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 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 compass-maker, in the year 1576, and he then found it to dip nearly 72o, 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° 0′

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, or both south, repel each other. 5. Poles of different names attract each other.

6. The loadstone is an iron oar naturally possessing magnetism.

7. Magnetism may be communicated to iron and steel,

8. A steel needle rendered magnetic, and fitted up in a box, so as to move freely in any direction, constitutes the mariner's compass.

Charles. I think there is a similarity be tween electricity and magnetism.

Tutor. You are right; there is a considerable analogy, and a remarkable difference also between magnetism and electricity.

ELECTRICITY is of two sorts, positive and negative; bodies possessed of the same sort of electricity, repel each other, and those possessed of different sorts attract each other.-In MAGNETISM, every magnet has two poles; poles of the same name repel each other, and the contrary poles attract each other.

In ELECTRICITY, when a body, in its natural state, is brought near to one that is electrified, it acquires a contrary electricity, and becomes attracted by it.-In MAGNETISM, when an iron substance is brought near

one pole of a magnet, it acquires a contrary polarity, and become attracted by it.

One sort of electricity cannot be produ ced by itself. In like manner, no body can have only one magnetic pole.

The electric virtue may be retained by electrics, but it pervades conducting substances. The magnetic virtue is retained by iron, but it pervades all other bodies.

On the contrary: the magnetic power differs from the electric, as it does not affect the senses with light, smell, taste, or noise, as the electric does.

Magnets attract only iron, but the electric fluid attracts bodies of every sort.

The electric virtue resides on the surface of electrified bodies, but the magnetic is internal.

A magnet loses nothing of its power by magnetising bodies, but an electrified body loses part of its electricity by electrifying other bodies.