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Steel and nickel retain the greater part of the induced magnetism, and are said to be permanently magnetic. © Cast iron also retains a large propor- tion of the magnetism imparted to it, Even so-called permanent magnets, however, lose a portion of their pow-' er gradually; but by "closing" their poles with pieces of soft iron; which thus become induced magnets with dissimilar poles in contact, the induc- ing effect of these pieces strengthens the magnetism; such pieces of iron are termed armatures. If magnetized steel is heated to redness, or is sub- ject to violent blows, it loses its mag- netism, That a magnetic needle points approximately north and south is due to the fact that the earth itself is a huge magnet, whose conditions accordingly relate to what is. called terrestrial magnetism. dus, - the north magnetic pole is not at present identical with the true north pole, but is situated within the Arctic circle. In consequence of the different posi- tions of the magnetic pole and the geographical north pole, a magnetic needle does not point true north and south, but a little to the right and left, according to the locality. This- is termed the declination of the needle, and on the great lakes varies from 8° easterly to 10° westerly. The amount of declination varies from year to year, and in this country is at present diminishing at the rate. of about 6' per annum, When a needle is bal- anced on a horizontal axle, so that it can turn in a vertical plane, and is then magnetized, it is found to set itself at an angle depending onthe locality, with the north-seeking pole pointing downwards if north of. the magnetic equator, and the south-seek- ing pole pointing downwards if south of the magnetic equator. This is termed the inclination, or dip of the needle, and a needle, thus arranged is termed a dipping needle. The amount of the dip varies in different places. Magnetic charts are maps on which are marked lines showing the distribution of the earth's magnetism. It is found that the three magnetic elements, as the declination, dip, and intensity of magnetic force are termed, vary not only in different places, but also in the same place, from year to year, from month to month, and even from hour to hour. Those changes which proceed gradu- ally for several years are termed secu- lar. Frequently disturbances which produce a temporary irregular effect on all the needles over a consid- erable area; these are termed mag- netic storms, and are often connected occur . _ poles, TAE Marine. RevIEW with manifestation of électrical phe- nomena, such as the aurora borealis, or a violent thunderstorm, and still more generally with those solar out- bursts known as spots on the sun, All such changes in the earth's mag- netism are now daily recorded at many stations by self-registering .ap- paratus, In the vibration experiments the amplitude .of arc by which the needle is drawn aside should be as small as possible, consistent with accurately noting the last of the vibrations across the starting point, which should allow the needle to pass two or three divisions beyond, in order to be sharp- ly defined. . The same amplitude should be used for each set of vibra- tions. : Other methods of marking or nam- ing the poles of a magnet are as fol- lows: The north pole is called the positive or plus pole, marked thus, +; and the south pole, the negative or minus pole, marked thus, -- SOME MORE GOOD EXPERIMENTS. For purposes of discussion, a theo- retical magnet is assumed, long and indefinitely thin and uniformly mag- netized. Such a magnet may be looked upon as a pair of poles united by a bar exerting no action, the whole mag- netic effect being concentrated at the When it is freely suspended, the line that joins the poles is called the magnetic axis, or the equator of the magnet. The force exerted at different dis- tances between the poles of the same magnetic mass is inversely propor-. tional to the squares of the distances. The force exerted at a given dis- tance between two poles is directly proportional to the product of the magnetic masses of the poles. With a given field, iron receives the greatest amount of magnetization, steel coming next. When the mag- netization ceases to increase, the sub- stance is said to be saturated. The magnetic field is the space sur- rounding a magnet in which such mag- net exercises its influence, and uni- formity supposes the lines of force to be parallel. Magnetize a piece of watch-spring about six inches long, and ascertain how large a nail it will Bring the two ends of the magnet into contact. The ring thus formed mani- fests no polarity, thus showing the equality of the opposite poles. Break the magnet at its middle, and test the strength of magnetization of the two new poles developed at the point of fracture. Nearly fill a slender glass tube with support. 29 steel filings, and close the ends of the tube with corks. Draw the marked pole of a strong magnet from the middle of the tube to one end, and the unmarked pole from the middle to the other end, and repeat the strok- ing several times. One end of. the tube will attract and the other will re- pel the marked pole of a suspended magnetic needle; that is, the filled tube has become a magnet. Thor- oughly shake up the filings; the tube loses its magnetic properties, as. if. the | actions of the many little magnets in the tube were neutralized through their indiscriminate arrangement. When. a magnet is broken each piece becomes a magnet, the newly developed pole being of strength nearly, equal to that of the original poles. The subdivision of the magnet may be carried on indefinitely and with the same results. This suggests that the magnetic property must reside in the smallest parti- cle capable of existing by itself, that is, the molecule. It is easy to imag- ine that if the steel particles in the shaken tube could be restored to their former positions, the tube would again act as a magnet. It may be assumed that the - molecules of a magnetic substance . do. not exhibit magnetic properties when*the magnetic axes of the molecules are turned indifferently in every direction; and that the pro- cess of magnetization consists in turning the molecules so that their \ axes point in the same direction. When the axes of all the molecules have thus been set parallel, the maxi- mum of magnetization has-been se-. cured, Magnetization: changes the length, . but not the Volume of the bar magne- tized. When soft iron is rapidly mag- netized and demagnetized, it becomes heated, and sometimes-a_ clicking sound is. produced at each change. Take a bar of very soft iron, about three feet long, and make sure by trial that. its ends will not attract bits of soft iron. Then hold the bar in the plane of the magnetic meridian, as nearly as you can tell, and--with- its north end depressed below the hori- zon a number of degrees greater than the corresponding latitude of the place of experiment, that is, giving it the' position of a dipping needle. Tap the rod on its end with a mallet or wood- en block, and test it for magnetic po- larity. The end towards the earth, in our latitude, would have north mag- netism, and. will attract the south end of a suspended needle. It will also support small bits of iron. A magnetic field is recognized by the fact that it gives a definite direc-