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The power of inducing magnetism in iron diminishes 'as the distanee increases. When the inducing magnet is slowly re- moved, the force éxerted by the ends of the iron bar undergo a dimunition ; corresponding and they cease. to exist' al- together when the magnet is wholly with- drawn, and the bar returns to its natural or neutral state. Thus the magnetism induced in iron is temporary only, | The foregoing facts enable us to account for the attraction of iron by the magnet, and to reduce the phenomenon to the general principle of the attraction of op- posite, and the repulsion of similar poles. When the pole of a magnet is brought near to a mass of iron, the immediate effect, we have seen, is to render the latter a magnet by induction, an opposite magnetism being developed in part of the mass which is nearest to the acting pole; and a similar magnetism in the part which is most remote, Now, as opposite poles attract one another, while similar poles repel, attraction will-ensue between the acting pole and the nearer parts of the iron mass, and the repulsion between the same pole and the remoter parts; and the former of these forces will pre- ponderate, being exerted at a shorter dis- tance. It follows from this that attrac- tion must always result, whichever pole of the magnet be presented to the iron. Since the magnetism which has been induced in a piece of iron imparts a sim- ilar condition to.a second piece in its vicinity, and that to a third, and so on, it follows that attraction must ensue be- tween these several pieces, although with diminishing energy, as they are more dis- tant from the acting pole; and the first will support a second, a second a third, and so on, if only the weights of the successive pieces be proportionate to the diminished force. A magnetic chain can thus be formed, having two poles. 'The following experiment illustrates the mat- er: Let a few pieces of iron rod, or a few soft iron nails be taken, If one of these be placed in contact with the pole of a permanent steel magnet, it is at- tracted to it and becomes a temporary magnet; it now, in turn, supports a sec- ond piece; this, in turn,,a third piece, and so on, until a chain of four or five pieces is built up. Each. of these pieces of iron is itself a. magnet. If it be the north pole of the magnet to which the pieces are attached, the end on the first piece in contact with the steel magnet will have south, and the other end north, magnetism, and this.last will develop in the nearest end of the next piece of iron, south magnetism, and so on. But if the steel magnet be removed from the top of the chain, all the rest drop off and are 'found to be no longer magnetic. .A TAE Marine REVIEW piece of soft iron laid with one end on the end of a magnet practically be- comes part of the magnet, simply ex- tending its length, and its further end being of the same polarity as that of the. magnet to which it is joined. POLARITY, The influence that gives directive force to the compass needle, causing it to point to the' magnetic north, is called polarity. It is customary to regard the polarity which exists,jn the, north end of the compass needle, red __ polarity, whence it follows that south, blue polar-_ ity, must pervade the regions of the ter+ restrial north, because it is a law of na- ture that bodies similarly magnetized will repel, while those dissimilarly poet ized will attract each other. SOFT IRON, _ As regards magnetism, it is iron, which becomes instantly magnetized to its full capacity, when exposed to the influence of any magnetized body, and which loses its magnetism instantly when the influ- encing body is removed. Now the earth may be regarded as a huge mag- net having the same properties as the natural and artificial magnet and_ pos- sessing the two poles (north and south polarities), and a neutral line, or mag- netic equator. The north magnetic pole of the earth's magnetism was sit- uated, at the time of discovery, in Boothia Felix, to the NNW, of Hud- son bay, in British North America, and -has actually betn reached by man, it being discovered by Sir James Ross, of the English navy, on June 1, 1831; while the other, lies to the south of Tasmania, but owing to impene- trable barriers of ice, has not been quite reached, but so nearly that its 'position was without difficulty ascer- tained by calculation. The magnetic poles of the earth are entirely distinct from its poles of rotation, and too, they are not diametrically opposite each other as the true poles of. the earth; the north magnetic pole being" situated about 1,200 nautical miles south of the true or geographical pole of the earth, and the south magnetic pole about 900 nautical miles north of the true south pole. Like other magnetic bodies, the two terminal portions of the earth's mag- netism are its poles, where the attrac- tion is greatest. geographical poles, which are repre- sented by a mere point, the magnetic poles include a. considerable area of the earth's surface, amounting per- haps to fifty square miles. These mag- netic poles are not permanent fixtures, but instead they shift slowly from west to east and back again after long' periods of time, but for what reason 'each other, meridian of 11° Unlike the true, or. 33. no one knows, it being due to some unknown law. The True or Geographical Poles are the extremities of the earth's axis, the two points of 90° north and south from the equator; or where all the meridians of longitude cross or cut one another, or the points where there is no longitude at all or 360° of it, just as you please. These -are also the points where there is only one direction; being sttuated on the true north pole of the earth, you could only look or go in one di- rection, namely, south, therefore, the direction south must commence at the true north pole of the earth. The conditions would be identical to an observer situated on the true south pole of the earth, except the one di- rection there, being north; the direction north must commence at the true south pole of the earth. An observer situated at either of these two points during the summer season of each, would always have the sun on the meridian, bearing either due north or due south, but if he should take one step from the pole, the above conditions would instantly change, for: he would then have all the directions of the coim- pass, and the sun would change _ its bearing readings, the same as other posi- therefore, 'tions on the earth's surface. The reader of this will probably wonder where the directions east and west commence. These directions, as well as all others, with the exceptions of north and south, have no commencing point; start from anywhere. MAGNETIC EQUATOR OF THE EARTH, Now, as the earth has*two magnetic poles, it must also have a magnetic equator, This magnetic equator is a they can 'sinuous curve ericircling the earth, and crosses the geographical equator in two places nearly diametrically opposite to something after the fashion of the ecliptic. The magnetic equator crosses the earth's equator on the east- ern side of the Atlantic, at about the west, and the other in the Pacific, about longitude 168° west. Its greatest divergence from the true, or earth's equatot, is in Brazil. Now, the earth's magnetic equator, acts the same as the equator does of any other magnet. It separates the two polarities of the earth's magnetism from each other. It is-a neutral. line whete there is no /vertical force issuing forth. This means that at any place on this line on the earth's surface, the dipping needle (to be explained later) rests in a horizontal position. D. C. Stover: & Co., Freeport, Hf, have undertaken the manufacture of marine engines.