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manent magnet. If, however, the bar be of hardened steel its magnetism will require a little time to arrive at its maximum intensity of magnetiza- tion, because of the coercive force causing it for a short time to resist magnetization. "Since the effect of inducing action is much increased by mechanical con- cussion, or any other cause which im- parts mofecular vibration to the bar while under the action of the induc- ing force, hence, to magnetize an iron bar we have only to hold it in the direction of the earth's magnetic force, and to strike it on the end while so held with a hammer. The bar will be found to acquire by" thi8 simple process a considerable amount of magnetism. As the direction of the - earth's magnetic force in higher lati- tudes is not far from the vertical the effect produced will be nearly the same if. the bar be held vertically. The end of the bar which was lowest being charged with north, or red, po- larity; and this magnetism is not tran- sient like the induced magnetism of soft iron, changing its' place in the bar with every change in the position of the' bar; but is constant like that _of a steel bar, retaining the same mag- netism whatever the position of the bar. By reversing the position of the bar and strixing it a few blows with a hammer, its magnetism is reversed. ' The magnetisnt ofthe bar so struck -resembles that of a magnet in all re- spects but this, that while, perhaps no change can be remarked in hours or days, it infallibly diminishes in a long time. To express this partially per- manent character the term subperma-. nent magnetism has been adopted. PERMANENT MAGNETISM. You have noticed when soft: iron is removed out of the magnetic field it. is no longer a magnet. A piece of hard steel, such as a knife blade, file, etc., retains its magnetism when re- moved out of thé magnetic field. Such a magnet is called a permanent magnet. Shocks, such as concussion, hammering, etc. have both the effect of producing and destroying magne- tism. If the. body,.is. highly magne-. tized the effect of concussion is to de- _ stroy - magnetism, if not magnetized. the effect is to increase magnetism. Another very peculiar thing about magnetism is in the construction of. ships of iron and steel. The quality of the iron or steel in each individual plate may be. such that it would not. retain magnetism' when by itself and, would act the same as an ordinary; bar of soft iron, but when these: plates, are riveted together. in the shape of a "TAE. MARINE. REVIEW vessel it was found that the hammer- ing, twisting and bending converted them into permanent magnets, the total ef- fect of which on the ship's compass is very large and very 'irregular. It was found by theory, and confirmed by experi- ment, that the total permanent mag- netism could always 'be resolved into two magnets, one along the ship's length, and: the other athwartships of the ship. This is the reason for .em- ploying two magnets in the adjust- ment of a"compass: ° 'After a few voyages of an iron ship a considerable amount of magnetism obtained during construction is lost. This is due to being® over-saturated with the magnetié fluid» the same as a magnet when it is first magnetized. Vibration, friction, heat, cold, buffet- ing the waves, change of geographical position, etc., etc, have each apart to play in the ship's magnetism taken as a whole.' After a number of years the ship acquires more of a permanent 'magnetic condition, but in no case is it such that it can be depended upon from 'year to year... A FEW DEFINITIONS. Magnetic Points of Consequence. . The points (really the magnetic poles of the earth) which occupy the center of lines of equal dip. : Points of Magnetic Indifference. A point near the. center of. a magnet where no effect is produced. -- - Magnetic Curves. A series of lines or directions which may -be graphic- ally denoted by iron filings scattered tipon a card or pane of glass placed horizontally upon a magnet and gent- ly tapped. The beautiful lines. into which the filings are thrown indicat- itig lines of magnetic. force. Magnetic Elements. Intensity, declina- tion and dip. aed Magnetic Intensity. The greater or less effect by. a magnet, usually meas- ured by its attractive force. This var- ies inversely as the square of the dis- tance. Magnetic Limit. A limit of tempera- ture beyond which iron or any other magnetic metal ceases to be affected by the magnet. Magnetic Needle. bar or plate of magnetized _ steel. The. needle suspended by a. metallic or jeweled center upon a hardened steel pivot. For other instruments needles are often suspended by fine silk thread or even spider-lines. The test of delicacy is the number of hori- zontal vibrations which the. suspended needle. will make before coming , to rest. ' Bee Magnetic North. A slender poised That 'point of. the 31 horizon indicated by the direction of the magnetic needle. Magnetic Saturation. The: state of'a bar or needle when it has received the greatest amount of magnetic force which can be pepe imparted tov its. Magnetic Storms. Magnetic disturb- ances felt simultaneously at places re- mote from each other.: Saturation Point. Every magnet when first made will lose: a certain amount of its magnetism, and finally settles to a permanent state called saturation point. Magnetic Metal; Any metal, such as iron, steel, tin, zinc, nickel, cobalt, etc., which may receive: the' oe of the magnet. ! Magnetic Screen. A shell of "softxison cutting off a magnetic needle from :the 'influence of a magnet. Magnetic Shield. A hollow: iron 'case in which a watch is permanently kept to screen it from derangement by ex- treme magnetism. ; Magnetism was known to the an- cients, having been first observed in 'the: loadstone, a species of iron ore found in abundance near the city. of Magnesia, in Asia Minor, whence. Ape magnet takes its name. | Magnetic Tick: A faint cick heave when a substance is magnetized or <de- magnetized. Called also magnetic click. a A MYTHICAL VOYAGE, Starting from the north magnetic pole. one would have to steer due south by compass to reach the true or geographical north pole, since the compass needle points mot to the true pole but to the magnetic pole. This does 'not look reasonable at 'first thought, but it is nevertheless a truth. To 'prove it try the following experi- ment. Lay a bar magnet on a table, flat side down, and lengthwise from you, with its south end or pole, from you, thus representing the earth's magnetism as at really exists. Now, take a small "magnetic needle suis- pended from a thread, and starting from the center (equator) of the bar. magnet carry it along the top of the bar magnet (be careful and not hold it too close to prevent dip, and possi- ble repulsion instead of attraction, but just far enough away so that its influ- ence will be gently exerted, though manifestly traceable) moving towards -- the north pole representing the earth's magnetism | (away from you). It will be seen that the north end of the sus- pended needle will remain parallel ' with and its north end pointing in the direction of the bar magnet 'contain-. ing south magnetism, just as though: