be opposite to the unmarked end D; then place the two attracting poles G 1, 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 horse-shoe 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 magnetized, will lose its balance by being magnetized, 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 Kamtschatka. The needle is continually traversing slowly towards the east and west.

This subject was first attended to by Mr. Burrows, about the year 1580, and he found the variation then, at London, about 11° 11' east. In the year 1657, the needle pointed due north and south: since which the variation has been gradually increasing towards the west, and in the year 1803, it was equal to something more than 24° west, and was then advancing towards the same quarter.

Charles. That is at the rate of something more than ten minutes each year.

Tutor. It is; but the annual variation is not regular; it is more one year than another. It is different in the several months, and even in the hours of the day.

James. Then if I want to set a globe due north and south, to point out the stars by, I must move it about, till the needle in the compass points to 24° west.

Tutor. Just so: and mariners, knowing this, are as well able to sail by the compass, as if it pointed due north.

Charles. You mentioned the property which the needle had of dipping, after the magnetic fluid was communicated to it is that always the same ?

:

Tutor. It probably is, at the same place: it was discovered by Robert Norman, a compassmaker, in the year 1576, and he then found it to dip nearly 72°, and from many observations made at the Royal Society, it is found to be the

same.

James. Does it differ in different places ?

Tutor. Yes. In the year 1773, observations were made on the subject, in a voyage toward the north pole, and from these it appears that

In latitude 60° 18′ the dip was 75° Ꭴ

I will show you an experiment on this subject. Here is a magnetic bar, and a small dipping needle: if I carry the needle, suspended freely on a pivot, from one end of the magnetic bar to the other, it will, when directly over the south pole, settle directly perpendicular to it, the north end being next to the south pole. As the needle is moved, the dip grows less and less, and when it comes to the magnetic centre, it will be parallel to the bar; afterwards the south end of the needle will dip, and when it comes directly over the north pole, it will be again perpendicular to the bar.

The following facts are deserving of recollection.

1. Iron is the only body capable of being affected by magnetism.

2. Every magnet has two opposite points, called poles.

3. A magnet freely suspended arranges itself so that these poles point nearly north and south. This is called the directive property, or polarity of the magnet.

4. When two magnets approach each other, the poles of the same names, that is, both north, vor both south, repel each other.

5. Poles of different names attract each other. 6. The loadstone is an iron ore naturally possessing magnetism.

7. Magnetism may be communicated to iron and steel.