The following methods are among the best for forming artificial magnets ;

1. Place two magnetic bars A and B: (Fig. 25.) in a line, so that the north or marked end of one, shall be opposite to the south end of the other, but at such a distance, that the magnet c, to be touched, may rest with its marked end on the unmarked end of B, and its unmarked end on the marked end of A. Now apply the north end of the magnet L, and the south end of D, to the middle of c, the opposite ends being elevated as in the figure. Draw L, and D asunder along the bar c, one towards A, the other towards B, preserving the same elevation: remove L D, a foot or more from the bar when they are off the ends, then bring the north and south poles of these magnets together, and apply them again to the middle of the bar c as before: the same process is to be repeated five or six times, then turn the bat, and touch the other three sides in the same way, and with care the bar will acquire a strong fixed magnetism.

2. Upon a similar principle, two bars A B, C D (Fig. 26.) may be rendered magnetic. These are supported by two bars of iron, and they are so placed that the marked end в may be opposite to the unmarked end D; then place the two attracting poles G I, on the middle of a B, as in the figure moving them slowly over it ten or fifteen times. The same operation is to be performed on c D, having first changed the poles of the bars and then on the other faces of the bars; and the business is accomplished..

The touch thus communicated may be, farther increased by rubbing the different faces of the bars with sets of magnetic bars, disposed as in Fig. 27.

James. I suppose all the bars should be very smooth.

Tutor. Yes, they should be well polished, the sides and ends made quite flat, and the angles quite square, or right angles.

There are many magnets made in the shape of horse-shoes, these are called horseshoe magnets, and they retain their power

very long by taking care to join a piece of iron to the end as soon as it is done with. Charles. Does that prevent its power from escaping?

Tutor. It should seem so; the power of a magnet is even increased by suffering a piece of iron to remain attached to one or both of its poles. Of course a single magnet should always be thus left.

James. How is magnetism communicated to compass needles?

Tutor. Fasten the needle down on a board, and draw magnets about six inches long, in each hand, from the centre of the needle outwards; then raise the bars to a considerable distance from the needle, and bring them perpendicularly down on its centre, and draw them over again, and repeat this operation about twenty times, and the ends of the needle will point to the poles contrary to those that touched them.

Charles. I remember seeing a compass when I was on board the frigate that lay off Worthing, the needle was in a box, with a glass over it.

Tutor.

The mariner's compass consists of the box, the card or fly, and the needle. The box is circular, and is so suspended as to retain its horizontal position in all the motions of the ship. The glass is intended to prevent any motion of the card by the wind, the card or fly moves with the needle, which is very nicely balanced on a centre. It may, however, be noticed that a needle which is accurately balanced before it is magnetised, will lose its balance by being magnetised, on account of what is called the dipping, therefore a small weight, or moveable piece of brass, is placed on one side of the needle, by the shifting of which the needle will always be balanced.

CONVERSATION XXVI.

Of the Variation of the Compass.

CHARLES. You said, I think, that the magnet pointed nearly north and south, how much does it differ from that line?

Tutor. It rarely points exactly north and south, and the deviation from that line is called the variation of the compass, which is said to be east or west.

James. Does this differ at different times? Tutor. It does; and the variation is very different in different parts of the world. The variation is not the same now that it was half a century ago, nor is it the same now at London that it is at Bengal or