If the tack in contact with the magnet be taken in the hand and the magnet suddenly withdrawn, the tacks will at once lose their magnetism and fall apart. [Illustration: Fig. 9.—A Magnetic Chain.] It will furthermore be found that a certain magnet will support a certain number of tacks in the form of a chain, but that if a _second_ magnet is placed beneath the chain, so that its south pole is under the north pole of the original magnet, the chain may be lengthened by the addition of several other tacks. The reason for this is that the magnetism in the tacks is increased by induction. *Magnets will Attract or Repel* each other, depending upon which poles are nearest. Magnetize a sewing-needle and hang it from a thread. Bring the north pole of a bar magnet near the lower end of the needle. If the lower end of the needle happens to be a south pole it will be attracted by the north pole of the bar magnet. If, on the other hand, it is a north pole, it will be repelled and you cannot touch it with the north pole of the bar magnet unless you catch it and hold it. This fact gives rise to the general law of magnetism: _Like poles repel each other and unlike poles attract each other._ [Illustration: Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.] Another interesting way of illustrating this same law is by making a small boat from cigar-box wood and laying a bar magnet on it. Place the north pole of the bar magnet in the bow of the boat. Float the boat in a basin of water. Bring the south pole of a second magnet near the stern of the boat and it will sail away to the opposite side of the basin. Present the north pole of the magnet and it will sail back again. [Illustration: Fig. 11.—A Magnetic Boat.] If the south pole of the magnet is presented to the bow of the boat the little ship will follow the magnet all around the basin. The repulsion of similar poles may be also illustrated by a number of magnetized sewing-needles fixed in small corks so that they will float in a basin of water with their points down. [Illustration: Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles.] The needles will then arrange themselves in different symmetrical groups, according to their number. A bar magnet thrust among them will attract or repel them depending upon its polarity. The upper ends of the needles should all have the same polarity, that is, all be either north or south poles. Magnetism flows along certain lines called *Lines of Magnetic Force.* These lines always form closed paths or circuits. The region in the neighborhood of a magnet through which these lines are passing is called the _field of force_, and the path through which they flow is called the