a north and south line, this result is the same as if the earth were an immense magnet, with one pole at the 'north, and the: other at the south. Hence, the theory is, that the earth is really a magnet, and the magnetic force of the earth compels the compass needle to take this -posi- tion. sis the compass needle has. two 'ends, and one in. pointing north must cause the other to point south. The terrestrial globe exhibits the properties of a magnet in the direct- ing power it exerts upon the magnetic | needle. Whether on sea or land, on mountains or in deep valleys, a mag- netic needle, if free to move, 'always so adjusts itself that its poles point © in a definite direction, along a line which is near-to north and south true or geographically. That pole of the needle which is at- tracted by the north magnetic pole of the earth, must be in an opposite magnetic condition.. Hence it is the proper south pole of the magnet, but ' since it points toward :north, it is des- ignated the north. pole. ~ MAGNETIZATION. The various sources of magnetism are the influence of natural. or . ar- tificial magnets, terrestrial magnetism, and electricity. The three principal methods of magnetization by the in- ductive action of permanent , magnets are known by the technical names of single touch, separate touch and dou- ble touch. : MAGNETIZATION BY THE ELECTRIC CURRENT. Much greater power can, however, be obtained by means of electro-mag- netism; and the following methods are now almost exclusively employed by the makers of magnets. A' fixed "electro-magnet is' employed; and the bar to be magnétized is drawn in opposite directions over its two poles. 'Each stroke tends to: de- velop at the énd of the bar at which the motion ceases the opposite 'mag- netism to that of the pole which is in contact 'with it. Hence strokes in op- posite directions over the two con- trary poles tend to magnetize the *bar the same way. When very intense magnetization is to be produced the electro-magnet must be very powerful, and the bar then adheres to it so strongly that the operation above described becomes difficult of execution, besides. scratch- ing the bar. Hence it is more, con- venient to move along the bar a coil of wire through which a current is passing. ; _. DESTRUCTION OF MAGNETISM. The capability of induction, and the correlative power of retaining the in- duced magnetism, as we have seen, are both greatly affected by any me- chanical or other force which excites a:'-vibratory motion in the particles of the iron. On the other hand, the mag- netism of a steel bar is weakened, or even wholly destroyed by the same means; the vibratory motion which is imparted to the molecules of the body favoring the recombination of the separated magnetism, and, therefore, the return of the body to the neutral state. It is thus that the power of a magnet is seriously lessened, or alto- gether destroyed, by a fall from a height upon a hard pavement. INDUCED MAGNETISM. All soft or cast iron rods or bars, or other elongated forms of cast.or soft iron, unless the position of their length is at right angle to the line of the direction. of the earth's magnetic, force, are immediately rendered mag- netic by induction from the earth, and the nearer the iron is to the line of force or dip the greater will be the amount of induction. When a bar of soft iron is held on the magnetic: meridian and parallel to the dip it be- comes immediately endowed with fee- ble magnetic polarity. The lower ex- tremity is an N (red) pole, and if the N (red) pole of a small magnetic nee- dle be approached to it it will be re- pelled. -If the bar is held vertically the lower end will Still be a north pole, but of less intensity. If the bar ts held horizontally. north* and south the north end will be a north pole but of still lesser intensity, the south end of a south pole also of lesser intens- ity. If we now turn the bar in the same horizontal plane 'its -magnetism will diminish, and if placed in an east and west direction it will lose its po- larity, and if we turn it still further, until its. position is reversed, the mag- etic poles of the bar will be re- versed. Magnetism resulting from induction -may be more or less permanent, or totally transient. 'Thus it' has been found that a bar of iron that has re- mained fixed for: along period in a direction parallel, or nearly so, to the earth's line of force becomes a per-